How-To: Size Your Pilot Jet, Pilot Screw Explained

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How do you tell if your pilot jet is too lean, rich, or just right?

The pilot jet is sized correctly if the pilot air/fuel screw(s) setting falls in the effective range, typically 1-3 turns out, with 1.5-2.5 being the sweet spot. This jet is also known as the slow jet, where it really impacts the 0-1/8 throttle positions.

 

What does a pilot air screw look like? What does it do?

Pilot Air Screw

A pilot air screw will be made of brass, gold in color, and will have a blunt tip compared to a pilot fuel screw. The pilot air screw meters air, so turning it clockwise/in will richen the fuel/air mix, while counter-clockwise/out will lean the fuel/air mix.

 

What does a pilot fuel screw look like? What does it do?

Pilot Fuel Screw

A pilot fuel screw will also be made of brass, gold in color, and will have a pointy “needle tip”. That is why you always want to “lightly” seat these, the tips can easily break! I use the smallest diameter screwdriver so I generate the least amount of torque. The pilot fuel screw meters fuel, so turning it clockwise/in will lean the fuel/air mix, while turning it counter-clockwise/out will richen the fuel air mix.

 

Where is the pilot air/fuel screw located?

Let’s define the carburetor butterfly or slide the dividing line between the air box side, and the engine side. A pilot air screw will be located on the air box side, while a fuel screw will be located on the engine side. This is true for most carburetors. I’ve seen the pilot air screw on the engine side on most Amal carburetors. When in doubt, remove the screw and see what kind of tip it has, that will determine what it meters.

 

What are some indications of a lean pilot jet and/or screw setting?

  • If your air screw is in further than 1 turn, you need a richer pilot.
  • If your fuel screw is out further than 3 turns, you need a richer plot.
  • Off idle hesitation or bog.
  • Revving the engine in neutral will result in hanging RPM’s, or RPM’s will slowly drop back to idle RPM.

What are some indications of a rich pilot jet and/or screw setting?

  • If your air screw is out further than 3 turns, you need a leaner pilot.
  • If your fuel screw is in further than 1 turn, you need a leaner pilot.
  • Off idle heavy feel or sputter.
  • Revving the engine in neutral will result in RPM dip below idle RPM, RPM’s will drop quickly.
  • Turning the fuel screw all the way in will not stall the engine.

 

How do you properly adjust a pilot air/fuel screw?

The proper way to tune a fuel/air screw is to make adjustments on a fully warmed up engine and at LOW RPM. Lightly seat the fuel/air screw and set to factory settings as a baseline. With engine running at low RPM, turn the screw in clockwise until the engine begins to stumble, then turn the screw counterclockwise until the engine stumbles, noting how many turns in/out both occurred. Now, adjust between these two settings to achieve highest RPM and smoothest running engine. Reset idle to recommended settings and test ride motorcycle. If engine bogs or hesitates during acceleration, richen idle screws 1/4 to 1/2 turn and see if that helps. Rev engine in neutral and note how RPM’s return to idle. If RPM’s hang, richen the mix. If they dip below idle, lean the mix.

Here is a short video I put together to help explain this information along with a FUEL screw adjustment procedure.

 

 

Here is an example of a an AIR screw adjustment.

 

 

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  • jim

    hi i have a yamaha grizzly 660 2004 what causes it to back fire when i let up on the throttle after driving at about 60 -80 km hr please and thankyou

    • Matthew

      Jim,

      When you close the throttle and decelerate, the engine runs lean, and can pop through the exhaust. Sometimes this is considered normal with a race engine and open pipe.

      However, if this is something new, check for exhaust & vacuum leaks, as they can cause this as well.

      Give me a bit more info: stock pipe, jetting, fuel screw set properly?

  • Michael

    Hi,
    I have an 82 Susuki GS650G, Canadian Model. The previous owner replaced the old air cleaner with four individual cone filters letting through allot more air. He also put on quieter exhausts. When he sold it to me he mentioned that I needed to increase the Jet size, Didn’t tell me which jets, and that I needed to raise the pins. Due to time problems I have yet to do either. Could these be the reason my Engine just randomly dies while on the Highway? Or could there be something more serious involved.
    Thanks in Advance
    Michael

    • Matthew

      Michael,

      The pod filters may or may not let more air in at idle. They do let more air in at higher RPM’s.

      Let’s start with the pilot jet. If your fuel adjustment screw ends up in the 1-3 turns out range, and have no bogging or hesitation off the line, it’s good to go.

      The needle clip position and main jet is something you will have to test trial and error and read the plugs. Usually, I would expect the clip to be one position lower than stock and the mains 1-2 sizes bigger than stock. Get a hold of a service manual and note stock jetting. Open up your carbs and write down what you find.

      Randomly dies on highway. Jetting may or may not be related to this issue.

      • Michael

        I’ll be sure to do that this weekend, what do you mean by “Read the plugs”?

        • Matthew

          Reading spark plugs gives you indication of whether it is running rich, lean, or just right. See my post on “How-To: Read Spark Plugs” which covers this topic and also how to perform a plug chop.

    • BH

      All things being equal and no other apparent or obvious issues/problems, the most common reason a motorcycle inexplicably “dies” on the highway (and nowhere else) is fuel starvation due to empty float bowls caused by clogged filter screens above float bowl needle/seat intake valve assemblies. Remove the needle/seat assembly and use a small screwdriver or pick to remove the tiny little filters and clean them. If you can park the bike on the shoulder for 10 minutes after it dies (giving fuel time to refill float bowls) and then start up and run fine again (for awhile)… dimes to donuts, that’s your problem. It only shows up on the highway because the increased fuel consumption at highway speeds outpaces the fuel flow slowly dripping into the float bowl(s) through dirty filters and empties the bowls before they can refill. The most common reason for this condition is rust in the gas tank. Tiny particles can get through the fuel filters above the petcock (which also needs to be cleaned) or other inline filters (replace). People park their motorcycles outside, especially in the winter, with untreated bare metal interior, part-empty fuel tank causing the inside surface of the tank to rust. Warmer days and colder nights affects air pressure in the empty space above fuel in tank which expands and contracts drawing in cooler moister air in the evening and overnight through overflow/breather = rust.

  • Matthew

    I have and 83 virago 500 which has 2 carbs. How does tuning two carbs differ from one carb if it differs at all?

    • Matthew

      Both carbs should be tuned to the same number of turns out on the pilot fuel or air screw. This will result in a smooth running engine. Obviously, when tuning, you can only work on one side at a time.

  • Joey

    I have a 2005 polaris predator 500. I removed the spark arrestor from the stock exhaust and this caused my quad to backfire frequently. it has stock jets. should i go to a higher jet i think its got a 150 main stock

    • Matthew

      Joey,

      When does it backfire? On deceleration with throttle closed? Have you adjusted the fuel screw after removing the spark arrestor?

  • Joey

    Mostly when letting off of the throttle. I have not yet I just replaced my valves and am debating if I will need to rejet

    • Matthew

      OK. I will tell you that some popping on decel is normal with an open pipe. I would try readjusting your fuel screw. If you are close to 3 turns out, bump up the pilot jet 1-2 sizes.

      On the main jet, you’ll have to a WOT plug chop and read the plug.

  • Joey

    What is a WOT plug chop? Eventually I’ll be upgrading to a k&n air filter eventually so I just want to get an idea of what my carb should be set at

    • Matthew

      WOT = wide open throttle. Search my site for “How To Read spark plugs”. With a K&N, you should probably richen the main by 2 sizes. Do you plan on running the air box lid? If not, maybe jump 4 sizes on main. The pilot is adjusted for max RPM and hopefully is in the 1-3 turns out range.

      It helps to have all mods in place as you only want to do this once 🙂

  • Joey

    Yeah I’m gonna keep the lid on. My airbox does have holes drilled into the back of it though

    • Matthew

      Sounds good. Check the fuel screw adjustment and report back. Main will need to be increased by 2-4 sizes for sure. You will be moving more air and need to compensate with more fuel.

  • Stuart

    I have a sherco 250 which is very flat/slow to pick up, would you advice a bigger pilot jet as adjusting the idle doesn’t seem to work too well.

    • Matthew

      Stuart, did you try to adjust the pilot air screw first? How many turns out? Does it bog? What does the plug look like after idling for a few minutes?

  • Stuart

    As it stands at the moment, the pilot fuel screw is at 2.5 out and the air is at 4.5, though I’m told anything past 3.5 makes no difference. Can’t say I’ve looked at the plug after idling but will do at the weekend. Does it bog ? No not really, just doesn’t pick up quickly from first applying the throttle.

    I should have said it is a sherco 250 trials bike.

    • Matthew

      Stuart, you should have an air OR a fuel screw, not both. There will also be an idle screw. Get back to me on what you have. Most likely an air screw.

  • Matt

    So not a tech question, but is that a Yamaha XS series bike in the video?

    • Matthew

      Matt, good eye. It’s a 76′ XS360

  • jimmi banke

    Matthew your the man, great site!

    I have the same xs750 “77” triple you have worked on.

    Im about to fine adjust the carbs, and want to know the settings you ended up with? just for a basic start point when i adjust mine. I too have some small filters on, but have a aftermarket exhaust with more flow through.

    cheers
    jimmi

    • Matthew

      Jimmi,

      Thanks for the compliments! The 77′ XS750 actually has an 850 motor in it that was punched out to 885 or 888. It has air filter pods and a MAC 3 into 1 exhaust. Pilot jet was one step larger (45?) and fuel screw 2.75 out. Needle in middle position. Main: 132.5. Runs good.

  • Michael

    Great site ! I watched the how to about adjusting fuel/air mixture screws. I am currently tuning my carb and would greatly appreciate some guidance. I’ll give some info first.
    2012 klr650 w/CV40 carb,just made changes from stock:
    drilled slide 7/64
    klx needle 2nd clip 1 #4 shim
    142 jet (from 145)
    new extended fuel mixture screw
    slip on mid pipe back Lexx exhaust
    race tip (big unrestricted one)
    original tube spark arrestor swapped with free flow Spark arrestor
    1″X 4 holes in air box
    new uni filter

    With all the changes, the bike transformed in attitude…great throttle response, and it just sounds mean!… Only thing is, I’d like to get it dialed in nice. It’s a little over poppy on the decel…( I expect some ) and the fuel mixture screw can be all the way seated and it doesn’t slow down idle.
    It does run pretty good across the spectrum of adjustment, but better towards the closed side of adjustment. I did not change the pilot jet.

    Any advice on getting this carb tuned in would be appreciated !
    Again, nice work on the videos, real clear information for people to understand. Thanks.

    • Matthew

      Hi, Michael.

      First off, thanks for the compliments, and the KLR is a cool bike BTW! I do quite a bit of work on a 87′.

      Popping is to be expected with an open “race” pipe. You can get rid of it by richening pilot/fuel screw, but sounds like you need to go the other way, leaner, based off your fuel screw position.

      Since your best idle is at almost closed fuel screw position, I would decrease pilot by 1-2 sizes and see if you can get fuel screw in the 1-3 turns out range.

      BTW, can you give me a link to the adjustable fuel screw? It’s impossible to adjust stocker.

      • Michael

        Hey Thanks for getting back to me, going to head to the bike shop in search of some pilot jets. I’ll let you know how it works out. As for the mixture screw, there are several KLR websites that sell the screw, but the same carb(cv40)is used on harleys a lot, so I searched outside of the KLR community. Found this place, the screw was cheaper, and the shipping was fast & free.

        Thanks again.

        http://www.cv-performance.com/products/

        • Matthew

          Thanks for the link, Michael.

          BTW, I find it odd that you have to go leaner on pilot, usually these bikes are lean from the factory. Have you verified the pilot jet size in service manual?

  • Jared

    Hi I am having trouble getting my quad to run and I’d like some advice.

    It is a 2000 Kawasaki Bayou 220, we cleaned the carb in a dip tank and
    made sure everything’s cleaned well, when we start it we use fluid because
    it wont start on its own 50% of the time.

    once it starts it idles for maybe 2-5 seconds and it dies, we’ve tried
    tapping the throttle to keep it running white we fiddle with adjustments
    but we have found no success, currently it sits at 2 turn out on the A/F
    screw, and the idle screw does nothing regardless of position..

    We’re at a complete loss as to what to do with it. I’m really hoping you
    can help me because I’m losing my mind with this thing and I’ve got 3
    more quads that wont run to fix still.. and those are:
    a 94′ bayou 220
    a 88′ Blaster YFS200
    a 87′ LT250R QuadRacer

    I’m sure I’ll have more questions for you since all of them are having
    carburetor related issues :/

    • Matthew

      Jared,

      If it will only run on starting fluid, then the carb is not delivering fuel. When you had carb apart, did you spray carb clean through all passages and verify they are clear? Did you hold all jets up to a light source to see if you can see through them?

      Have you performed a compression test?

  • Allan

    Hi,
    The first time I cleaned out my carbs I didn’t know I would get them out of tune. I got it working really well afterwards, a lot of power, but I heard a clicking sound after about 2 months and I thought maybe it was something I did in the carbs. I opened up my carbs once again and cleaned them. When I put it back together, the pilot fuel screw wouldn’t make a difference when I would turn it and my motorcycle only runs max 55 miles now and has very little acceleration. I can’t seem to adjust it. My pilot fuel screw is different from the picture though, mine has a spring on it and it screeches when I turn it. Supposedly its normal, unless it’s not a fuel screw. Should I keep playing around with it or throw it at the junkyard? I don’t know what to do. If I can’t fix it myself, I don’t want to keep it. I don’t trust mechanics, I took it to one and he charged me 400 dollars and there was no change to it. He didn’t even give me a receipt. Supposedly he worked for Universal Studios fixing all kinds of machines… anyway thank you for help. Oh and I didn’t know about the air screw. I don’t think I ever saw that on my bike, is that inside the carburetor? Do we adjust that too??

    Thank you,

    Allan

    • Matthew

      Allan,

      The fuel and pilot screw will have a spring on it. In most cases, the fuel screw will have a washer and o-ring also. The purpose of the images above is to illustrate the differences in the tip, which will indicates what the screw meters.

      If you here “clicking” in your engine and the power dropped off, perform a compression test. Also, hold a screwdriver to ear and engine and try and locate the noise.

  • Mark

    Hello, i have a kawasaki mule 2002 with the 620 motor. it never has run right at idle so after some time replaced carb with a new one from kawasaki. the new carb seemed a little better it gave me some idle adjustmant for a while but still seems to be quite lean at tickover. now it has the idle screw right in and strugles to idle and stalls. the top end revs seem good and there is power. also when punching the throtle it bogs. i have looked on various sites and they say there is only one size of pilot jet and there non adjustable due to some regulations?

    • Matthew

      Mark,

      There probably is some way to adjust the fuel mixture via a pilot air or fuel screw. It may be “hidden” under a welch plug or cap. I’ll send you an email and maybe you can send me a pic of the carb.

  • Mark

    Thanks for the reply, success i have found a pilot fuel screw hidden under a plug and it is running much better now. thanks

    • Matthew

      Great job!

  • Manie

    Hi Matthew,
    I’m from South Africa.I got hold of a Sachs Madass 125 with 3000 k’s on the clock.As far as I could see on the internet, these bikes are fitted with Mikuni carbs,whereas mine has a TK carb on (round/slide).Main jet=94,Pilot jet=40.Could you please tell me how many turns out, the air screw should be?Can’t get it to idle propperly THANKS

    • Matthew

      Should be about 1-2 turns out as a baseline. You will have to adjust while engine is HOT and tune for highest RPM.

      • Manie

        Thanks a lot ,now I can work from there!!

  • Brent Pfeiffer

    I have a 97 gsxr 750 sport bike that has 4 CV carbs. Do the setting for them have to be the same or different. I know the bike is running rich for sure(throwing fuel out) and it has a off idle hesitation. Happens a lot while cruising around town.almost feels like I’m riding a horse. I have tried every gear and all different rpms and its no better no worse. Seems like any gear or rpm if I hold the throttle at the same spot it hesitates. It also does it really bad if I pull in the clutch then get back on the throttle lightly. My manual says crazy numbers for the factory setting on these carbs. Its something like 4 1/4 outer and 2 3/4 inner. Runs terrible at those settings. I’ve got mods and the carb was professionally jetted. But it was done at a way different altitude then where im at now. So I’m hoping to just adjust the mixture to fix it. Thanks!

    • Matthew

      Brent,

      Alright, let’s take this step by step.

      All carbs should have the same fuel screw setting, for example, all should be 3 turns out.

      Please explain what you mean by rich and throwing fuel out?

      The hesitation, is it a bog (lean) or rich (sputter)?

      List all mods and what your current temp and altitude are.

      Sounds like you’ll have to take carb apart and record all jet #’s and compare to stock and go from there.

  • Brent Pfeiffer

    Thanks for the reply man I really appreciate it. The bike has a full Yoshi exhaust header and silencer. The silencer has been cut shorter as well. It has an ignition advancer +2 and k&n filter. My main jets are 132.5 outer and 130 inner. Stock is 130 outer and 125 inner. My needle is on the middle clip with a shim under it. And my pilot jets are all 2 steps up from factory which is 12.5 so they are all 14.5s and my mixture screws are all messed up. Bike runs amazing if I’m flogging it. But off idle it has a stumble I guess. How can you tell a stumble from a hesitation? I’m 100% sure its not a bog let’s just put it that way lol. And by throwing fuel out I mean if I rev it I get some black smoke just a little bit but then it puffs a lot more once it comes down to idle. And I had it at the drag strip last night and right at every shift it puffed black is what my brother was telling me. I’ve tried every setting I can try with the mixtures besides going way in like between 1-3 because of what my manual says they should be at stock. Should I just disregard the manual? Thanks again for you response I really appreciate it!

    • Matthew

      Brent,

      Lightly seat all pilot fuel screws and turn them all out the same, about 2 or 3 turns. With the bike HOT and idling around 1100 RPM or so, turn each 1/4 turn in at a time until idle begins to stumble. Turn each out 1/4 turn at a time until it stumbles. You should be able to get the highest RPM somewhere in between.

      All screws need to be the same amount of turns out.

      Do this and we go from there.

  • Brent Pfeiffer

    Also my temp is around 70s right now and altitude is 1400s above sea level

  • Brent Pfeiffer

    Sounds good. I will do that tomorrow. Should I put in new plugs? Since I know it has been super rich for quite a while. Could to much fuel have fouled them or be throwing off all my work if I do try to set the mixture? Thanks

    • Matthew

      If the plugs are not fouled don’t bother changing them until you have it running much better. I like to spray a bit of water on each header pipe to see if the cylinder is firing. Water should “sizzle” off at the same rate. No sizzle mean cylinder is not firing.

      • Brent Pfeiffer

        I am positive all cylinders are fireing. I have a temp gun and all header pipes read in the same spot closest the the head in the 300-500 range so I know they aren’t foul’d completely just I’m sure very black from a few years of extremely rich mixture. But I will do what you told me to do today and see where the mixture screws land. Thanks again for all you time. Just wish that Suzuki manual didn’t lead me so far off where it should be lol.

        • Brent Pfeiffer

          OK so I did what you told me to do. My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall. Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary. Thanks

      • Brent Pfeiffer

        OK so I did what you told me to do. My highest and smoothest idle was between 1 1/2 and 1 3/4 the difference between those two settings was about nothing. Any lower than 1 it starts to idle rough and stumble I went further to 3/4 and it was almost gunna stall. And out if I go any further than 2 3/4 it idles rough and 3 or further it is about to stall. Hope this tells you something. Havent taken it around the block yet because at this setting the bike idling in the garage needs the fan to stay at 225 degrees and that seems higher then I feel comfortable with. Should that high of a temp worry me? It is liquid cooled with a fan switch if I needed it but that made me weary. Thanks

        • Matthew

          Brent,

          Good to hear you made progress. I would take a test spin at 1.5/1.75 setting and see how it runs. The temp should creep while standing still. What is the temp while riding it? Report back after test ride.

          • Brent Pfeiffer

            I took a spin around the block and it is going in the right direction for sure. I had it at 1 3/4 and it still stumbled just a little bit here and there. So I took it for a long ride. Temp is 70 degrees here and the temp at 60 down the highway was about 170-180s and in town it would creep up to 200+ one thing I did notice is that I was able to slowly ease off the throttle without it bucking and kicking and it was much smoother at cruising speed in 6th gear at 60 giving like a 1/16th throttle. So I’m confident were heading in the right direction. I really appreciate your time and expertise! I’m gunna go down to 1 1/2 and see if its better. I’ll keep you posted. Thanks!

          • Brent Pfeiffer

            *Update*
            Went to 1 1/2 turns out and it still is doing the stumble. About the same as 1 3/4 honestly. I’m gonna try another 1/4 turn in. I’ll keep you updated. Thanks!

          • Matthew

            Brent, if it is stumbling/bogging/hesitating, you will need to go richer. Explain how you get it to stumble? Are you flogging it when rolling slowly?

          • Brent Pfeiffer

            When I get it to stumble I am either just taking off from a stop at very little throttle. Or if I’m rolling down the road and I pull the clutch in then let it back out and ease on the throttle. Always at around a 1/4 throttle or less. It will also do it in between gears while shifting right when I get back on the gas. Wide open there is no bog. I can cruise in 6th and go wide open and she gets right up and goes. Its just low speed and very little throttle that it does it. Thanks

          • Matthew

            Brent,

            OK, let’s talk about this stumble. We have to determine if it’s too rich or lean. If you add more throttle during stumble does it go away? If so, it might be a rich spot. If it is a lean bog, more throttle will not help it recover.

            You can also install new plugs and go ride it around and cause it to stumble over and over again and pull plugs to read them.

            It seems this stumble is occurring when on the pilot 100%. The fuel screw setting at idle trims the pilot fuel, and considering the fuel screw ends up at 1.5/1.75, the 1/8 throttle position (all 100% pilot flow) will be rich. You might want to drop a pilot jet size.

          • Brent Pfeiffer

            More throttle does make it go away. But different gears and different rpm doesn’t.

            Its a really weird kinda studder I guess really. It almost seems like a misfire in a car. If I hold the gas at the same spot while its doing it I can get it to buck and kick and carry on as long as I want. Sounds like wah wah wah wah wah all the way down the street until I give it a little more gas. I have tried a plug chop before to no avail. But then again I was also like 4 full turns further out on the mixture screwes thanks to the Suzuki manual lol. Thanks for all your guidance. I’m gonna try 1 1/4 and see if its any better or worse. I’ll keep you updated.

          • Matthew

            OK. Keep in mind that at 1/8 throttle you are fueling the motor on 100% pilot jet SIZE, the fuel screw is out of the picture by then for the most part. You can experiment with different fuel screw settings, however most likely it is all pilot by then.

            Also, is the timing advance adjustable? Too much advance can cause bucking like you described.

          • Brent Pfeiffer

            The timing advance is just the factory key for Austria. The bike when it was sold in the us had a retarded timing to meet regulation. And no its not adjustable. It is sold by a company called factory pro. And from what I hear they are pretty replutable. And it also did this before I did the advance. I have the next size down pilot jets so I will try that if the mixture change dosnt work. How exactly does the pilot size not get metered at 1/8th throttle? Thanks

          • Matthew

            The pilot jet actually feeds fuel to at least 2 passages. For example, with throttle blades closed, one passage will be on engine side, while the other on airbox side. The engine side passage is further “trimmed” or “metered” by the fuel screw. When throttle blades open to 1/8 or so, it uncovers the other passage and it will be 100% on pilot. So if it needs to be leaner or richer at 1/8 you have to swap the actual jets.

            Does that make sense?

  • Brent Pfeiffer

    Yes it does. Thanks. Now here’s a question. If my idle was set to high would that maybe cause the blades to be open all the time and possibly cause this? Because I was idling it at 1500 before when my mixtures were way out. but since adjusting the mixture it seems to idle smoother at lower rpm where it was opposite before. So could my idle be set to high and cause samething?

    • Matthew

      If your idle is set too high (throttle blades open too much), then any adjustments made by fuel screw may not be noticed. I like to have idle as low as possible when tuning fuel mix screws. Small RPM changes are much easier to notice.

      • Brent Pfeiffer

        My idle was below 1000 rpm when I made the fuel screw adjustment. But right when I was done I put it right back by habit lol. I’m thinking when my fuel screws were super rich I had to idle it up to keep it even running. Because now that I leaned them out I can idle smooth at way lower rpm. Before if I tried to idle lower it would surge the whole time. So I’m wondering if that would be opening that un-meatered hole and causing my stumble this whole time. Once it stops raining I will test my theory and let you know. Thanks

        • Matthew

          Alright Brent, please report back your findings.

          • Brent Pfeiffer

            So the weather in Ohio sucks and its been raining or to cold to ride ever since my last comment. But finally I was able to go another 1/4 turn in and take it for a ride. So here are my findings. Idling lower like in the 1100-1200 rpm range it is almost perfect at 1 1/4 turns out on all 4 screws. Any higher and the hesitation comes back. I also noticed something else that is different then before. The tip of my pipe Is turning brownish tan instead of black the way its been since I got the bike. So I’m thinking that is a good sign. I will be taking it to the track tomorrow to see if its any faster or easier to get off the line and will report back. Thanks so much for your help Matthew I really appreciate it!

          • Matthew

            Sounds like you are headed in the right direction. So the stumble was probably a sputter, not hesitation? I say that because you went leaner and things improved.

            Now that your fuel screws are nearing 1.25 out, you can get a leaner cruise (1/16-1/8) throttle by dropping pilot jet size. You will have to readjust fuel screws and they will end up about 1/2 further out. If you decide to do this, your will be at the same fuel ratio and the bike will be getting a little less fuel when 100% on pilot.

            Does that make sense?

          • Brent Pfeiffer

            Makes perfect sense. I will be putting in the next smaller pilot jets this weekend. I will report back my findings! Thanks!

          • Brent Pfeiffer

            So I put in the next size smaller pilot jets and it took a massive leap to the better side. I did exactly as u said and went a half turn out to compensate and now the only time it sputters is when trying to go like 5-10 mph which I would consider exceptable from where it was at. I also noticed it smoother in general throughout all rpm range and stretch links and all off the line I’m on the back wheel to the 60ft. Thank you so much for all your help!

          • Matthew

            Good job Brent! Glad to hear it worked out. Are you going to move on to the needle and main?

  • John

    I have a 1999 Honda Shadow Aero 1100 with Shotgun pipes with the baffles cut down from 12″ to 4″. About 2 months ago I had the carbs cleaned due to a drop in fuel mileage, from 120 miles to about 85 miles. Since the was done, my mileage has increased but, when starting from a stoplight, there is hesitation and an occasional stalling. Also it is now backfiring in the pipes on decal that wasn’t there before the clean up. I am not mechanically inclined and the mechanic that originally did the work in no longer in my area. What are your thoughts on this. Thanks in advance.

    • Matthew

      John, it sounds lean. Go ahead and lightly seat your fuel screws noting how many turns it took. Get the bike HOT and tune the fuel screws for highest idle. Report back how many turn out you end up with and whether it helped.

  • Dom

    I have an 82′ Yamaha xs400 heritage special…It hasn’t run since I got it, its got a brand new battery, the carb was recently cleaned, but before I got it he took out the air filters and put in foam air filter cones…we got it to start and run for a few seconds while holding the choke but then it would die…I’ve tried adjusting the pilot screw and it hasn’t done anything…the other thing that concerns me is the rubber hoses connecting the carb to the engine are slightly cracked, could that be an issue as well?

  • Dom

    The boots…I read that if there is a leak in the boot that putting electrical tape over them will temporarily fix the problem? Is that an accurate way to see if a leak in the boot is what’s causing my problem?

    • Matthew

      Dom,

      If the boots are cracked, replace them. Electric tape, RTV, etc..never seem to hold up.

  • tony turmoil

    Hi there ive got a vespa scooter and when I asjust the air screw all way in or all way out does nothing at all..125 cc with simmoni pipe.been told by mechanic its got a air leak

  • Darren

    1985 honda shadow 700.

    I have k&n filter, and the chamber between muffler pipes has been removed and holes closed off.
    I have 135 mains, and 45 pilots i believe…

    the popping is finally gone (about 3 turns out on pilot screw). Or would popping on decel be normal with that exhaust setup?

    Bike tops out at 95mph, when it could easily do 105 last year before i played with jets to get rid of popping. And also the bike stalls when gearing down and coasting to a stop sign at times (never used to do that). Buddy said it smells rich when he rides behind me…. Is this a pilot issure or high speed jet issue. And will pulling the baffles help to correct the problem by leaning the mixture out a bit and allowing me to turn a/f screws in without having popping. or is popping something i have to learn to live with?

    Hope that didn’t break your brain

    • Matthew

      Darren,

      Popping with open pipes is to be expected. You can somewhat remedy this by continuing to richen pilot jets, however will consume more fuel at cruise. Try going up one size on pilot.

      I know that the stock air cleaner is very restrictive. Did you replace the air cleaner or just the element? Any needle changes? What are stock main sizes.

  • Andrej

    Matthew, first off, great site and really great and easy to undrestand articles.
    Secnodly, I have a problem. I have a 2008 Yamaha XT125 with a 150 ccm big bore kit and an Arrow exhaust and I’ve recently fitted a Stage 6 R/T Dellorto VHST 28 caruburettor on the bike. The Dellorto carb doesn’t seem to use the same system as is described in the article as it has a metal Air mixture screw with a spring on it (I could link you a picture for a better idea, but I’m not sure if your site allows it).
    My problem is that I have this screw screwed all the way in and the bike behaves very weird in idle/low RPM. In idle, the bike is very loud, doesn’t run smoothly and sputters a bit, but when I give it a bit of throttle (around 1/8, it sputters quite a bit more, sometimes the RPM fall down a bit and after I let go off the throttle they hang for quite some time and decrease very slowly back to idle RPM.
    I’m guessing I’ll have to buy a smaller pilot jet (idle jets as they are called on the manufacturers website).
    Any ideas/suggestions?

    • Matthew

      Andrej,

      I believe that carb is fitted with a pilot air screw on engine side of carb. To verify, remove the screw and it it has a blunt tip then it meters air.

      Have you tried to turn the screw out to achieve highest RPM?

      • Andrej

        Matthew, thank you for the swift response.

        The screw is blunt, however the diameter of the end is less than a milimeter. The most stable state, if you could call it like that, with the highest RPM is when the screw is fully screwed in.

        • Matthew

          That screw meters air. If it idles best all the way in, it wants more fuel.

          Does it run better with choke on? Try blocking the air intake with masking tape (50%) and see if it runs better to confirm lean condition.

          • Andrej

            The funny thing is that it runs worse on choke, also dies with a minimum amount of choke added. It runs better without the air filter, and also if I turn the screw out the exhaust starts to smoke heavily.

          • Matthew

            OK. What color smoke? How does the plug look?

  • Andrej

    The smoke is mainly a darker shade of gray to black. The plug is wet.

    • Matthew

      Andrej,

      I’m not too familiar with the carb you have, however did a bit of reading and that screw meters fuel, not air like I stated before. Since you can turn in all the way in and it will still run, I would recommend dropping pilot jet 3-4 sizes. They are cheap so buy the the next 4-5 smaller sizes to experiment.

      • Andrej

        Thank you for everything. I have already ordered 3 of them, the one that is currently in is a size 45 (the biggest sold for this carb), and I’ve ordered a size 40, 38 and 36. Will report how it went after I get them, maybe it’ll help anyone else. Thanks again 🙂

        • Matthew

          No problem. It will be trial and error, but I’d start with the 38 and go from there. Post back with results!

          • Andrej

            The jets came in tomorrow, but as it turned out, the 36 is still too big.
            It runs much better now, but still won’t die with the screw fully screwed in.
            I have ordered another pair, this time sizes 34 and 32 (32 being the smallest one available) hoping one of these will be the right one. If not, I don’t know what else I can do.

          • Matthew

            Install the 32 and report back. Hopefully it will work for you.

  • John

    Hello Matthew,
    I have a Virago 535 2002 with popping from front cylinder, Bike has Highway Hawk Fat Pipes ( think they are highway hawk, no marking on them)
    the exhaust had very small baffles in which I have replaced with 8inch baffles and baffle wrap ( makes chuffing sound now instead of loud bang) apparently the previous owner had a Dyna jet carb kit fitted at same time, carb sync looks near enough, neither mixture screw has much effect, front is 3 turns out and rear is 2 2/3 out popping seems to ease a little if front mixture screw screwed in until it is 1 1/5 turns out but neither effect engine revs, when the front mixture screw is fully seated with engine running then loosen it engine revs rise slightly but only momentarily then even out again. spark plugs good colour not sooted up or white. thought I had a vacuume leak sprayed carb cleaner on rubber carb mount to engine on front cylinder, engine revs rise dramatically,, stripped it down twice now and reseated/sealed all I could,, I`m now at a bit of a loss as to what to try next.any suggestions.
    How it sounds below:-

    http://vid1277.photobucket.com/albums/y493/jgl9/yammissfire_zps2db14cb6.mp4

    • Matthew

      Hey John,

      You mentioned vacuum leaks. Are they all gone now? If not, you have to take care of them.

      You should be able to get carbs synched perfectly, since there are only 2 of them, it’s even easier. Hook both gauges up to one carb to see if gauges both read the same. If carbs are hard to synch or won’t synch, then you have more vacuum leaks.

      The fuel screws should be responsive. They should both be turned out the same amount. If you can close them and engine still runs, the pilot jets are too big.

      An IR temp gun can be handy for aiming at pipes to read temps and help understand what is going on.

  • Andrej

    Hey Matthew, finally got the jets. Well it finally dies when I screw the mixture screw all the way in. I have it out by about 2 turns now and it runs quite smoothly. The only issue I’m still having is that the bike is quite prone to stalling when I add very little gas (as in from a standing start at a traffic light). Also when I’m riding it at about full gas and high revs, when I let go off the gas the bike is a bit nervous, makes a bit of a different sound and likes to kick forward a bit as if there is still some extra gas that ignites in the cylinder even though there shouldn’t be any (if that makes any sense written like this). Both of these symptoms were worse with jets that were too big. Do you have any idea what could this be?

    • Matthew

      Andrej,

      If you slowly roll on the throttle will it die? Is the stall from a bog? Remove air cleaner and tape all but the bottom 1/8-1/4″ of carb inlet and see if conditions improve.

      • Andrej

        I will try and post back with results. Also, I’ve just heard from my parts seller, it appears that my carb is made for a 2-stroke but my Yamaha is a 4-stroke. Apart from the atomizer and its bush, the idle jets are also different. The ones that I’ve tried and have currently fitted in are all for a 2-stroke. The ones for the 4-stroke appear to have larger side holes. Could it be that this could be causing my problems?

        • Matthew

          Yes, the jet needle/emulsion tubes are different. I have no experience with swapping 2&4 stroke carbs. Here is some reading:

          http://www.dotheton.com/forum/index.php?topic=29838.15

          See if different needle jets are available.

          • Andrej

            I’ve read your link and I think it makes sense, especially seeing as it almost always gets flooded when going from half to 3/4 gas even though the needle is in the lowest position and the main jet is the right size (no problems at full gas, high revs).
            The parts seller gave me info on which the 4 stroke parts should be (the atomiser with its bush, if I’m not mistaken, this is supposed to be the needle jet) although they aren’t sure whether the bike will run better with these 4-stroke parts.
            I think I’ll go for it, hopefully it will finally work as it should 🙂

          • Matthew

            Andrej,

            I think you are almost there. Try the the 4 stroke needle jet and report back.

  • John

    Hello Matthew,
    It appears that I have solved the air leaks ( no engine rev rise spraying carb cleaner now),, Carbs are synced perfectly,, I put my exhaust gas analyser in each exhaust pipe in turn, I then put both fuel screws too two turns out and the front cylinder is ok ( the front one is popping/chuffing at idle ) (before I put the larger baffle in and baffle wrap the front exhaust popped/crackled and banged ) but the rear cylinder is showing way way too rich so I turned it in one full turn and the reading came down significantly but still richer than the front. Turning the fuel screws in or out makes no difference to the engine revs at all, you cannot hear or see rise on the auxiliary tacho fitted for the purpose of tuning.
    I`m confused as to why the front cylinder is the only one chuffing/popping. I understand that the pilot jets appear to be to large but when the previous owner had the exhaust fitted she also had a Dyna Jet kit fitted,, Would this not mean that the dyna jet pilot jets fitted would be correct for the new exhaust gas flow ( it appears that with the Dyna Jet kit you get new main jets and pilot air jets,, would it be this pilot air jet that needs to be reduced in size , there are two pilot air jets in the virago carbs, the Dyna Jet kit replaces the number two pilot air jet,,, or would it be the “regular” pilot jet that needs down sizing ) and weather or not it is,,, why do you think only the front pipe is popping/chuffing,, I`ve tuned loads of bikes over the years but have never had this problem. Your help, opinions and views are greatly appreciated,, I would love to solve this issue.
    Think I shall look see what Dyna Jet jets are fitted.
    P.S whilst syncing the carbs I turned ( unscrewed ) the adjuster screw right out and the engine revs increased dramatically,, is this normal ?.

    • Matthew

      John,

      Let me back up a bit and ask what the compression is on each cylinder and are the valves adjusted if applicable?

      Have you had these carbs apart? If not, maybe it would be a good idea to double check the previous persons work. Also, can you send me a link to the jet kit? Maybe I can go over the instructions so I can make better sense of what was changed out.

  • Brent

    Hi, Mathew it’s Brent again don’t know if you remember me but my gsxr is running weird again. After I talked to you it ran good for about a month and then it started to run bad again. But it was more of a lack of throttle response. I bought a emgo sync tool and sync’ed they were way off. And now the throttle response is fixed. But the weird thing is I reset the mixture screws and at 2 1/2 turns in it dies and the idle keeps going up all the way up to 4+ turns out. My stock jetting is 127.5-125 main jets 12.5 pilot. My jetting in it now is 1 step up on everything so 130-127.5 mains and 15 pilot. It don’t make any sense to me why I would need to go up 1 step on mains and 2 on pilot for it to run right. Any help is greatly appreciated!

    • Matthew

      Brent,

      If it ran good at a certain setting and all of a sudden started running bad, it is not the settings. Something else is going on. Explain in more detail the symptoms you have.

      • Brent

        Not exactly. I’m thinking the carbs had never been synced and my adjustments were on it before it was synced properly. It did run good for about a month but then I started to lose throttle response so I synced and that problem went away. But when I left it right where it was. Which was 2 1/2 turns out on the mixture screws it idles terrible. Like constantly surging at idle. So I tried to adjust and I find to reach the smoothest and highest idle I am like 4+ turns out.

      • Brent

        Is it possible for my main or needle to be lean and cause my mixture to have to be way rich to compensate? Thanks for helping me man. This bike is going to be the death of me lol

      • Brent

        I should also add that with the mixture at 4 turns out when I take it for a test ride the lower rpm and smaller throttle openings are smooth and extremely responsive (more then it ever has) but since the sync the mid has been kinda lack luster. And at wide open throttle it pulls like a mule all the way up to Rev limiter. That’s what is leading me to believe the needle setting may be where I’m off.

        • Matthew

          Valve adjustment and carb synch should always be done before carb tuning. Also no vacuum leaks.

          Now that you have them synched and you best idle is 4+ turns out, you need to go larger on pilots. Probably 2 sizes larger.

          The needle does not come into play until 1/4-1/2 throttle, so no need to worry about it now.

          • Brent

            Lesson learned lol. I was always under the assumption that carbs our of sync would lack throttle response and I always though it was very responsive. Until I synced then it was way more responsive. What I still can’t wrap my head around is why do I need so much more fuel at idle and my mains are only 1 step up. Don’t those normally coincide to an extent? Oh and I have the next size up and it’s 17.5 should I go 2 steps and get the 20? Thanks

          • Matthew

            Brent,

            Carb synch is most important at idle and least important at WOT.

            Sometimes bikes are very lean from the factory. I’ve taken stock bikes and had to up the pilot 2 sizes and still ended up 3 turns out. Give the bike what it wants and don’t get too tied up in the difference from stock.

            Try the size up you have and see what happens.

            If you carbs synch is dead on, you should be able to tap the start button with no throttle and it should fire no problem.

  • Brent

    Gotha I will try the one I have and see where it lands me mixture wise. Thanks again for all your help.

    • brent

      Well here is a little update on the tuning progress i made. I put in the next size bigger pilot jet (17.5) and it made it run quite a bit smoother. I reset mixture screws after the change it smoothest idle was atleast obtained. It was 3 1/2 though so im thinking i will be putting in 20s next because it rode smooth and nice for about 10 minutes then it went back to its figgity self and was hesitating again. Still better then with the 15s but not fixed yet. I checked and rechecked synch and it has been spot on since i first did it and i still cant understand why this bike needs so much fuel at idle. Im only half a step up on main and when i put in the 20s that will be 4 steps up on pilot. Thanks for all the help i would never be even this close without your help amd write up.

      • Matthew

        Brent,

        I would cruise around and cause the bike to hesitate like you mentioned. Try to have the bike do it over and over again and then pull the plugs (plug chop). Maybe the color will indicate what is going on.

        Can you double check for vacuum leaks? Spray carb clean everywhere from cylinder head to airbox!

        • brent

          I put in the 20’s and i put the mixtures at 2 1/2 to start off. It started and ran pretty good and i let it warm up to try to dial in mixture and my highest idle and smotthest idle was 2 3/4 which seemed alot more like where it should be. It was way to late to do a test ride so i shut it off and waited till the next day. I tryed to start it the next day and was experimenting with a no choke start and it turned over 3 maybe 4 times and then hydro locked or atleast so it seemed. So i held the throttle wide open and it turned and fired atleast. Smoked for a few minutes. Wondering if maybe i had a float stuck and not all the way seated and filled the cylinder full of fuel or something. Started fine next morning and has been good ever since. But after that hard start i rode around the block and then re-tuned the mixtures ended up at 2 1/4 turns out and went for a half hour ride to really see how it would run. It ran good. Could tell it was alittle lean. Popping on decel real bad and had quite a bit of ping. Plus end of pipe was turning light greyish white. If i reset mixture and its over 3 turns out im gunna cry lol.

          • Matthew

            Brent,

            You probably had a stuck float. Have you pressure test the needle and seats? I like to take 3-5psi of air and blow into fuel inlet. All should seal if float is depressed. High float levels could cause headaches.

            Since you shortened the silencer, decel pop may be normal. Don’t try to eliminate this by increasing pilot jet size. You’ll end up way too rich at idle and cruise.

            Has anything been done to CV slide and/or spring? Maybe the stumble you are trying to solve is the transition between pilot circuit and throttle valve. I know jet kits in some cases drill the CV slide and replace spring so they lift easier giving the engine more fuel/air. Something to think about.

          • brent

            Yes i did do a pressure test and all was good after that one little hiccup. Has been starting fine ever since. My float hight is factory which is 7mm from the gasket surface with the float just touching but not compressing the needle spring. As far as drilling the slide i know i havent done it but thats not saying someone else didnt. It was jetted but not with a jet kit by a shop i do know that. Im not looking to get rid of all the pop on decel but it was excessive in my book. And the tip of the pipe being white/grey was another lean indication i was thinking. Thanks again for everything. You have been a huge help btw. I tryed for leaks again like you told me. I sprayed everywhere and it is leak free. Ive actually done that like 5 or 6 times before just to make sure i wasnt missing something. Because to me it dont make any sense to need 20 pilot jets when stock is 12.5 but all tests i do come back negative. And compression is good and everything keeps pointing me to pilot jetting. I guess this bike just came mega lean from the get go but how didnt it hessitate when new? Or maybe it did who knows. But its only got 13000 miles on it so it should be able to run and ride smooth i would think.

          • Matthew

            Brent,

            Sounds like you covered all bases.

            I would drill slide hole as a last resort, however maybe someone drilled it out too large?

            Maybe it would be worthwhile to find needles that have a smaller starting diameter (portion closest to clip grooves.

          • brent

            If i were to drill the hole out what size would i need to drill it to. And what size would be to big? Also what is the gain from drilling the hole? Would i need to start all over again with my pilot jetting? Im kinda wondering if this is some of the problem. Because when doing the carb synch i noticed the slides would flutter alot when revving. Would not getting enough air cause that?

          • Matthew

            Brent,

            I don’t know if drilling the hole is a good idea or not. I’ve read both sides of the argument. I did however drill slides per instructions on some jet kits, which supply new springs as well.

            However, it would be a good idea to check yours to see if someone has drilled them out. Most kits supply a 1/8″ drill bit for this, so stock should be smaller.

            A larger hole will cause the slide to lift slower.

            If your slides flutter, make sure they are perfectly clean and operate smoothly. Polish them with crocus cloth if needed.

          • brent

            Ok this bike is insane. It sat for about a week because it was raining. So i went out and choked it and boom hydrolocked again. So i held the throttle open and it started just like before. I checked needles and they hold pressure fine. So next day it sits over night and i try no choke. It starts and runs fine starting cold with no choke. So i check my mixtures and they are at 2 1/4. Still from that last long unsucessful ride and i start to adjust. If i get down to 1 or less turns it stumbles and dies. Anywhere 1 1/2 turns or more the idle stays the same till about 2 1/2 no change at all. Then above 3 it starts to stumble really rythmicly. So i think ok awesome im sure i got the right pilot 100% so i then put it at 1 1/2 to aim tward the bottom side of that range and see what happens. I take it around the block and it had been idling in the garage so it was 200+ degrees reading the temp guage. Ran amazing. I mean absolutly perfect…….until i started moving and temp dropped to 160-170 where it normally runs and boom stumbling and studdering like crazy so bad a fe2 times off the take off i thought it was gunna die out completely. How can the engine temp make such a huge differnce? I mean enough to make it run perfect then like crap just like that. This bike drives me insane. I could swear its a nitro rc car.

          • Matthew

            Brent,

            Try tuning the fuel screws at engine temp of 160-170. Point some box fans at radiator to keep it cool. Stock thermostat is 180 correct?

          • brent

            Thats a good idea using the box fans. I always turn on the fan that i have wired on a switch bypass and it still creaps up to over 200 degrees. The fan dosnt come on till 225 the way the factory has it and drag racing it i always have that fan on to keep the temps down. As far as the t.stat goes i my fsm says its a range of 168-176 that it opens completely. Also with the mixture down at 1 1/2 like it is now i did a cold start try with the choke and it fired up super easy and the choke worked better than it ever has. With choke fully on the idle was at like 3k and as i eased it off it went down to 1200 perfectly. Before with the other jets if i choked it all the way the idle would be kinda low like 1k ish then as i eased it off it would go up to say 3k then come back down to idle. So im pretty sure i got the correct pilot size now. Its just tuning it in and i should be good to go. Thanks again for everything.

          • Matthew

            Cool. Sounds like you are VERY close if not there. So what size pilots are you currently at?

            The bike should want some choke for cold starts. If it starts fine w/o choke then it’s too rich.

          • brent

            20’s and im at 1 1/2 turns. Trying a 1/4 turn more fuel tomorrow. I know its between here and 2 1/4 because at 2 1/4 with choke it hydrolocks when i try to start it and thats obveously not right lol. Ill keep updating until i get it. Just incase someone else has these issues and we can save them alot of headaches.

          • Matthew

            Brent,

            I just want to say good job on not giving up and fixing it yourself! I’m sure you (and I) have learned a ton from this.

            Leave an updated comment once you put some time/miles on the bike.

          • brent

            Ok now this one has got me scratching my head. Since my last post it has been to cold to ride until today so i went out and went a half turn more fuel. Which puts me at 2 turns out. I put alittle bit of sea foam in the tank and topped it off with vp110 and went for a ride. It will no longer accept full throttle. It breaks up really bad at wide open throttle. Almost like its lean on the main jets. But the only thing i have changed is the mixture. Im stumped. But plan on trying 2 1/2 but cant figure out how that would make any difference at wide open throttle.

          • brent

            Just as i had thought the mixture chnage made no difference. Still will not except full throttle. Seems like its running out of fuel or something. I did find something that may be throwing a wrench in the gears tho. I pulled off the vacuum line to the fuel valve and it was wet and had alittle bit of fuel in it. Im wondering if that diaphragm has a hole in it. And maybe thats why it seems to need so much fuel at idle when the vacuum is low. Just a thought. Let me know what yours are. Thanks

          • Matthew

            Hey Brent,

            The fuel mixture will have very little effect at WOT.

            Do you always run VP110? Probably overkill for this engine.

            Interesting discovery on the petcock vacuum line. If there is an internal leak, then it will cause problems. During decel, vacuum is high, so you might suck up extra fuel and it may stall the engine. At WOT, vacuum is low, and you have a leak, so fuel delivery will suffer.

            Hook up a vacuum gauge to petcock and see if it will hold vacuum. If it bleeds off, replace it.

          • brent

            Yeah i always run vp110 it is a drag bike primarily. So i run race gas almost always. I didnt figure the mixture would effect it i was just grasping at straws lol. And i will put the mitivac on it tonight and see if it holds vacuum. How else would fuel end up in the line tho? If im thinking right the only way is leaking diapragm i may be wrong tho. Im wondering if the whole time that has been leaking and what was making me need such massive pilot jets. Is that possible? Thanks for the input.

          • brent

            Well heres the update. I put my mitivac on that line and left it all noght and it never budged. So im thinking that diapragm is good. Not sure how the gas got up in the line. So i was testing a few things here and there and find the throttle position s3nsor’s resistance tested way out of spec. So now im ordering a new one. This bike will be the death of me. Could that be the cause of everything i have been experiancing? It says its supposed to read between 3.5-6.5 k ohms and mine was reading 16.28 ish. And when i losened the screws to adjust it anyway i went it made no difference at all. I didnt even know a carbed bike had a tps in the first place. Kinda seems like it really wouldnt control much or as much as a fi bike would. Just an update. I will update again when i get the new one on and test ride it. Thanks

          • Matthew

            Brent,

            Honestly, I don’t have much experience with TPS’s. On a carbed bike, they usually have it to monitor throttle position and possibly control ignition timing and maybe a fuel circuit. On my dirt bike, there is a fuel cut solenoid and timing curve associated with TPS settings.

            Let me know what happens when you get the tps working properly.

          • brent

            Well that did not go as smoothly as planned. I tested the new tps and it tested good and way different then my old one. So i installed it per the manual. I put the 15 pilot jets in and also put the needle back in the middle as a base line starting point. I took it around the block and it ran terrible. Bog off the line so bad i had to rev it and slip the clutch to get it to even move. Once i got above 1/4 throttle it ran smooth and clean tho. And the jerky throttle response is gone now. But this bog is what has me confused. My mixture is at 2 3/4 and if i go leaner it seems like the idle drops and isnt as smooth. I do know for sure that the tps not working made the bike run way rich because i pulled the plugs just grasping at straws and wow were they fouled. Black covered in soot. But i changed them with new ones and it still didnt completely fix it. It almost sounds like the tone of the exhaust has changed also. Way deeper almost v-twin sounding until i get past 1/4 throttle then it hits a turbo and most of the time blows the rear tire off. Kinda scary the couple times it did that lol.

          • Matthew

            Brent,

            I can’t gather whether you are rich or lean on pilot. You mentioned plugs are black (rich) and it bogs (lean?).

            Now that you have the TPS sorted out, reset your fuel screws for highest RPM, and note how many turns out you end up. That will tell you whether to move up or down on pilot jet.

          • brent

            Well heres an update. I tryed to set the mixtures and it was all over the place 1/4 turn idle up. Next 1/4 turn idle down. So i was talking to my father in law about it and he looked at the fould plugs and said for it to be dumping this much fuel in and not burning it like it was there would more than likely be alot of gas in my oil. Sure enough he was correct. I looked in the view window and it was way over full. I drained alot of gas mixed with a small amount of oil out of the crank case. It was probably 75% gas. No joke. How screwed am i? Would it ruin the clutch? Blow the engine? Or can i let it drip out for a while then fill with oil and call it a day? From what my father in law said it would make everything seem rich burning it off in the engine like that. Thanks for your advice man. Your a life saver.

          • Matthew

            Don’t sweat the fuel in the crankcase. Give it 2 drain and fills and it will be OK.

            If you had that much fuel in the crankcase, the float became stuck open due to debris of worn float needle. Remember when you had to hold throttle wide open during cold start? That was probably due to all the fuel flooding the cylinders.

            At this point just replace the float needles. They can give owners so many headaches with high float levels or flooding that you are experiencing. And yes, it will cause the bike to run super rich is the float needles do not seal properly and result in high float levels.

            Another thing, you checked the vacuum petcock? Do you have a prime “PRI” setting? That setting bypasses the vacuum actuated setting and always allows fuel flow. Don’t use this setting other than when the carb bowls are dry to quickly fill them.

  • Andrej

    Well Matthew I honestly don’t know what else I can do. Installed the 4 stroke needle jet size 268 (smallest one available). It has 8 side holes on the top end, whereas the 2 stroke version doesn’t have any (it was a size 266). The bike only starts with a bit of gas, quickly dies without it, and smokes very badly (white smoke in lower revs, gets a bit darker if I rev her up). I had also installed a 4 stroke pilot jet of the same size as before (32) which has bigger side holes than the 2 stroke one and the smoking was much worse, which is why I switched back to the 2 stroke one and she runs like I explained in the beginning. The only thing that makes sense to me now, is that gas is coming in through these holes on the 4 stroke needle jet rather than air, which would happen if the gas level is too high. Maybe it’s related to the fact that the carb is in at a bit of an angle?
    So yeah, she runs worse now, and I really don’t know what else I can do.

    • Matthew

      So with the jet needle change only it will not idle? What is the white smoke all about? That part does not make sense to me.

      • Andrej

        Yeah, I don’t get it either. Only the needle jet (atomizer with its bush) were changed, the idle jet is the same again. The only reason that would make sense to me why it dies would be that the excess fuel when I let go off the gas floods it to the point of dying alhough that’s weird because apart from the terrible smoke (very light gray if not white), it revs up without too much hesitation.
        Also what bugs me is that I can’t adjust the idle screws because it won’t idle, but if I adjust them with the minimum amount of gas needed to not die, the revs are the highest when the mixture screw is all the way in (it meters fuel).

      • Andrej

        Well finally some good news (sort of). My theory proved to be right as far as it seems. I took some tape and closed the side holes on the needle jet, and it runs like it did before, just some sputter from 0 to 1/4 gas but other than that quite normal and also it idles with no trouble again. Also now I’m certain that the fuel level indeed is high enough to pour into these side holes as the needle jet was soaking wet to the top when I took it out, aside from the fact that it runs pretty well with the holes closed. The smoke is appearantly due to the mixture being way too rich.
        So the next order of business for me is to lower the fuel level in the float chamber.
        I currently have a needle valve #300 and a float with the weight of 9.5 grams and the fuel goes almost to the top of the needle jet.
        I know that I’m streching the original post a bit, but do you happen to have any reccomendations as to how much smaller would be appropriate?

        • Matthew

          What size is the throttle valve cutaway? They are usually stamped on the bottom. Here is more reading on the topic:

          http://www.howtomotorcyclerepair.com/how-to-tune-throttle-valve-cutaway/

          If you don’t have a float level spec, I like to set the level about 2mm below the carb body and float bowl mating line. You can insert a piece of clear tube in main jet plug or float drain if equipped. Run the tubing in a “u” above fuel level and that will help you measure level.

          • Andrej

            The throttle valve cutaway is a size 45.
            The fuel level is about 5 milimeters above the mating line.

          • Matthew

            I’ll be honest, I’m not too familiar with the carb you have.

            I’d follow the carb mfg spec for float height and stick with that. I usually never change float height to mask a jetting issue.

            I know throttle valves are pricey, but it may be worthwhile to try a larger/leaner cutaway with the 4 stroke needle jet. It will pull less fuel from pilot and jet needle at 1/4 throttle.

  • Andrej

    That seem to be the case, yeah.
    I know this should be a little hard to say, but how much of a change does one usually go with the cutaways, considering I have the 45 and the others being 40, 50 and 55?

    • Matthew

      If you go back to the link I provided on tuning cutaway, there is a graph from Mikuni that shows each size changes fuel flow by 10%. On that particular bike, I stated with a 3.0 (lean) and went to 2.0 (rich) and it made a drastic difference. I ended up going to the 2.5. So I experimented with a 20% difference.

      In your case, I would start with 2 sizes leaner. Plan on revisiting pilot jet and it will probably end up richer in size since throttle valve will make signal weaker.

  • brent

    I have an update on the gsxr. Its cold here so i have it on a battery tender and i went out after the oil filled with gas catastrophe to try to start it. I tested the needle valves with 5 psi and they held perfect. I have been watching oil level like a hawk to make sure it hasnt changed. But im starting to think i either have a battery issue or a starter issue. I go to start it and it turns over 2 or 3 times slower than normal and then stops. If i hold the starter button i can hear the starter motor almost sizzling but i only do that for a second. Instantly my battery tender goes from green light to charging and then it charges for about a half hour and green light comes back on. I go over and try to start it again and boom turns over fine and fires right up. Im not noticing any change in oil level so im not thinking its hydrolocking from filling a cylinder with fuel. Any thoughts? Thanks

    • Matthew

      Hey Brent, try shutting the fuel off and running the carbs dry before letting the bike sit overnight. In the morning, see if you have the same issue to rule out your theory.

      • brent

        Ok good idea. I will try that. And let you know what i find. Thanks

  • Tyler

    I’m having a problem finding the adjustment screw on my 2000 Kawasaki Vulcan classic 800. Please help.

  • Kami Penhasi

    Hello. Thank you for the information. I have a 1980 Honda Xl 250S. I start the bike and it runs but when I drive it and push the gas the bikes turns off. When I do not push the gas it doesn’t turn on. DO you have any idea what the problem is?

    • Does the problem get better of worse with choke partially on?

      • Kami Penhasi

        It gets worse. As soon as the choke is on and want to start the bike , it wont start. Without the choke it will start. Once it is one and I put the choke the bike turns off completely. Sorry for the late reply.

        • Pull the spark plug and take a look at what color it is. Email me a pic.

  • Anthony Hovanjec

    Hi great work on your videos, anyway been trying to tune a ktm 300 exc it’s a 2stroke, just off idle trying to smooth out the power, it’s a little bit hesitant, or not smooth, bit crackly I guess.. Thanks

    • Anthony, I will be coming out with a 2 stroke jetting guide soon. In the meantime, email me through my contact page and I can help you out.

      -Matt

  • Jarrett Hunt

    How would this work on a bike with two carbs? I’m trying to tune my 73′ Cl350.

    • Yes. Both pilots and fuel/air screw settings need to be the same.

  • Bruce Branstad

    I resurrected this 1985 Honda XL600R from the ruins for my nephew. The previous owner tried to make an attempt to do the same thing, but failed miserably. Too many armature mistakes almost caused this bike to be offered up as a eBay parts sacrifice. Some people should not even try to do something like without the correct knowledge and experience. (https://www.youtube.com/watch?v=5AqqQ6jnR5I)

    Currently is hard starting and pops through the carburetors while trying to start it. I rebuild the carburetors, however the jetting is not stock due to the changes the previous owner tried to do, such as drilling holes in the stock muffler and modifications to the air box. Per the advice of my local Honda Dealer, I installed 122 & 188 main jets. I had a 62 and 70 slow jet. I decided to put in the 70 due the mods to both the air box and muffler. I do have a 58 and I am considering installing that to see if it helps with making it start easier. Any advice regarding this would be appreciated.

    • Hi Bruce,

      I would look up and see what the stock pilot jet size is. I bet you will find that 1-2 sizes larger than stock is what the bike would want. Try adjusting the pilot fuel screw and see if it is responsive. If so, how many turns out for max idle RPM and smoothness.

      Bike looks real nice!

      Matt

      • Bruce Branstad

        Matt,

        Thanks. The tank is from my 87 XL600R.

        https://www.youtube.com/watch?v=FDRg49vzdxk

        My bike has a 87 XR600R motor and carbs. I have been having issues with it running lean, white spark plug. I went from 122 mains to 125 mains and the color on the plug is headed in the right direction, light tan to pinkish. My next jump will be 128. I do believe the 45 slow jet is not enough to get the bike started reliably. I have not replaced the Air Cutoff Valve (ACV) yet. I have been reading that a faulty diaphragm can cause havoc. That’s what I have been experiencing with this bike.

        Any advice will on this bike would also be appreciated.

        • Bruce,

          The main jet is responsible for 3/4 to full throttle position running. You mentioned hard starting and idling issues? Well, that is all pilot jet/fuel screw. Get that sorted out and move on to the needle and main.

          • Bruce Branstad

            Matt,

            I had to replace the pilot jet. The carbs are off of a XR600R using all the XL600R air box and exhaust. The primary carb had a 45 and I went to a 58. The bike now starts a lot easier after getting the air mixture right. Your video and additional information helped me to find my solution to this issue. Next will be getting the main jets where they need to be.

            THX… BB

  • steve buschemi

    Hey Matt,
    I’ve got a quick question for you. Could you perhaps elaborate on the difference between a lean hesitation/bog/stumble off-idle as opposed to a rich stutter? Thank you!

    • Sure, what specifically? How to fix it? Or?

      • steve buschemi

        Oops, sorry for the ambiguity…but more like the actual symptom itself. Like, how does a stumble/hesitation ACT compared to a stutter? How does each sound? How do they affect rpms as they are occurring?

        I’ve got an off-idle….issue…if you will, and I believe I know which way of Stoich I am. But before being certain I just wanted to see your description to see if I can more definitively put a label to the behavior I am experiencing.

        • A lean bog will have the engine fall on it’s face and not recover. A rich condition will cause the engine to sputter, and emit black smoke from exhaust. Watch this video as it has a lean bog to get an idea of what it sounds like: https://youtu.be/6b9dMhgZT-c

 

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