How-To: Jet a 4 Stroke Dirt Bike

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Year, Make, Model

The dirt bike featured in this blog post and How-To video is a 2006 Yamaha YZ250F. All the 4 stroke dirt bikes are equipped with Keihin FCR carburetors, so this post should guide you in the right direction no matter what brand you own.

 

how to 4 stroke dirt bike jetting

 

Circuits We Are Going To Jet

  • Pilot circuit – This is effective from 0 to 1/8th throttle positions.
  • Accelerator pump – This circuit is responsible when the throttle is quickly opened, and squirts fuel in engine when slide opens.
  • Needle & clip position – This circuit effects ¼ to ¾ throttle positions.
  • Main jet – Main jet is ¾ to wide open throttle (WOT)

 

Pilot Jet & Fuel Screw

I like to start jetting a carburetor by getting the pilot jet sized correctly and the fuel screw set properly. Reason being, the pilot jet contributes to all other circuits by a small amount, and can cause the following symptoms if not set correctly. 

Some of these symptoms include

  • Hard starting HOT or COLD. Using the hot start lever often.
  • Hanging idle which indicates lean mixture.
  • Decel pop. Do not try and richen the pilot circuit to eliminate decel pop. Check the silencer packing, which may help. 

I cover pilot jet sizing here in detail. You want the fuel screw to fall between 1 to 2.5 turn out max. Any further out, and you run the risk of the fuel screw falling out. I personally like to have the fuel screw fall at 2 turns out so there is some adjustability left either way. This adjustability may be needed for temperature changes, etc. If the fuel screw ends up at 1 turn or less, decrease pilot jet size. If it ends up at 2.5 or more, increase pilot jet size. 

 

Also, do yourself a favor and install an adjustable fuel screw. The stock screw is impossible to get at and you will usually burn yourself.

 

adjustable fuel screw fcr mx

 

 

 

Below is a video showing you how to properly tune your fuel screw. I would run through this video and set up your fuel screw before removing carb, so you can make the decision on whether to change the size of the pilot jet.

 

Accelerator Pump Circuit

The accelerator pump (AP) squirts fuel into engine only when when you whack the throttle really fast. This is needed to compensate for the lean condition of the slide opening. If you roll into the throttle really slow, the AP does not squirt any fuel at all. I’ll explain this later on. There are 2 adjustments that can be made to the system, fuel volume and timing of squirt. 

First let’s talk about the AP timing. The AP timing screw (see image below) is located near the throttle wheel can advance or retard the squirt timing. I usually do not adjust this from the stock setting. If you need to modify timing, turning the screw out will advance the squirt. The opposite is true for turning the screw in. It seems most desirable for the timing to be as soon as possible and it must miss the slide as it opens. In the past, I have adjusted the timing to just miss the slide. You can test this with air box removed and engine NOT running. Whack the throttle and pay attention to the squirt. 

 

o ring mod ap timing screw

 

Next is the leak jet. Changing the leak jet is the ticket to eliminating the bog. Here is how the leak jet works: 

Fuel bowl > leak jet  > AP squirt nozzle 

The above represents the path the fuel takes (left to right) from the fuel bowl to AP squirt nozzle and eventually to the engine. Notice the leak jet is in the middle. Imagine a garden hose hooked up to your spigot at one end and a sprinkler at the other end. These represent the fuel bowl and AP squirt nozzle. Now go poke a hole somewhere in that hose between the two. What happens? Less water will squirt out the sprinkler. The poked hole represents the leak jet, except it will bleed fuel back into the fuel bowl. So a numerically larger leak jet will have a larger orifice and bleed more fuel back into bowl and less fuel to squirter. The opposite is true for a numerically smaller leak jet. Also, if you open the throttle slowly, all fuel will be bleed to bowl via the leak jet. There is no need for a squirt of fuel if throttle is opened slowly.

 

leak jet location float bowl 4 stroke mx fcr

 

Now that you know how a leak jet works, they usually come from the factory with a 70-100+ size. Since these bikes are meant to run WOT, and they are probably trying to conform to some environmental standards, they are usually too lean from the factory and it that is why they all BOG when you whack the throttle from a roll. 

I typically install a 45-55 leak jet in most of the 250F’s. You will want to decrease leak jet number size until the BOG is gone.

Now, there are several companies that sell adjustable leak jets. The nice thing about these aftermarket kits is the ability to adjust the the leak rate by turning a screw with a screwdriver that is super easy to access. Given how much of a PITA it is to remove a carb on a 4 stroke, this may be a better option for you if you only want to tear the carb apart ONCE.

 

Another AP improvement that is super easy and inexpensive is the “O-ring mod”. The o-ring mod simply couples the AP pump arm to the adjacent shaft (not sure what it is called). This will move the pump arm quicker and result in a much stronger pump shot. You can go to any hardware store and ask for a #78 o-ring, it fits perfect. You can also use safety wire, however I prefer an o-ring since there is a little compliance if needed, reducing any binding.

 

o ring mod ap timing screw

 

Needle & Clip Position

To be honest, I don’t have any sound advice on how to select needles. I usually surf the web and see what all the talk is about, and then usually try it for myself. Forums and magazine articles is usually where I start. 

As far as the needle clip position is concerned, I use my butt dyno to see what change feels the best for ¼ to ¾ throttle positions. I typically move the needle clip up and down to see what runs best. Remember, the needle clip position is always referenced from the top. Moving the clip up lowers the needle causing a leaner mix. Moving the clip down raises the needle causing a richer mix. 

This 06’ YZ250F came stock with a OBELQ needle with clip in 3rd position. In 2008, they changed the needle to a NFPR with clip in 5th position. The 08’ needle is a dual taper that claims more bottom to mid performance. If interested in this needle, just click the ad below and navigate to the OEM parts finder, 2008 YZ250F, carburetor diagram, and order the needle.

 

Main Jet

I have a post here on tuning a main jet. The main jet is ¾ to WOT positions. You’re going to need some open space to perform this test! As you can see in my video, I was not able to do so.

 

Comments

Has this blog post & How-To video helped you? Please comment below.

 

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  • dennis

    Do you think the 08 Yamaha needle will help my 06 honda crf?

    • Matthew

      No, don’t run that needle in your bike.

      Let me know what modifications you have, temp, altitude, and humidity you ride in.

  • Carlos

    4HB9sIU4pE5h

    Hi Matthew, first I wanted to say you’re doing an excellent job with these videos. I have a few questions, I own a 2012 yz250f with the 40mm Carb and the stock leak is a 95 my local dealer had a 65 only so i installed it, from my understanding every time a different leak jet is installed you must visit the ap timing because a small leak advances the timing and a larger leak delays it, so I adjusted my ap timing by eye getting to the closes possible point of it not hitting the slide, (1 full turn cw (delaying)) I still have a little bog under load on the stand I have no bog. I’m going to order the 45 leak from your link, did you change your ap timing after installing the 45 and if so how?

    Thanks in advance,
    Carlos

    • Matthew

      Carlos,

      Did your bog improve by going from 95 to 65? I would also do the O-ring mod, which gives a more powerful shot.

      I find that the stock timing is usually pretty good and don’t have to mess with it. I get a ton of carbs where the screw has been tampered with, and in those cases I adjust it to just miss the slide.

      Your carb is bigger than most 250F’s, so it may want a leaner leak jet then bikes with smaller carbs. Buy a few different sizes to experiment with.

      BTW, don’t use a no bog on stand as any indication of correct jetting. It matters when you ride it.

      Hope this helps.

      • Carlos

        Thanks Matthew for the reply,
        I do have the o-ring mod and my bog did improve with the change but not completely gone I ordered a 45 and 55 leak jet.
        I just want to get my understanding of the ap timing straight, my understanding is that every time we change a different size leak we need to adjust the timing because the leak jet sizes control volume and a small bit of timing, smaller leaks advance it and larger leaks delay it so in my case stock 95 timing when switched to 65 had me way ahead of the slide that I needed to delay it one full turn to just barely miss it.
        Should I try it with the oem timing even though I’ll be hitting the slide? If the bike in the video’s leak was changed with out adjusting timing shouldn’t it be hitting the slide?
        I’m going to the track tomorrow I think I’ll try to put the oem timing just to test it and hopefully with my new leaks I can eliminate this dreaded bog.

        • Matthew

          The beginning of the pump shot is controlled by the AP timing screw, not leak jet size.

          The duration of the pump shot is controlled by leak jet size and pump bowl size. A smaller leak jet will “leak” less back to fuel bowl so the pump shot should last longer, since more gets squirted into the motor.

  • Carlos

    Thank you so much for clearing things up, should I put my timing back to oem despite it hitting the slide or always try for it just missing the slide?

    • Matthew

      I’d start with stock AP timing and do the o-ring mod and numerically smaller leak jet until bog is gone.

  • Huseyin

    Hello Matthew
    I have a bog problem with speed throttle open. Husaberg FE450 carb is Keihin FCR MX41 . When the cold choke is at open position, it is not bogging. Zero bog.
    I see that needle clip position is 2nd but manual says it has to be 6th. It can releted with this or do i have to look at also leak jet and using o ring method?

    Thanks

    • Huseyin,

      If it bogs when you open the throttle really fast, that is usually an indication that the accelerator pump circuit is too lean. Yes, the needle clip position can play a part in the bog, however I would look into the leak jet & o ring mod to eliminate bog.

      For the needle clip position, how does it run if you slowly roll into the throttle? When the throttle is moved slowly , the acc pump does not squirt any fuel.

      • Huseyin

        I have not leak jet on my carburetor as you show on your photo. There is another thing and it is not shown on part diagram. 🙁 It looks like my bog problem is not releted with this.

        • What year? Let me look at the parts diagram.

          • Huseyin
          • Yes, the parts diagram does not show a leak jet available. Either your bike has a plug or the parts diagram simply does not show it available. My advice would be to take the carb float bowl off and see what it has and we can take it from there.

          • Huseyin

            Adjusted carb float.
            Tried to reduce pump squirt time but it is squirting 1.5 seconds and my bike is worse than before. With fast throttle open not too fast it is stopping.

          • Why did you adjust carb float? Was it out of spec?

            How did you reduce squirt time? The timing screw retards/advances the pump shot.

            Do you have a leak jet in the float bowl?

          • Huseyin

            Yes carb float was out of specs and it was overflowing. I was supposed timing screw was for squirt time. I just screwed it a bit more. :S

          • The timing screw will retard/delay the squirt if screw is turned in, or advance/earlier if turned out. Try turning the screw the other way.

            I apologize I did not see the image you posted earlier. Since you have no leak jet, another option to get more more fuel to squirt is to change the pump diaphragm. There is a rivet on the diaphragm that limits how much fuel is squirted into motor.

            Here is more info: http://crfsonly.com/howto/ap-mod/professor-redbeard450-ap-mod-how-to-guide.php http://crfsonly.com/howto/keihin-fcr-carb/honda-crf-fcr-carb-update-how-to-install/how-to-install-honda-accelerator-pump-update-crf450r.php
            I know the info is for Honda’s, but the bog is due to not enough fuel. Can you send a pic of your rivet size?

            Matt

          • Huseyin

            Rivet size is tiny as the right one of photo. It seems no prob with fuel squirting. It is squirting too much. Maybe squirting overmuch?

            Thanks

          • That is a possibility. You mentioned a squirt of 1.5 seconds? Most try to achieve 0.8 seconds.

            Here are a few things you can try:

            1. When you whack it and it bogs, do not let go of throttle, does it ever recover from bog? If it does, it is too much fuel, or rich.

            2. Leave choke on and try and make it bog. Is it better or worse? If better, it wants more fuel, or is lean.

            Let me know.

          • Huseyin

            Heyy Matthew,

            I tried to set ap timing and i think i successed .

            Tried these,

            1. Whacked the gass and did not let go of throttle. It was blowed up and stopped.

            2. Leaved choke and it is working with no bog. I setted iddle and tried whack the gass soo fast but no bog. It is perfect with choke.

            But there are some sempthoms on bike.

            1.Bike is working on high rev when i choke. It was not working so high before. I changed needle clip 3rd to 6th. It may cause this?
            2.When i gas open and close, rev is not dropping as before. It is revving down slowly.

          • Huseyin,

            The choke on throttle whack test was to simply find out if you are lean or rich on acc pump circuit. Since it runs better with choke on, your pump shot is lean.

            Have you adjusted your timing to just miss the slide? Have you installed the o-ring mod? If these 2 items do not solve it, maybe an aftermarket adjustable leak jet with larger capacity pump shot is needed.

            Matt

          • Huseyin

            Fuel is missing the slide. I checked it carefully. I also installed o-ring mod but it is still bogging. You mean i need a Boyesan Quickshot 3 ?

            It is a bit expensive so it is the last stand for me.

            Thank you for replies

          • It doesn’t look like the QS3 is available for your model? I know they are pricey, and I always avoid spending that much if possible, however they sure are a nice piece for easy adjustment. I’ve installed many of them to cure bogging.

            Have you tried to hop on some forums pertaining to your model for some advice on possibly swapping parts from another make or year? I’m not that familiar with FE450’s.

            I’m assuming that your diaprahm is in good shape? No holes or internal leaks?

            All I can gather is that it is lean and your pump shot seems too long. You should be in 0.8-1.0 sec range.

          • Huseyin

            I removed carb, cleened and adjusted ap timing. You know there is not leak jet on carb. there is one way fuel system so pump has to be strong squirt but not. i dont know. Try to decreese squirting time. With choke it is working with no bog.

            QS3 isnt for all FCR models?

          • Yes, the QS3 is for FCR carbs, however, not all FCR’s are created equal. Most have leak jets, yours does not.

            Read this:
            http://husaberg.org/fuel/6529-leak-jets.html

            You might want to post over there and see what it would take to swap parts from another bike.

            Another option would be to talk to a company who mods these carbs, like Tokyo, and see what options are available: http://www.tokyomods.com/carburetion/carburetion.html

          • Huseyin

            Thank you matthew, i opened a topic there. And i recorded a video.
            https://www.youtube.com/watch?v=BnN2TDJ3Iqg&feature=youtu.be

            Please look at that.

          • Huseyin,

            For starters,are you doing all your testing in neutral? Keep in mind that this is kind of pointless. I can make any bike bog in this situation. Does it bog when you are slowly riding at a low RPM and then whack the throttle?
            Secondly, you have a hanging idle, which means you are lean. The symptom of the idle not coming back down is due to the lean fuel screw setting. If you hit 2.5 turns out, and still lean, increase pilot jet size. Incorrect settings will make the bog worse.

            Thanks, for posting the video, it helps diagnose the problem.

            Also, give me a link to forum post. I’ll help out in any way I can.

          • Huseyin

            Matthew thank you

            Full neutral at all testing. Tried fuel screw full closed, 3 turn out and same.

            http://husaberg.org/fuel/19738-fe450-2005-bogging.html

          • Huseyin

            Needle clip 4th position, fuel screw 4turn out. More stabil but soo much backfiring.

          • Here is what I would do. The hanging idle needs to be fixed:

            1. Make sure you have no vacuum leaks at carb boot. Check with carb spray. 2. Make sure hot start plunger is clean and seating in carb correctly. Double check cable slack adjustment.
            3. Make sure there are no exhaust leaks at head pipe and muffler joints.
            Once all above checks out, readjust fuel screw. If hanging idle persists, you need a larger pilot jet. I would order the 1 & 2 sizes larger. Majority of the bikes I jet respond well 1-2 sizes larger in my climate.

            More fuel in the pilot circuit will help the bog to some extent.

          • Huseyin

            Hello Matthew
            Good news dude. I found a 42pj and tried to start with it. Needle 6th clip, 42pj, 3.5 turnsout fuel srew and bike is working better with this setting. I ordered bigger pilot jets. I will try with them when i get those.

            http://youtu.be/iSEVPMq2Frk

            I also wonder that. Hot start has rubber inside as cold start?

            Thanks

          • Sounds much better! Notice with more fuel the RPM’s do not hang up and drops back down to idle instantly.

            Keep increasing pilot jet size until you end up at 2-2.5 turns out on fuel screw.

            The hot start is usually metal with some sort of seal on it. After watching this video, I don’t think you have to look at the hot start as the problem. It responded very well to the increased jet size already.

            Keep me posted on a 45 PJ.

          • Huseyin

            Hello Matt,
            Finaly i found a 45pj. Needle 6th clip, 45pj, 1,75 fuel screwout and bike is working wonderfulllll .. Thank you for your effort and support.

            I also saw that my timing chain tensioner was almost last teeth. I decided to topend replacement besides piston.

            What do you thing about valve floating? I read many thread at husaberg forum about valve springs. Oem springs can break at 200 hours and everyone offers dual valve springs from ktm. Most users are using 650cc and 650cc can turn 8000rpm but 450cc 11.000rpm. Ktm springs can go around 10.000rpm 🙂 it can be float at 11.000rpm i really wonder.

          • Huseyin,

            Good job! I’m glad that your issue is finally solved regarding the pilot jet.

            Regarding the valve springs, I can’t comment on what to run on your model. If you have that many hours and suspect a stretched chain/worn tensioners, and the possibility of valve springs breaking, it seems like you need a full top end rebuild. New valves and valve job as well.

          • Here is more reading. Looks like guys are swapping Honda parts in:

            http://husaberg.org/fuel/5143-40-fcr-spruce-up.html

  • You should try using a SL-30 lift from HMC Industries. It gives the bike more stability and allows you to work on projects and any height you want. http://www.hmcindustries.com They also have great gas caddy’s.

  • Brayden

    I have a 2007 crf150rb, it doesn’t idol well and if I give the throttle a whack at low rpms it bogs out. I have FMF TI Powercore slip on and stock jetting. Any suggestions? Thanks, brayden.

  • Ron Howell

    Will the nfpr needle work on a 07 crf250r

  • Nick Ferby Barringer

    i bought this video before trying to figure out how to get in to watch it again

  • Nick Ferby Barringer

    i ride a trx450er we talked awhile back found out bonestock gotta a dynojet kit recommended by mechanic at dealer installed as instructions said new needle with clip in #4 and dropped back to a 42 kehin stock pilot and c126 dynojet(138 Kehin jet) and 1.5 turns out on fuel screw should i try to adjust fuel screw out another half turn or so or run a bigger main and pilot elevation is 1500ft roughly and i gained the bottom end i was missing but bogs hard up top now but it will start when hot now so thats a plus

      • Nick Ferby Barringer

        followed the video i fall at 2 turns out my quad does the same as this bike but she hates high end 3-4 wide open throttle couldnt get out of second gear im thinking with my needle is to low or my main jet is to small? what do you think

        • Sounds like your pilot circuit is tuned well. If you are having issues at WOT, then it is your main jet. You bought a jet kit correct? Do you have a link to the instructions?

          • Nick Ferby Barringer

            http://www.dynojet.com/pdf/Q118.pdf
            i followed these paper copy ima change that mainjet tonight or tomorow

          • Nick Ferby Barringer

            WOT is a lot better now backfires on deceleration and i think it needs a lil more didnt mess with the needle or fuel screw changed from cj128(k138)
            to a cj132(k142) not sure if i need to move needle up or go a size or 2 bigger

          • Backfire on decel is considered normal to some extent. Make sure you have no exhaust leaks and new muffler packing. Make sure your fuel screw is tuned and hopefully it ends up at 2 turns out, indicating a correct pilot jet size.

  • Lonnet

    I have some questions about my 05 rmz 250. The throttle hangs for 1-2 seconds before going
    down in rps after I rev it and the idle has to be set REALLY HIGH otherwise it´ll die. Do you think I need a bigger or smaller pilot jet? Or isn´t it even the pilot jet I need to change?

    • Lonnet

      I have some videos on my channel where you can here/see what i mean but I just did a top end rebuild so I can post a new video if you want to see how it runs after the top end rebuild. Thanks

      • Lonnet
        • Lonnet

          How many sizes up or down do you think i need? 1-2?

        • Take the carb completely apart and clean it. While apart, record all the jetting and we will compare to stock.

          • Lonnet

            Today I just wanted to check if the intake boot was leaking air and if that was the reason behind the weird idle. I wanted to warm it up a bit so I rolled around for about 5 minutes and when it got hot it just died. When its cold again you can start it but as soon as it gets up to temp its like the ignition shuts off. I noticed that the hot start plunger was a bit loose, do you think that might be the problem?

          • Lonnet
          • Yes.

          • Lonnet

            What i meant when i said that the hot start was loose was that it wasn´t even attached to the carb, the threads on the plastic plunger are broken

          • It’s possible that the hot start plunger is being pulled out as a result. Repair that before doing anything else.

          • Lonnet

            It had a 40 pilot jet, what do you think i´ll need?

          • Buy a 42 and 45, they are cheap.

          • Lonnet

            I went to the shop but they didn´t have 45 so i just bought the 42. I could lower the idle maybe 200-400 rpm but its stil high so i think i´ll buy a 48 and test

          • Lonnet,

            How many turns out are you with the fuel screw with the 40 pilot jet? And the 45? The fuel screw setting determines if you should change pilot. Did you fix the hot start?

            Matt

          • Lonnet

            I fixed the hot start. The fuel mixture screw was still 3 turns out with the 42 jet

            and i could only lower the rpms by 200-400 rpms. I am going back to the shop to buy a 45 and 48 jet because they did not have them in yesterday but they orderd new ones to the shop so i´ll go there next week to see..

          • OK. Also, what do you mean by lowering the RPMs by 200-400. Are you saying it does not hang as bad?

          • Lonnet

            The throttle doesn´t hang as bad but what i meant was that i could turn the idle screw so the rpms went down. Before i had the idle screw way out otherwise it would die but now i could turn in the idle screw and lower the rpms without it dying on me

          • Gotcha. Was the engine RPM increasing and smoothing out as you turned out the fuel screw?

          • Lonnet

            I don´t have an adjustable fuel screw, right now its sitting on 3 turns out. I tested turning out the fuel screw when i had the 40 pj installed and the idle was smoother but the rmps were still roughly the same. But with the 42 pj its even more smooth but the rpms are still very high

          • Do your self a favor and buy an adjustable fuel screw. You have to adjust while running.

          • Lonnet

            Went to the shop and bought a 48pj and now its too rich. But atleast i can kickstart it with the hot start

          • Lonnet

            Okey, I couldn´t get it to run good, no matter what i did. So i took apart the carb today and these are the jet sizes: Main jet 178, Pilot jet 48, Leak jet 60, Starter jet 68 and Slow air jet 100. All of them are stock exept the pilot jet which is 40 stock. I got a question aswell, what does the starter jet and slow air jet do?

          • Starter jet feeds the choke circuit. Slow air jet is an air bleed that feeds the pilot circuit. Do not change either of these. I also like to install a 45 leak jet in 250f’s.

            Make sure you clean all passages really good. Remove the fuel screw and pilot jet and clean all those orifices really good.

            I would install a 42 or 45 pilot with fuel screw 2 turns out and start from there.

          • Lonnet

            Im starting to suspect that its not even the carb thats acting up, i think it has to do with the valves. When i changed piston i saw a LOT of soot. Thats because it was burning oil before i changed piston. So im guessing the valves dont have a good seal agaings the valveseats. Today when i get home ill lap the valves. NOTE: the idle was like shit before i did a top end rebuild so its not because of that

          • Do not lap the valves! They are coated and you will ruin them, causing them to wear faster. If you suspect valves, perform a leak down test:

            https://www.howtomotorcyclerepair.com/motorcycleleakdowntest/

          • Lonnet

            Well shit…already lapped the valves

          • That sucks. Take it to a machine shop for a valve job. New valves need to be installed and seats cut.

          • Lonnet

            What will happen if i dont get new valves or the seats cut?

          • They will wear extremely quick and require more frequent adjustment.

          • Lonnet

            i hope its okey if i swear. FUCK!

            I DID NOT SEE THAT THERE WERE DIFFERENT VALVE CLEARANCES FOR INTAKE AND EXHAUST SO THE EXHAUST CLEARNACE WAS MILES OFF THE RECOMMENDED SPECS. FML

          • I do have a valve adjustment video here: https://www.youtube.com/watch?v=VK_BJu9AE6Y

          • Lonnet

            i know how to adjust the valves, just finished with it but its a bummer i didnt read the manual thoroughly before i started. I did all that for nothing, damn valves…

          • Lonnet

            The idle is perfect now and it got so much POWWWAA, wheelies as soon as i hit the throttle. THANKS FOR HELPING ME OUT MATT!! Props to you 🙂

          • Awesome. What pilot did you end up with?

          • Lonnet

            I left the 48 in but I took your advise and bought an adjustable fuel/air screw but I can´t remember what i set it to

          • Hot start should have some slack in cable. It should also move freely, as they often freeze up. At idle, hit the hot start and the bike should rev due to it getting more air.

    • Lonnet,

      This is a classic sign of a lean idle. Either your current pilot jet is dirty, too small, or your fuel screw is set too lean. I recommend cleaning carb and starting with one size larger than stock. Adjust fuel screw and get back to me with how many turns out you end up.

      Matt

  • Paul Capper

    excellent explanation on these for carbs mate, hopefully it’ll help me out

  • Paul Capper

    fcr not for

    • Paul, thanks for the compliment. If you have any questions let me know.

      • Paul Capper

        I’ve got a 41mm keihin mx carb from a ktm 450 sxf which I’ve put on my ktm lc4 640 supermoto , so miles away with jetting etc. now having a better understanding of the carb thanks to your write up I should start making some progress. my only worry is having a lean setup and causing engine damage.

        • I wouldn’t worry about running it lean. They run like crap lean and it will be obvious that it needs jetting work. Keep me posted.

          • Paul Capper

            mat, sorry to bother you, does the size of the air jet ( in the carbs bell mouth by airbox) effect low speed performance? mine has an 80 in it at mo.
            kind regards
            Paul

          • Paul,

            I’ve never had to mess with the air jet. Leave it stock.

  • Trevor Paquet

    Hey Matt,
    Just watched about all your videos on jetting and carb setup as I’m trying to figure out my freshly rebuilt 01wr250f. Got a few things going on and can’t seem to narrow them down. I’m about 3 turns out on the fuel screw running the stock (42 I believe) pilot jet so I think I may need to go a little smaller with that. But the real issue I’m having is i can’t get my bike to idle no matter what i do to the idle or fuel screw. I think i am to lean just really looking for a definite answer before I have to rebuild it again! Lol #leanismean

    • If you are 3 turns out, then you need to go bigger on pilot (45).

      Regarding idle, make sure the entire pilot circuit in carb is clean.

      • Trevor Paquet

        Thanks for the response! I think this is the first time I have actually heard back from one of these! Props man! Definitely be hearing from me more. lol So I figured it out a little more today. I actually ended up at 4 turns out and it runs great. I know that’s not right so anything else besides bigger pilot? And do you think bigger than 45 if I’m 4 turns out?

        • If you are 4 turns out, then I would jump up 2 sizes to a 48. However, before you make the change, make sure you check to see if you have any vacuum leaks and that the hot start is clean and functioning properly.

          • Trevor Paquet

            Thanks again. I will check that out.

  • Chris Souppa

    Hi Matt have you ever tried the leak jet delet ?

    • No. However some KTM’s don’t even have a leak. I never had a bike that would not bog after installing a 45 leak. They sell them down to 35.

      • Chris Souppa

        I just tried just happened to have set screw laying around. It’s way better then be for still slight bog from slight throttle (just off idle) to WO any suggestions ? I’m also used to Fuel injection this is my sons 07 yz250f

        • Make sure pilot jet and fuel screw is set up properly. I found a 45 pilot jet and fuel screw at 2 turns out works pretty good on that bike. Have you adjusted the AP timing and oring mod?

          Make sure to check this video out: https://www.howtomotorcyclerepair.com/fcraptiming/

          • Chris Souppa

            Yes have oring mod ap timing is good is good it has 45 pj

          • Is it bogging when revving in neutral or actually riding it?

          • Chris Souppa

            Riding

          • Does it go away with choke on? If not, maybe it’s too rich with a plugged leak.

 

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