How-To: 4 Stroke MX Fuel Screw Adjustment YZF CRF KXF RMZ FCR
It is very important to have the fuel screw on 4 stroke MX bikes set properly. More so than any other motorcycle engine by far since they do not seem too forgiving. Starting one of these bikes when it is running too lean or rich can really be a PITA, not to mention there is a hot start feature to complicate the entire start up procedure.
Symptoms of fuel screw not adjusted properly
- Hard to start hot or cold
- Hanging idle means it is lean
- A rich setting will make it difficult to start
- Decel exhaust pop or backfire denotes lean condition
Fuel screw adjustment procedure
The proper way to tune a fuel screw is to make adjustments on a fully warmed up engine and at LOW RPM. With engine off, lightly seat the fuel screw and set to factory settings as a baseline (usually around 2 turns out). 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 dirt bike. Screwing it in makes it leaner, screwing it out makes it richer.
How-To Video
In the video above, I adjust the fuel screw on a 2006 KX250F. It is equipped with a adjustable fuel screw, which I highly recommend buying. Below is the first video I came out with for fuel screw adjustment, however it has some audio issues, and why I chose to post a new video. Anyway, here it is:
Hard to reach the fuel screw?
If your fuel screw is hard to reach by hand or a small screwdriver, replace the fuel screw with an adjustable one that will make it easy to adjust with no special tools (and might keep you from burning your hands).
How do I know I have the right pilot jet size?
On FCR carbs, the fuel screw should be between 1.0 to 2.5 turns out. If you end up at 1 turn or less, decrease pilot jet
You can expect one jet size to change fuel screw by .5 to 1.0 turns. For example, the YZ250F in video had a 45 pilot and fuel screw setting at 1.0 turn out. With a 42 pilot, it ended up at 2.0 turns out. Below is the pilot jet
Need more jetting advice?
Alright, this post tackles one circuit of your carburetor only. See my complete 4 stroke jetting guide here.
Valve adjustment
I just want to mention that tight valves may also cause hot start problems. If you followed the above and continue to have hot start issues, check out my YZ250F Valve Adjustment Video.
Comments
Has this post/video helped you? Please comment below.
Hey man, I made a mistake and pour 10w40 oil in my fuel tank on my crf80f
I don’t know what to do!!!
It turn on idle for about 20 seconds and shuts off 😞😞
You will need to clean and flush the tank. You will also need to take the carb apart and clean/flush away the oil.
Hey man I don’t know what jets to use on my Kx250f 2009 according to my weather and elevation. Is there a graph or something similar to know what to use properly. My exhaust is backfiring when releasing the throttle.
Greetings from Mexico!
What temps and altitude do you ride at?
Some of these specs will apply:
https://howtomotorcyclerepair.com/how-to-kx250f-jetting-specs/
Matt
At 3600 feet with dry 85-100 degrees
I watched you video on adjusting the carb and it mentioned about a replacement pilot screw that is more easily adjustable.
Mine is on the bottom of the crab right up near the engine and the 90° screw driver is a pain.
Can you tell me to get replacement pilot screws.
thanks
Mike
Here it is: https://www.rockymountainatvmc.com/parts/tusk-fuel-mixture-screw-p
I have a Kx 250f 2009. It runs great but hot or cold on the first rev it puffs a cloud of unburn fuel. Adjusting the fuel mix screw doesnt seem to improve the issue. Any input would be appreciated.
Most likely the accelerator pump causing that. What leak jet is installed?
Hey Mat,
I have an 07 kx250f and I want to go bigger jetting all around. Where can I buy the larger Jets.
Thanks Johnathan
Rocky mtn atv
I am having trouble figuring out what parts are meant for my carburetor. If you have some time would you be able to drop me a link for a larger pilot jet, main jet, and leak jet. What is the o ring mod? Thank you so much for your help
Pilot jet
https://www.rockymountainatvmc.com/parts/pro-x-n424-21-series-keihin-pilot-jet-p
Main jet
https://www.rockymountainatvmc.com/parts/pro-x-99101-357-series-keihin-hex-main-jet-p
Leak jet
https://www.rockymountainatvmc.com/parts/keihin-n424-52-series-leak-jet-p
Oring mod
https://youtu.be/hq0OE3bCm5E
Thank you
My secondary main jet is a 68 what should I change that to? Thanks
I think you are referring to the jet that supplies fuel to the choke. Don’t change that one.
Matt, can you tell me what size leak Jet you’ve found to work best for an 04 yz450f?
I don’t have specs for that model. Do you have a bog when you crack the throttle fast when riding?
If so, first make sure pilot jet and fuel screw are tuned. Then decrease leak jet size until bog disappears.
Hello. My CRF450R exhaust pipe goes red. Wats the reason? Carburetor is cleaned. Fuel screw 2.5 turns out.
Any exhaust leaks? If you turn fuel screw 2.5 to 3 turns out, does the engine RPM increase?
I have a 2015 honda cfr 250X I bought it used, its major problem was it will not idle and will run on choke, so i cleaned the carb and replace all new stock jets, and ran 50 miles and some how after that it started getting sluggish and after that ride it does not want to take the throttle any more and it will die.
I tried to play with the fuel screw but it will still not take the throttle, it only takes the throttle for 1/10 of the throttle turn and then it will die, what i dont understand is why it ran for 50 miles and now it does not work.
I am trying to get your input on what to do, i have removed the carb and cleaned it three times before the last run of 50 miles.Thanks for your help.
Sounds like you have to pull the carb again and double check your work. Did you tighten all the jets? Maybe one fell out?
Hey man I have a 2010 Kawasaki kx250f accidentally adjusted my idle adjusting screw while my bike was not running and cold and now it has a hard start and when it does start it has a wicked high idle, any tips on how to get it back to normal
Turn the idle screw CCW to lower idle speed.
Recently picked up a 2000 YZ400F, bike will start and run as long as the choke is on. If the choke is off the bike work start or idol. As soon as the choke is disengaged the bike will turn off. Took the carb apart and cleaned everything and reassembled it. Still having the same issue. Any suggestions?
Did you clean the pilot jet? Verify it was clean by holding up to the light? Did you remove the fuel screw and clean that passage well? Hot start plunger port clean and moving properly?