How-To: 4 Stroke MX Fuel Screw Adjustment YZF CRF KXF RMZ FCR

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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 size. If you end up at 2.5 or more, increase pilot jet size. After 2.5 out, you risk the fuel screw falling out. Besides, the fuel screw also begins to lose effectiveness that far out.

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 used in FCR carbs.

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.

 

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