FAQ: Where is the Idle Mixture Screw?

The idle mixture screw, or pilot screw, on an FCR carburetor controls an emulsion of fuel and air. That means that turning the screw out makes the mixture richer, and in makes it leaner, the reverse of common practice on two stroke and older four stroke carbs, but the same as on most automotive carbs. The screw is located under the carb, up inside a well in the float bowl, just forward of the jet access plug:

IMG_0633.jpg

Here, with an extended screw in place, is where it is, viewed from the side:

IMG_0632.jpg

The large, bright metal screw just below the fuel line is the idle speed adjustment screw. In is faster, out, slower.  On YZ426 and earlier models, the idle speed knob is black plastic.

Additional info on adjusting the pilot screw:

http://www.thumperfaq.com/jetting.htm#PJ

http://www.thumpertalk.com/forum/showthread.php?t=327405

Guest
This topic is now closed to further replies.