How the Idle Adjustment Affects Everything


Jul 29, 2000
South America

The idle mixture hole is at the bottom of the carb throat just below or in front of the front edge of the slide (if it were allowed to close fully). This means the idle mixture is always part of the whole mixture no matter what the throttle position is. That is why all carb adjustments begin with the idle adjustment (air screw and possibly changing the idle jet).

Adjust It:
With the engine warm go ahead and adjust the air screw till you have maximum idle RPM. If it is really high for idle RPM then lower the slide a bit by unscrewing the slide stop screw. Then turn in the air screw 1/2 turn so that the RPM lowers a bit. The idea here is to have the idle mixture a bit rich to offset the instantaneously lean mixture that results from cracking the throttle open. When the air screw is set right then the engine exhibits a nice crisp acceleration from idle, both with the transmission in neutral and while riding. You don’t want it so rich that the engine sputters any while riding at slow speed with the throttle closed or just barely open.

When to Change the Idle Jet
If the final air screw position is more than 2 turns out then you need a smaller idle jet. If the air screw position is less than 1 turn out then you need a larger idle jet. Until you get hold of another jet just stick to the air screw adjustment that you got when following directions above.

Adjust Before Each Ride
If you live where the humidity/temperature changes a lot from week to week then you should adjust the air screw before each ride. This adjustment will help correct the overall jetting throughout the whole throttle movement. Like I said before, the idle mixture is always fully on no matter what the throttle position. So if some hot humid weather comes then adjust the air screw out and that will affect all your jetting, making it all leaner. Of course you should still change the idle and main jet out for each major seasonal change but still by doing this little trick you can have a happier engine each time out. And a happy engine means a happy rider!

The Hardest Step
Getting the idle and main jet right is easy. What is hard is selecting a different needle with a different taper if the mid throttle running is leaner or richer than it should be. My guide to tuning and available software to make it easier is here —>

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