Adjusting the Idle Mixture Screw

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To adjust your carburetor's idle mixture screw, ride the moped until its all the way warmed up and the low end is at its worst. Then put it up on the stand and reset your idle mixture screw like this:

Set your idle speed to be faster than normal. Unscrew your idle mix screw a quarter turn at a time until the engine starts choking a bit. It usually takes about 3 or 4 seconds after each quarter turn until the engine starts reacting to the change. As soon as the engine starts choking, screw it back in a quarter turn. Now you know where your max setting is.

From there start turning in the idle mix screw a quarter or eighth of a turn at a time until the engine runs as smoothly as possible, i.e. the RPMs are very consistent and solid. Remember to wait a few seconds between each adjustment to let it take effect. Now reset your idle speed back to normal and turn off your engine. Write down how many turns out your mixture screw is from all the way in for future reference.

Now go for a spin and see how the low end feels. If the low end still doesn’t seem good, try turning in the idle mixture screw another quarter turn and see if it gets better or worse. If better, try another quarter or eighth turn in and see if it gets better or worse again. If worse, back it off to where it was best before. Eventually you’ll find the ideal setting.