I pulled this from another post:
Anyway... the proper way to set the idle mix is to turn the smaller screw (idle mixture screw) in either direction until you get the highest rpm you can. If it goes too high use the larger screw (idle speed screw) to compensate, and repeat this till you get the best "tune" you can.
Once you achieve this turn the idle mix screw 1/8 - 1/4 turn counter clockwise if the screw is closest to the engine (intake) side, or 1/8th - 1/4 turn clockwise if the screw is closer to the air box side of the carb.
What that does is Slightly enrich the idle to prevent a lean running condition.
This procedure is best done w/ a new or clean air filter/fuel filter/new spark plug.
As a base line setting these small carbs run at or near 2 turns out from seated. This is just a base line, and not etched in stone.
Give it a shot, and see how it does.