The screw on the carb is an idle mixture screw just like on a cars carb. If you turn the screw in (clockwise) it makes the mixture richer. My carbs screw is 2 turns out. I have found that the carb needs adjusted if the temp outside changes much. I have tried to mod the airbox and it made it run bad. The only thing I did do which did not seem to do much was open the air inlet hole to a bigger size = 19mm.