There is a small screw in the throttle barrel. On the side of the carb that has the arm that opens and closes the throttle barrel, there is a small srcew in the centre of the pivot point of the throttle control arm. That's your idle mixture screw... you'll need a fairly small screwdriver to get in there... adjust it an eighth of a turn at a time at most. Don't try it with the motor running... it's way too close to the prop!