Should be a lean/rich fuel mixture valve that might need to be adjusted. It is located where the fuel hose attaches to the carb. Either on the top side or could be on the bottom side of the housing. Round valve with two flat sides for the wrench to fit. Usually takes an 11mm open end wrench to make the adjustment. Start it up and hold the throttle pedal down while turning it in either direction. Adjust it till it revs up the way you want it. Good Luck.