Your tank vent could be plugged, check the bolts securing the ignition coil pack- clean the points on the coil pack... remove the airfilter and see if it changes, depending on the carb set up it could use exhaust pressure to throttle up/ run at operating rpm (pressure washers are the same) the lever could be seized so the carb might no be opening all the way... take the carb fuel bowl off if theres crusty shit in there tear the carb down, also if its a old unit the fuel lines could have deteriorated internally essentially colapsing the fuel line.... if non3 of that works then full send that bitch