check reeds, and make sure your jet is good and clean you may need to give the carb a good cleaning. does it rev high in neutral? or just when you pull the clutch? in neutral you clutch is engaged, and it could be slowing it by friction, and you only notice it when you disengage the clutch, but its mor than likely dirty, sticky or just jammed open, your throttle cable slide ok or could it be stuck? try lubricating everything, and cleaning your carb. if that dont work, im at a loss. good luck