You have an air leak somewhere. Try this..Take the pressure line from the exhaust, plug the throat of the carb with your finger,and blow into the pressure line, chances are youll find your leak. You could also, if you cant find it like that, get a spray bottle with water and a little dish soap in, spray common areas known for leaks (back plate, carb, where it mates to the block, the primer on the tank ect.) Do that without fuel in the tank of course