This may sound funny but its probably a leak, A cracked fuel line from the tank to the carb or the exhaust to the tank or even the tank its self. If its older check the O ring and make sure its sealing on the lid to the fuel tank. Those are the usual places. After that its off with the Exhaust pipe and poor alcohol threw the pressure port and clean it out with a paper clip till you can blow threw it. Thats about it m8 [8D]