Sure sounds like you have an air leak somewhere in your system. Clamp the carb line, remove the vent line at the muffler nipple and pressurize the fuel system with a 50cc syringe. Clamp the vent line and remove syringe. After 30 mins, release the carb line clamp, and see if there is any air that rush out of the line (you may need to place your ear near the line to listen). If there is, then your fuel system is OK.