replace the fuel lines, both of them (from the tank to the carb and from the exhaust to the tank). Also look at the tank real good, is it cracked? Does the cap fit tight? If your running a fuel filter (and you should be, if not shame shame) make sure its installed right and it too is tight. Those few things should fix that problem for you. Try blowing into the tank via the exhaust tube. See if you can blow it to the carb. An air leak is hard to run down but those are the first things to try, and cheapest.