I'm assuming you are using a two line fuel system with the clunk line going to the carb and the vent line going to the pressure bleed from the muffler? You've checked the the fuel system and using appropriate sized fuel line. Does the enging have decent compression when you hand turn the prop? Another issue I've seen with the GMSengines is the O-ring seals they used for the carbs is the cheap chinese crap rubber which after some time dries out and crack. When that happens you have air leaks all around the base of the carb. Test with some oil and see if it draws/sucks it in around the base of the carb.