First off if you are using as gasket between the muffler and the head get rid of it. Clean both mating surfaces and apply a thin bead of Hi Temp Red Silicone to one face and after a few minutes bolt them together using some red locktite on the muffler bolts and that will be the end of your muffler coming loose problem. With your engine willing to only burn off the prime and not run sounds like the carb is not pumping or sucking fuel. When did you rebuild the carb, after it started giving problems or before, and did you replace the pump diaphram. Check that the pulse hole from the engine to the carb is not blocked. I'm pretty sure that others will chime in with other possible solutions.

Karol