I had the same problem. it would get loose up in the air, but it would misfire so bad because of exhaust fumes being pick up by the carb, what I did is to gut-out the inside of the muffler or drill the inside plate looking from the exhaust inlet with several 3/8" holes and some in the bottom at the exit, or grind out the sides of the muffler take everything out then weld it back together you will notice a big increase in power, also install the gasket that goes in between the muffler and the engine to alleviate some of the vibration effects and put some red locktite on the threads if the threads are stripped go ahead and re-tap them with a 1/4" 20 or 24 tap and also use red locktite(you can also use the blue stuff) those are the things that really took care of my muffler getting loose problem. good luck.
Bill Pomplun A.K.A. Aero Nut.