If you cant see any external flaws on the muffler, then there probably is nothing wrong with the exhaust... are you Absolutely sure there are no pin holes in your lines..... pin holes cause lots of bubbles which can make a motor to run horribly..... sometimes they are hidden... i busted my a** trouble shooting a motor because and it turned out to be a punctured line.... If you motor is really running that bad, its worth pulling the external lines off and do the pinch suck and tongue method