I've had problems with the muffler cap a few times.. now before it's installed I drill where the muffler cap meets the outer tube and crank in a (red) loctited screw, and do the same at the top end for good measure. No problems since.
thats a good idea, unfortunately Ive lost the cap so I need to get another one or a new muffler. lol, its always something, new tire and tubes are installed and the thing rides great so...