i rigged up a muffler if you will for my 49cc 4 stroke, i wanted a bit more power and the stock pipe is to restrictive, but unlike a 2 stroke running without a baffle and decent back pressure will fry the exhaust valve, so i rigged something to provide just enough back pressure but to be more free flowing than stock, so far im liking it! i know it looks like crap and i am working on it, i bought a buncha stuff today. anyways let me know what you think
YouTube - custom exhaust on 49cc 4 stroke
YouTube - custom exhaust on 49cc 4 stroke