I've never owned a 50cc scooter, so I am no expert.
That being said, I do know that the gasket, that goes between the head and the flange on the exhaust pipe, is advertised (by some sellers) as fitting both the 139 and 157 gy6's. Also the exhaust outlet on the 150's muffler can't be more than 1/2" in diameter. So there shouldn't be a nickles worth of difference in the 50cc and 150cc stock exhaust systems.
My 150 runs fine with the original exhaust system, even though I put a K&N cone filter, and rejetted to a 115 main, over 3 years ago.
Maybe someone who has both a 50cc and a 150cc, will enlighten me as to the difference in the two systems. Til then, I can't se why an 85 cc engine would be held back too much running a stock system.
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