Re: i think my "80cc" has issues. top speed only 28mph with shifter kit...
Excellent advice Joe. Start small and work your way up. So far my engine is completely stock. I have done nothing to the engine. I have matched my intake and exhaust headers. Added a tuned pipe and air filter. I have also rejetted the carb. I can now do 35 easy and can hit 40 if there is no wind. Once you get to around 30 resistance, mostly wind resistance, plays such a huge roll and increases exponentially with every mile an hour faster you go.
Have you tried different gears for top speed. For instance I have a 6 speed cassette and my bike will actually get a higher top speed in 5th gear. 6th is just too tall for the engine to pull. I will do some proper porting soon. Plan on trimming the piston skirt to unblock the intake as well as lower and widen it slightly. I will also raise the exhaust port slightly. But like Joe said a little at a time.