Re: i think my "80cc" has issues. top speed only 28mph with shifter kit...
You say you sealed your intake and exhaust manifolds but did you match them. You should have noticed that the intake and exhaust manifolds are round and the ports are not. This causes them to be partially blocked. That is the first thing I would try. I would then check to see if your exhaust pipe if flowing freely. Make sure there is nothing blocking or hindering he exhaust. Then I would start playing with fuel mixture but like was mentioned check your plug. It is the only way to tell how your mixture is without an exhaust gas analyzer.