I have an 80cc (chinese) motor mounted on my mountain bike. It initially ran great, for about 50 miles. I got a max of 36.1 MPH. Now on mile 120, I can't get over 20 MPH. I have tried a bunch of different things.
I have replaced the spark plug.
I have checked for an air leak.
I have replaced the plug cap.
I have cleaned the carborater and verified proper working order.
I have replace gas/oil mix with 30:1 blend.
I have checked that all screws are tight (mounting, carb, chain,etc.).
It's a mystery.
I have noticed there is a 'squeek'. With engine off and clutch engaged (free rolling), I hear a 'squeek' in the motor casing.
At 3/4 throttle, it gets the shakes and bogs the motor down. I no longer get full rpm's.
Here is a pic of the engine, opened up. I tried to wipe off the top of the piston with a rag, but some build-up is still there. And now that I've taken the top off, I'll have to replace the gasket, right?
Now some pics: