I installed the 80cc motor kit on a nearly new mountain bike. Everything is straight, well lubed and runs true. After about 100 miles I decided to test the top speed. Up to 25mph everythings great. Plenty of throttle left and when fully open engine sounds like it is going to gain speed rapidly. After about 27mph the engine acts like it is being controlled or held back by some kind of restrictive device. My son in law has the same engine kit and it responds the same way. I have installed a new carbon plug wire, plug and carb. o-ring. I modified the muffler by removing baffles and cutting the small exhaust pipe to about 3 inches. I would appreciate any ideas or suggestions to engine can continue reving. Thanks, Mike