I have a very sturdy mountain bike with a 66cc engine on it. I built it from the frame up. I am using a 38T sprocket. When I cruise at about 75% throttle it is fine and I think this around 20-25 mph but when I go any faster the bike gets squerly. The front tire wants to shutter and the whole bike feels like it is wiggling. My question is this normal or do you think something is wrong? I spent 5 months building this bike very maticuolously using high end parts. It is a steel GT tripple triangle frame and I have built custom wheels for it with heavy duty hubs spokes and double walled rims.The tires are high end specialized kevlar tires. I am running avid v-brakes front and rear. It has a heavy duty sealed headset holding a nice suspension fork. The stem and handle bars are aluminum with the bars that taper to a larger diameter where the bar attach to the stem. The bottom bracket is a sealed cartage type with shimano cranks. I was able to set it up with no tensioner on the engine chain. I am running a stock engine that I port matched to the intake and exhaust. I am also running an air filter and expansion chamber from SBP. The engine runs well and smooth. I mounted the engine very solidly to the fram with no rubber just right up against the metal and I made sure the front and rear mounts are lined up with the frame tubes to the right angle so they are flush. This thing is like a solid tank. The only thing I can think of is the concept of a gyro. I used to play with this toy designed to excercise your hand and wrist. It has a neon ball in a clear plastic case and you get the ball spining real fast and you can twist it around in a circular motion to keep the ball spinning for a long time and it was called a gyro. On the bike since the wheels are spinning so fast maybe if it starts to shudder the gyro affect makes it get thrown off and it gets worse and worse until you slow down or you try to correct it with the strength in your arms. I hear of guys going 30+ mph on here and some up to 40mph all of the time but no one mentions this problem. Just wondering if its my bike or a common problem here Thanks for your help guys!