Catastrophic aluminum bushing faliure.

[Fabian]

my buddy's harley shakes worse than my 66cc. I think a lot of people just have sensitive bottoms or something

I agree.

My chainsaws "all" vibrate more than the 66cc engine. After 4 hours on the chainsaws, my hands go tingly numb, but i can ride all day on my bike and they never experience that sensation.

That said, reducing the normal operating rev range of the 66cc engine greatly reduces vibration to where it is not even a concern. You just need to boost torque output to compensate for lost power and then to change the bicycle/jackshaft gearing to give similar road speed as to what you had before.