Wow Bootlegger; sure sounds like you've been having a bit of trouble with that thing ( no disrespect ) I've never experienced vibration with mine, I'm just on 2000 Ks. As far as studs and threads go I've swapt all mine with stainless steel ones from my local engineering firm, much better fit, tighter tolerances. I've also had to jiggle with that float needle thing, I reacon it's the forks, I've bent mine down a bit to increase the force on it and drilled out all the metal fuel delivery components to increase flow, seems to work for me. By the way my engine seems to be perfectly ballanced. I got my motor from a guy who imports them from Australia. These engines are made at the Jangsu university in some province in northern China as part of a university project. Although these engines my look alike there is a vast difference between their manufactures.
Don't give give up on it they are after all a lot of fun. I've had mine up to 57.4KPH thats about 35.9 MPH and she was just singing, but thats with quite a bit of tuning though. If I ever want to go faster than that I'll do it on a motor cycle, bikes ain't ment to go that fast.
Regards Fourfeathers.