Re: Vibration rpm limit
I have had a couple that were ALMOST as smooth as a Japanese dirt bike engine....others that would go 40 mph, but vibrated so bad, you could hardly hang on.
Going to a 40T sprocket will help, so will balancing your wheels.
I have gone to mounting all engines with small 1/16" thick sheets of lead between the mounts and the frame (4 pcs. total)...works like magic!