Just bought a rebuilt s5 t2 motor from a guy and have some vibration issues. I know its motor related as it vibrates when I free rev it. smooth at idle but get higher in the rpms and vibrates pretty bad. My thught is counterbalance and flywheel mismatch but a little concerned that maybe he mismatched parts inside. Am I on the right track with the flywheel and counterbalance weight or am I screwed and need to pull this motor apart and balance it.

 

These are the markings pulled from the flywheel castings that I could read with a mirror.. HY1 and 1a3n. Do those tell me anything?

 

I ran into that exact issue with a 12a (mismatched flywheel). It vibrated quite bad, bad enough that I knew right away that something was wrong. The thing is, I didn't have to rev it very high before it started shaking. Since yours is only doing this at higher rpms, maybe you have a different issue...

I think the flywheel is the place to start though (assuming you've already gone over motor/tranny mounts).

Good luck.