Thanks for the f/b Mr Cub, Ive got a few ideas to try now (including running w/o prop/spinner for a short while just to make sure they are not contributing to the vibes - they are balanced but maybe some imperceptable runout in the shaft...I dont have a DTI...?).

I mounted the motor on some neoprene standoffs but didnt help. I can also not feel vibration if I touch the crankcase, but only if I hold onto the stand that its mounted to (so its fairly rigidly mounted). This makes me think that the oscillation is centered around the main shaft which points to the flywheel/counterbalance/piston as being the problem. The max vibes seems at half throttle, it smooths out a bit at the top end. Mixture at both ends seems ok ito. pickup at idle and plug colour at full.

Anyway, I'll investigate and report back any findings just in case anyone else wants to mix and match parts between the two capacity motors.

BTW, how do you get the clutch off the main shaft? Two large screwdrivers? and, to remove the main shaft from the bearings/case?

Thx
Mark