Thanks for your replies. The props were all hand-reamed and perfectly balanced (checked on two other balancers). Mounting the wing makes no difference. The little H9 Cap doesn't have an engine box per se. I tried a prop aligned perfectly with the piston, - no difference. The engine was hard mounted.

I'm going to take the engine off and run it for a couple of hours on a stand to see if it smooths out. If it doesn't I'll send it back. I thought these were renowned for their smoothness!!

KD