I would think that a vibration at low RPM's is more the engine than the prop or spinner. Doesn't an imbalance there show up at higher RPM's or at an RPM that is more resonating than it would be at lower RPM's? Just wondering here because I'm not exactly sure.

I do know that if you are using a "soft mount", it will vibrate like crazy at low RPM's, but settle out at higher RPM's.

CGr.