My brother has a Saito 1.25 that has a vibration at a certain RPM but is very smooth at all other RPM. It's not at idle on his engine, which is 2000 or so, maybe around 2800. He tried balancing the prop, and a different prop, but the vibration at that rpm is always there.

I thought about removing some of the tapered portion of the cones so they do not touch but never bothered to do it.