Annoying vibration

Check the rubber isolation monts in the engine basket. Are you sure it is RPM related? Could it be an airspeed related airframe vibration?

Arlan Allen
N3989H

I know you said it started all of a sudden but do make sure the prop is indexed correctly and check the torque on the engine mount bolts.

Kevin

Well, here is the update..local AP thought aha, I believe it is a broken piston ring...so I authorized pulling the cylinder..nope..that's not it...it's like the Whack-mole game and each whack, I throw
More $$$$. Into the hole...

I have the C-85, metal prop. Yesterday I checked the mounts and could find nothing obviously loose.
The strange thing is it developed one day as I was flying pattern work, suddenly on one of my down-winds there it was.

Which engine?

As soon as I burn some oil off I will do that ! Today I put 2 oz of MMO in each tank.
Flew it once around but the winds came up, plus I had an Erickson Skycrane in the pattern with me..always a joy...