Well, my prop may be a bit unusual but, even so, that doesn't explain some low freq airframe vibration that I have been trying to get rid of.  Some of the problem was solved when I installed larger diameter hinge pins.  Generally there is more vibration upon leveling than in a climb.  I have begun to believe that there is too much shaking in the tail.  I also stopped some old endurance tests when the rpm was reduced to about 2300 because of increased vibration.

 

So, yesterday I noticed that there is a "looseness" at the center hinge point of the rudder. Removing the rudder didn't reveal anything spectacular, but the bushings that retain the rudder pins (1/4" bolt nubs) are awfully short and somewhat sloppy (I do have about 700 flight hours) - sloppy enough that slapping the center of the rudder or vertical stab with the flat of my hand produces a resonant vibration and a clatter.   Try yours.

 

I had some similar 1/4 ID, about 3/8 OD bushings around (about 5/8 in length) gotten at the local Ace Hardware.  I felt these would stabilize the pins by capturing more of it in length.  I got the old ones out with a 3/8" pin punch and pressed in the new ones (just the top two) with parallel-jaws pliers and a socket (see pic). Today, I had reduced vibration in level flight and even operating at 2250 RPM wasn't bad.  I might even be able to actually operate occasionally at 2350 for some weird purpose not related to racing. 

 

Anyway, I think I need to stiffen the tail cone or somehow dampen the empennage resonance or just change it.  Any ideas?