Every gas engine will have an RPM at which it puts out a noticable vibration freq, usually is a few clicks above idle. Try to avoid that RPM. My newest airplane is a 33% Hangar 9 CAP with a ZDZ 80 single, Bisson pitts muffler, and Bambula 26-10. The performance of this comb (still not broken in with about 3 gal through it) is OUTSTANDING!!! 22.5 lbs, takeoffs, and torque rolls are done at less than half throttle, and general straight and level is done at 1/8th to 1/4 throttle (just above that bad vibration RPM). I cant wait until this engine is broken in... I plan to switch to a Mejzlik 26-10 or a Menz S 26-10 soon. I highly recomend the 80 single, you wont be dissapointed.

Best of Luck,

Bobby