Engine performance mystery

[Eric Brown]

Gentlemen, I have an update. Thru the generosity of another airplane enthusiast, I was offered the use of a Tru Tach ll shoot thru the prop digital tachometer. Guess what ? ........ the engine runs fine. The Tru Tach shows 2560 RPM in the climb while the airplane tach shows 2350. In level fight, the airplane tach matches the Tru Tach after 4 min. When I climb again, the airplane tach drops to 2350 and the Tru Tach shows 2560. Again, after 4 min in level flight, the 2 tachs match each other in the 2000 to 2700 range. I have had the tach tach drive cable out of the tach drive housing. No anomalies. I re-greased the tach cable and reinstalled. I changed the cable housing routing just slightly. No difference. Rather odd behavior for a tachometer. Whadda you guys think ?