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Thread: Engine performance mystery

  1. #11

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    Might be prop getting vortex ring at static and needs forward speed to get a good bite.

  2. #12
    FlyingRon's Avatar
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    Quote Originally Posted by Bill Berson View Post
    To get the 2700 rpm in cruise on a 150 you might need to climb an extra 100 feet above cruise altitude and push the nose down for a short dive to get the 105. My primary instructor showed me that 45 years ago but I haven't flown a 150 since.
    Called get it on the "step".
    Mostly OWT. Doing the pushover may get you to the steady state faster but the steady state is going to be the same whether you level off at the altitude or overshoot and dive.

  3. #13

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    Engine runs hotter with friction until after 20 hours or so.

  4. #14

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    Sep 2014
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    Check exhaust for obstruction. Same for carb heat box. Check fuel system for proper venting and flow. Verify carb fuel inlet screen is clean. If carb is older, might think about exchanging for factory rebuilt. Or barrow a different carb to see if makes any difference. Check throttle shaft to throttle arm security. Last time I saw a similar RPM reduction problem on takeoff, it turned out to be a crankshaft bearing issue. #2 bearing support was found cracked. Most likely this is not the case since engine was just overhauled. May want to try an oil sample looking for elevated aluminum or bearing material. This is a tough one. Looking forward to see what it turns out to be.

  5. #15

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    May 2012
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    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 ?
    Last edited by Eric Brown; 10-18-2021 at 07:40 PM.

  6. #16
    rwanttaja's Avatar
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    Quote Originally Posted by Eric Brown View Post
    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 ?
    Eons ago, I noticed some tach anomalies in my C85 (which uses the same tach setup). In my case, it turned out that the end of the tach drive shaft (on the back of the engine) had cracked.

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    From your description, I'd say this isn't your problem...but the next time you disconnect the tach cable, slip a camera in and take a picture of the drive shalt.

    Ron Wanttaja

  7. #17

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    Could remove and spin the tach with a drill motor and confirm the changes with angle of attack.

  8. #18

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    May 2012
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    Another update.
    I found an old used tach and installed it in the plane. It's not to accurate, but it was steady. It showed NO rpm variance in take off and climb. That, to me, eliminates the tach drive system.
    I rigged up a table top tach drive test cable using a drill motor to drive it (as suggested). Connected the suspect tach to it. The tach DID show a variance in rpm when tilted in the up and down pitch angles. Connected an old tach to the test cable and it did NOT show a variance when pitched.
    The problem tach is a new Mitchell Aircraft Products unit, p/n D1-112-5025 / model 98480-25. A new tach is on order. Will continue to update.
    Last edited by Eric Brown; 10-23-2021 at 09:44 PM.

  9. #19

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    Cool. Easy fix.

  10. #20

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    Last update
    The problem has been resolved. The new tachometer has eliminated the problem. To me, the tach came under the least suspicion because it was new. One for the record books. Never have seen a tachometer act in such a manner. Thanks for all of your suggestions.
    Last edited by Eric Brown; 11-10-2021 at 01:46 AM.

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