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Thread: Engine performance mystery

  1. #1

    Join Date
    May 2012
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    Engine performance mystery

    I have a Cessna 150L with a newly overhauled O-200 engine. The engine has been flown twice. On the second flight I noticed that the RPM, during climb, was only 2300 RPM. The engine ground static run with the overhauled McCauley 69-48 (climb) prop is 2500 RPM. Right in the middle of the C150 TC specs. After the climb out, I level out to cruise configuration with the throttle wide open all this time. The engine will only turn 2500 max. NOW, the really strange part. With the throttle still wide open, I can dive the airplane until the RPM reaches 2700. I then level out to cruise configuration and the RPM now stays a 2700. If I climb the airplane again where the RPM drops to 2300 and then level out, the engine will again only reach 2500 RPM, max. All this with a continuous wide open throttle. If I dive again to bring the RPM back to 2700 and then go back level cruise, I now close the throttle to reduce to 2300 RPM and then advance the throttle back to wide open and the RPM will return to 2700. I did these scenarios several times. Each time the RPM would be limited after a climb, but not limited by throttle movement.

    Never in my 50 yrs of working on these things, have I ever seen a problem like this. Nothing in the Harry Fenton files. It's not a tachometer problem because I can hear 200 RPM engine noise difference. I have one possible theory, but I don't want to influence the thought process.

    Hey folks .......... any ideas ???????

  2. #2
    FlyingRon's Avatar
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    Have you looked at the induction system for loose or collapsing parts that may be transiently restricting airflow in these circumstances?

  3. #3

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    Well Ron, you were close. When I wrote this thread, I had not performed any investigation as it was the end of the day. It appears that I cried "mystery of the month" too soon.
    What I found today does not fully explain all of the events, but it was more than likely the problem. The RH exhaust tailpipe brace clamp had slipped down the tailpipe and dragged the brace rod with it. The throttle control runs under the brace rod. With the brace rod lowered, the the throttle control contacted the rod when the throttle was pushed open. This in turn would not allow the throttle to open fully. There was a 3/16" gap between the throttle arm and the full open stop on the carb with the throttle pushed full fwd.

    I had to reshape the brace rod and reconfigure the rod attachment to the tailpipe clamp to make plenty of gap between the throttle control and the brace. I'll try to attach some photos. Should have taken the Harry Fenten advice that, low power issues are usually found in the engine control rigging. Will check out on next flight.
    Attached Images Attached Images    

  4. #4

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    I guess my hopes were to great. The RPM problem still exists. I had lots of time to evaluate on recent 2 hr flight. It still turns only 2350 RPM during climb and only 2500 in level flight after the climb. If I reduce power to 2000 RPM, in level flight, and then advance the throttle to full, it will then turn 2700 RPM. If I climb and level again, only 2500. I re-checked the throttle linkage clearance and it's OK. The carb throttle fully opens. Now, in my experience, the climb RPM should be at least the same as the ground static RPM. In this case 2550. It acts as though the prop changes pitch. Mixture leaning had no effect.
    My thoughts are coming back to my original theory concerning a possible crankshaft thrust bearing problem. The RPM decreases after the prop has been loaded (climb) and then increases with then the prop is momentarily unloaded (pwr reduction). An oil screen check showed to contaminates. Engine now has 4 flight hrs since OH.

    At a Loss
    Last edited by Eric Brown; 09-15-2021 at 07:14 PM.

  5. #5

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    Here's a late thought. Has anyone had experience with the Marvel carb. one piece venturi and the type of fuel nozzle used. My carb. is a 10-4894 that has the one piece venturi mod, but retained the original fuel nozzle. Precision Airmotive SB MSA-7 only recommends the newer nozzle when roughness, caused by a rich mixture, is detected. As I found, leaning the mixture made no change, however there is an increase in engine vibration above 2300 RPM.
    I did experience the richness problem in a C-172 (O-300) back when the one piece venturis were first mandated. In that case the richness was easily recognized.

    Whadda ya think ?

  6. #6
    What is the airspeed doing at 2500RPM/level Vs 2700RPM/level?

  7. #7

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    Quote Originally Posted by PJZajkowski View Post
    What is the airspeed doing at 2500RPM/level Vs 2700RPM/level?
    100 mph IAS at 2500, 105 mph IAS at 2700.

  8. #8
    Quote Originally Posted by Eric Brown View Post
    100 mph IAS at 2500, 105 mph IAS at 2700.
    How about this idea... Your RPM (and power) is obviously a function of airspeed. Once you get the airspeed to 105 mph in a dive, you have 2700 rpm , more power (compared to 2500), and the airspeed can stay stay at 105 mph when you level off. Put it into a climb and the airspeed drops, rpm drops, power drops and when you level out, you can't get back to 2700 rpm and 105 mph because you have less power.

    I remember someone telling me long ago that you can cruise faster if you climb slightly above your desired altitude and then descend. You get a little more airspeed, which gives you a little more rpm, which gives you a little more power. Of course this would only be for fixed pitch props.

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