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Thread: Mystery noise at low rpm…?

  1. #1

    Join Date
    Dec 2019
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    Hatcher Pass, Alaska
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    Default Mystery noise at low rpm…?

    Wondering if anyone here has ideas about what this could be…

    When I pull back the throttle, an irregular low thudding noise develops. It’s noticeable at around 2000rpm but slightly louder at around 1500rpm.

    Airspeed makes no difference. Does it with throttle pulled back both at low and high air speeds. Never notice it at run up. Carb heat makes no difference either.

    Plane is a pa22/20, 150hp, engine overhauled last year with ~175 hours on it now.

    Let me know if y’all have any ideas!

    Thanks in advance,
    Brendan

  2. #2
    rocket's Avatar
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    Dec 2010
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    Talkeetna AK
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    Default Re: Mystery noise at low rpm…?

    Check your screen or filter bore your next flight.

    what you are describing could be a main or rod bearing about to go.

    couod be nothing but please check regardless.

    in flight or on the ground? Is it more noticeable if you pull the power back from full to idle?

    Rocket

  3. #3

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    Default Re: Mystery noise at low rpm…?

    Hey Rocket,

    It only happens in flight, not on the ground. Yes, most noticeable when pulling power from full to idle/lower RPMs.

    I noticed it a little while ago. Last time I flew it seemed to be louder (problem potentially getting worse?).

    Thanks,
    Brendan

  4. #4
    Vagabondblues's Avatar
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    Default Re: Mystery noise at low rpm…?

    Quote Originally Posted by MountainMind View Post
    Hey Rocket,

    It only happens in flight, not on the ground. Yes, most noticeable when pulling power from full to idle/lower RPMs.

    I noticed it a little while ago. Last time I flew it seemed to be louder (problem potentially getting worse?).

    Thanks,
    Brendan
    At Idle the throttle valve is closed and all the fuel is metered through three holes above the throttle valve. If your idle mixture is set too rich a condition called after fire can occur. That is when you close the throttle quickly excessive fuel is pulled through the intake valve that is normally overlapped with the exhaust valve on the exhaust stroke. The rich mixture is pulled into the exhaust, igniting in the exhaust making the noise you are hearing. Warm you engine and check idle mixture. If correct you should see 25 rpm rise when pulling the mixture out slowly and completely. On the back of the carb is a round wheel with a L and arrow and R and arrow. Adjust rich or lean until you get 25 rpm rise.

  5. #5

    Join Date
    Dec 2019
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    Default Re: Mystery noise at low rpm…?

    Vagabond blues you were spot on- mixture issue.

    Hardly any increase in rpm when pulling out mixture at 1000rpm.
    Then tested it at 1500rpm and got a nearly 100 rpm increase when pulling the mixture out.

    Leaned it out a lot on approach and that made the sound go away at all power settings.

    So, just need to adjust the screw then? Any advice on how sensitive that screw is? Or just adjust until I only get a 25rpm increase instead of 100rpm increase at ~1500rpm

    Thanks again, great to be learning from y’all!

  6. #6
    59pacer's Avatar
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    Feb 2010
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    Mulgoa, Australia
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    Default Re: Mystery noise at low rpm…?

    My recollection is that you warm the engine, close the throttle to get idle RPM (about 600 ?), then lean the mixture while watching the tacho. If the idle mixture is correct, you should see about a 25RPM rise before the engine begins to wind down. If the rise is greater than ~25, then lean with the carby screw. If there is no rise, then richen the mixture with the screw until you get the required rise.
    During the process, it may be necessary to adjust the idle speed back to ~600.
    I seem to recall a suggestion that between attempts to check the RPM rise, you should accelerate the engine.

  7. #7
    BeefSteak's Avatar
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    Apr 2017
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    Anchorage, AK
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    Default Re: Mystery noise at low rpm…?

    Thanks for brining this up, and the reply's. I believe I have the same issue.

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