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Thread: Strange engine noise

  1. #1

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    Default Strange engine noise

    I have been noticing that after landing, during the roll out and taxi there is a noise coming from the area of my engine compartment. It sounds just like the squelch is being broken on a radio, in fact I was a first certain thatís what it was. It is the exact same frequency and tone as that. However, it is not the radio. It seems to be emanating from the right side of the engine compartment and only after the plane has been flown. Iíve never heard it before flight. It happens when the plane is both moving and static and at idle of near idle power. Itís pretty loud when heard from outside the airplane. Any thoughts?

    Thanks!

    Kevin

  2. #2
    Brian's Avatar
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    Default Re: Strange engine noise

    Do you have a dry or wet vacuum pump? Carbon vanes in a dry pump could possibly sound like that.
    Brian
    Monrovia, CA

  3. #3

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    Default Re: Strange engine noise

    It is a dry vacuum pump

  4. #4

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    Default Re: Strange engine noise

    Any chance it is a vacuum leak? Maybe one of the intake gaskets, or rubber intake tube connectors. If you have the nerve, (& can do it safely) with the engine idling, (& making the noise) spray WD40 around each of the 'intake tube to head' gaskets, and around the rubber connectors. If one of them is leaking, and you do 'hit it' with the WD40, you will know it; the noise will either stop immediately, or at least change significantly.

    It is very hard to diagnose without actually hearing the noise. Does it go away at (near) full throttle during ground runup?

  5. #5
    Brian's Avatar
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    Default Re: Strange engine noise

    Link to a similuar issue with noise after landing:

    https://www.euroga.org/forums/mainte...-front-magneto
    Brian
    Monrovia, CA

  6. #6

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    Default Re: Strange engine noise

    Interesting link, I wonder why he didn't disassemble the magneto, surely whatever caused the screeching would have been obvious.

  7. #7

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    Default Re: Strange engine noise

    Thank you for the info. We will be looking at everything.

  8. #8
    Vagabondblues's Avatar
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    Default Re: Strange engine noise

    The old slick 4200 series magneto had primary magnet laminations that stuck up from the rotor about a sixteenth of an inch. The laminations on the frame also protruded about a sixteenth of an inch. As the bearings wore, the possibility of the two laminations contacting increased as the new manufacturing tolerances changed. To prevent the "sudden stoppage" of the accessory gears should the laminations contact a brass sheer pin was installed between the rotor laminations and the rotor shaft. If you have ever internally timed a Slick 4200 series magneto you will understand how close the two laminations was. The 4300 rotor had flush laminations to fix the problem.

    Also, if you have ever overhauled a Bendix 20/200 series you know that you have to set the bearing end load. There should be some end play on the rotor shaft when the two case halves are bolted togther. Too much preload on the unsheilded bearings could cause a howl as well as a spinning rotor head on the Slick4200
    Last edited by Vagabondblues; 12-18-2018 at 05:59 PM.

  9. #9

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    Default Re: Strange engine noise

    Quote Originally Posted by Vagabondblues View Post
    Also, if you have ever overhauled a Bendix 20/200 series you know that you have to set the bearing end load. There should be some end play on the rotor shaft when the two case halves are bolted togther. Too much preload on the unsheilded bearings could cause a howl as well as a spinning rotor head on the Slick4200

    My Bendix S20/S-200 Overhaul manual states "When proper bearing preload is achieved, magnet shall spin freely and there shall be no perceptible axial or radial play of the rotor shaft." To keep from incriminating myself, I would prefer not to mention the publish date of my manual...

    I have always 'interpreted' that statement to mean that when the case screws reach torque specs, the end play should be removed (& not before). Full disclosure; nowhere in the manual does it mention "end play", nor does it give any 'concrete' clearance specifications; but the term "perceptible" is batted around a lot.

    As I am about as 'perceptive' as a blind elephant, I have always used a dial indicator to check bearing "preload" (Bendix's word, not mine); and have never had any difficulty obtaining more than zero, and less than .0015 "end play". With less than .002 end play, I cannot "perceive" any movement of the dial indicator pointer when it is setup to measure radial play, so I am calling that 'by the book'. Of course I do constantly confirm the magnet is turning freely as I tighten the screws.

    If my procedure is contrary to accepted practices, please let me know.

    Thanks

  10. #10
    Vagabondblues's Avatar
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    Default Re: Strange engine noise

    I should have used the term "preload". Axial and radial. You adjust it be putting a shim under the lower inner bearing race and then measure it as "end play" with a dial indicator. I don't remember what is acceptable play.

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