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Yanmar 2GM20F knocking sound

Discussion in 'Mid-Size Boats' started by Pizzazz, Feb 26, 2018. Add this thread to a FAQ

  1. Pizzazz


    Joined Sep 11, 2015
    28 posts, 8 likes
    Hunter 31
    US Marina del Rey
    Could someone help me diagnose this knocking sound coming from my 2GM20F engine> Halfway through the recording I am playing with the decompression levers to see if the sound changes.

    1) When I lift the the decompression lever for cylinder 1, I get a pulsating feedback on my finger.
    2) When I turn the crankshaft by hand, slowly, at a certain position, I can hear a spring sound, such as tsk-tsk-tsk, once per revolution.

    Thank you,
    SV Pizzazz

    Attached Files:

    Last edited: Feb 26, 2018
  2. nveater


    Joined Feb 27, 2004
    76 posts, 6 likes
    Hunter 335
    US North East, MD
    I must not be hearing it but it really sounds like a normal 2 cylinder diesel to me- sorry:(

  3. Cowpokee


    Joined Jan 8, 2015
    179 posts, 50 likes
    MacGregor 26S, Goman Express 30
    US Kerr Reservoir
    Yep, sounds just like my engine.
    That tsk-tsk-tsk sound could mean your valve lash could use some adjusting.

  4. kloudie1


    Joined Nov 6, 2006
    7,558 posts, 398 likes
    Hunter 34
    US Mandeville Louisiana
    I don't hear a problem either except as Cowpoke says, check the valve clearances..
    Sometimes a phone recording will not catch some low freq transients.. could be you are hearing a tired motor mount that is not coming through on the recording?

  5. Claude L.-Auger

    Claude L.-Auger

    Joined Dec 14, 2003
    971 posts, 16 likes
    Hunter 34
    CA Lake of Two Mountains, QC, Can
    Was engine warm or was the recording done just after start-up. My engine (3GM30F) sounds like yours until it has reached normal operating temp.

  6. Whatfiero1


    Joined Mar 29, 2017
    159 posts, 27 likes
    Hunter 30t
    US littlecreek littlecreek
    Owners Manuel requires adjusting valve lash every 100 hr. Because there are no hydraulic lifters solid pushrods require maintained clearance.

  7. Bill Roosa

    Bill Roosa

    Joined Jun 6, 2006
    6,933 posts, 133 likes
    Hunter 40.5
    US Harrington Harbor North, MD
    Kinda hard to tell which sound is which without some narrative on when things are happening. with that said, if the engine was warm I'd check the valve lash, if the engine was cold then warm it up and see if the noise gets less loud. Just as a diagnostic tool the valve noises are at half the frequency of the piston noises so if you have a view of the crank you can tell which system is causing the noise. 600 RPM is 10 revs per second on the crank and 5 / sec on the valve train. You can actually count that with some practice BTW.

  8. pateco


    Joined Aug 12, 2014
    1,657 posts, 357 likes
    Hunter 31 (1983)
    US Pompano Beach FL
    It sounds just like my 1983 2GM.

    How hard is this to do?

  9. Whatfiero1


    Joined Mar 29, 2017
    159 posts, 27 likes
    Hunter 30t
    US littlecreek littlecreek
    Adjusting the valves is easy especially if you can get good access to top engine. There's. you tube videos on it and lots info on line

  10. Amos Cape

    Amos Cape

    Joined Feb 1, 2010
    206 posts, 15 likes
    Hunter 33.5
    US El Dorado Lake, Kansas
    I’m no mechanic I did it to my 2QM15. It made a big difference on how it ran. I also changed the injectors at that time.

  11. Ralph Johnstone

    Ralph Johnstone

    Joined Jan 4, 2006
    2,420 posts, 255 likes
    Hunter H-310
    CA West Vancouver, B.C.

    This is copied from the Yanmar shop manual.

    Yanmar Shop Manual.JPG

    Please state and show your sources.

    pateco likes this.
  12. Ralph Johnstone

    Ralph Johnstone

    Joined Jan 4, 2006
    2,420 posts, 255 likes
    Hunter H-310
    CA West Vancouver, B.C.
    This is from the manual but a little difficult to read. It's a slow picky job which just takes patience. The fact that the adjusting screw is national coarse makes it a little more challenging.

    Attached Files:

    pateco likes this.
  13. Whatfiero1


    Joined Mar 29, 2017
    159 posts, 27 likes
    Hunter 30t
    US littlecreek littlecreek
    I stand corrected. But still a necessary service

    pateco likes this.
  14. Allan12210


    Joined Jan 22, 2008
    1,375 posts, 122 likes
    Hunter 34
    US Alameda CA
    Best to purchase a new valve cover gasket and the O-rings for the hold down knobs as well.
    It's an easy task and if it hasn't been done in a long time you will hear a noticeable change in the rattle these engines make.

    When you peek through the hole in the back on the clutch cover and locate the timing mark, use a magic marker on the crank shaft pulley to make a reference mark. Then when you rotate to the next cylinder position make another mark. It makes locating the top dead centers easier the next time. I think I got that idea from Ralph. It was a picture he posted.

    If the rockers (both exhaust and intake) for the appropriate cylinder are still tight you have to rotate the engine one more revolution to get that piston to top dead center. Then proceed with the adjustment.

    I used a 1-1/16" socket on a breaker bar to rotate the engine. I can't remember if once the valve cover is off if that releases the compression. Maybe. It makes it easier to turn.

    Attached Files:

    pateco likes this.
  15. Ken Cross

    Ken Cross

    Joined Oct 24, 2010
    1,501 posts, 162 likes
    Hunter 30
    US Everett, WA
    I pull the decompression lever to make turning it over easy.


  16. Sailm8


    Joined Feb 21, 2008
    1,428 posts, 128 likes
    Hunter 29.5
    US Punta Gorda

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