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  1. #1
    New Member NCacres's Avatar
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    Franklinton, NC
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    YM 155

    Default YM 155 Decompression issues

    Hi . I just bought a YM 155 about 6 months ago. bought it real cheap, knowing it needed a few things fixed. I really enjoy the little tractor and have used it a lot on a brushhog and blade. I've been on here alot, and you guys have been a godsend. I really appreciate the help and knowledge on this site. Have been getting parts from Hoyle and they are helpful. I now have come up against a problem i need some help on. My decompression does not work. I know it is a needed item but it hasent been a problem yet. It started fine untill the temp got down below 25 deg. Now i need the decompression. The decomp rods do not touch the rockers like they are supposed to. My question is, Is there spacers that go on top of the valve stems to lift the rockers up, allowing the decomp rods to touch? thanks

  2. #2
    Veteran Member
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    Jun 2010
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    Southern California
    Tractor
    International Harvester 284, Yanmar 1401D, Yanmar 240, and others...many others...

    Default Re: YM 155 Decompression issues

    I have a generic I&T manual that is supposed to cover the older US Yanmars from the 135 through the 330. Under the "Valve Clearance Adjustment" section it says to adjust the clearance on the lever by removing the small covers atop the rocker arm cover. Then, with the crankshaft positioning cylinder 1 at TDC, it says to activate the compression release fully, then loosen the locknut on the screw under that small cover. Then adjust the screw until it contacts the valve but does not move the valve. Next, turn the screw one full turn inward and tighten the locknut. Turn the crankshaft 180 degrees and repeat the process on the number 2 cylinder.

    Having said that, it only says that this is for 195, 240, and 330 models. There is no process covered for the other models, but I don't see why they wouldn't be similar/identical. It does specify valve clearances, 0.2mm/ 0.008 inches; this is slightly different than the other models.

    Were the tractor mine, I'd follow the procedure listed above, and perhaps try it at 3/4 of a turn in, then adjust if needed.

  3. #3
    New Member NCacres's Avatar
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    YM 155

    Default

    There is no adjustment on the compression release. It is built into the valve cover.

  4. #4
    Elite Member
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    gilmer tx
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    yanmar 2002d

    Default Re: YM 155 Decompression issues

    Note the head parts drawings attached. http://www.partsandservice.com/index.html#home Note part #36. The list is calling it a retainer. I believe that is a mistake. It should be called a valve cap and is on all other Yanmar drawings I have seen.

    Only pointing this out to make sure your valve caps are in place. If not, your rocker arms would be way down causing your release to not make contact. Make sense?

    Sending you a pm.

    The copy over of the parts list didn't do just right. However, if you work with it you can bring up all the parts drawings of the 155 tractor or engine.

    And welcome to the forum.

    Copied over the parts list a little different, maybe make it easier to access.
    Last edited by winston1; 12-01-2013 at 09:51 AM.

  5. #5
    New Member NCacres's Avatar
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    Franklinton, NC
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    YM 155

    Default

    I had seen the "retainer" in the only drawing I had found. My tractor does not have these.

  6. #6
    Elite Member
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    yanmar 2002d

    Default Re: YM 155 Decompression issues


  7. #7
    Member
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    Location
    Lake Limestone Texas
    Tractor
    Yanmar 1300 d

    Default Re: YM 155 Decompression issues

    I have the same motor and I took the caps or retainers off when I did a valve job. I guess I should say they wouldn't stay on. My decom works without them There is no adjustment that I remember. The only thing that I would check is the thickness of the gasket or wear of the cam that push's down on the rocker arm and they turn inside the cover.

  8. #8
    Super Member California's Avatar
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    5,679
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    Sonoma County
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    Yanmar YM240, YM186D, and another YM186D

    Default Re: YM 155 Decompression issues

    One last point: Yanmar designates #1 cylinder as the cylinder nearest the flywheel.

  9. #9
    Elite Member
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    gilmer tx
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    yanmar 2002d

    Default Re: YM 155 Decompression issues

    Quote Originally Posted by kwin View Post
    I have the same motor and I took the caps or retainers off when I did a valve job. I guess I should say they wouldn't stay on. My decom works without them There is no adjustment that I remember. The only thing that I would check is the thickness of the gasket or wear of the cam that push's down on the rocker arm and they turn inside the cover.
    Curious here. These caps fit over the valve stem, much more than .008". If the valves are adjusted correctly I fail to see how they could come off.

  10. #10
    Member
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    Lake Limestone Texas
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    Yanmar 1300 d

    Default Re: YM 155 Decompression issues

    Winston the new valve was tapered on top, not flat. When I rotated the motor by hand the caps hit the keepers I knew I had the wrong valves, I needed the ones without a radius on the top. Wrong! yes but I went with it. Adjusted valves to .007 instead of .0078. I have around 300 hrs on it and no trouble that I know of.
    -135-155_03_15-gif
    Last edited by kwin; 12-04-2013 at 12:48 AM. Reason: added note

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