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Thread: Yamaha Low Compression is One Cylinder Question

  1. #1
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    Default Yamaha Low Compression is One Cylinder Question

    Looking at a boat that has an older 70 hp Yamaha 2 stroke with 115-115-95 compression across the cylinders. A couple of questions:

    1. Is it a big deal?
    2. If left as is, what would I notice in performance?
    3. If left as-is and I continue to use it, will it damage any other part of the motor?
    4. In round numbers what would it take in $ to rebuild it?

    Thanks, MG
    Dum Spiro Spero

  2. #2
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    Jan 2011
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    Maybe just be the head gasket with it not being far off. But you will see power issues and it will eventually get worse. Getting one rebuilt depends on the mechanic but you can rebuild one normally for under 2000 depending on if there is alot of other damage
    Hancock 70 Yamaha

  3. #3
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    The rings may be seized. A few treatments of ring free may be worth a try.
    Honey...I'll do it after the season is over.


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  4. #4
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    Nov 2013
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    It will not like cold starting or idling as well but you prob won’t notice as much difference on plane. Wsm rebuild kit amd decent mechanic and you’ll be good to go.

    ***Make sure you figure out the root cause of the low compression before it does the same thing on a new rebuild.

  5. #5
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    10% difference is the rule.
    Take the plugs out and put some seafoam directly in the cylinder, tilt motor up so it gets around the rings a little better, let it soak
    Then with plugs out, spin it over to blow the liquid out after a long soak to avoid hydro-locking.
    Then hook up a portable tank with a full can of Seafoam to 2 gallons of gas, run it for a few minutes til warm and shut it off.
    Let it sit 30 minutes or more and then run it again for 5 minutes or so and shut it off and let it sit, keep repeating.
    It will smoke more than Sammy Potkins but that is the carbon and crap burning off
    This works great and I have seen nice increases doing this method numerous times,
    I would take my bore camera and look in the cylinder for wall scars and problems on the piston.......I actually did one with my camera today.
    IMG_3149.JPG
    IMG_3150.JPG
    Last edited by ecu1984; 09-07-2020 at 10:41 PM.

  6. #6
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    PS. Most folks don't do a correct compression test from my experience.
    Run the motor til warm
    Pull all the spark plugs out
    Open throttle wide open to get air pulling into the block
    Pull the kill switch so there is no spark.....this will avoid damaging the power pack
    Must have a fully charged battery that will spin it over fast.
    Last edited by ecu1984; 09-07-2020 at 10:46 PM.

  7. #7
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    Quote Originally Posted by ecu1984 View Post
    PS. Most folks don't do a correct compression test from my experience.
    Run the motor til warm
    Pull all the spark plugs out
    Open throttle wide open to get air pulling into the block
    Pull the kill switch so there is no spark.....this will avoid damaging the power pack
    Must have a fully charged battery that will spin it over fast.

    Agreed Ive never seen anyone do a compression test correctly (warm engine) they always yank the plugs and pop the gauge in
    I am a nobody, that met somebody, that can save anybody.

  8. #8
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    Quote Originally Posted by ecu1984 View Post
    10% difference is the rule.
    Take the plugs out and put some seafoam directly in the cylinder, tilt motor up so it gets around the rings a little better, let it soak
    Then with plugs out, spin it over to blow the liquid out after a long soak to avoid hydro-locking.
    Then hook up a portable tank with a full can of Seafoam to 2 gallons of gas, run it for a few minutes til warm and shut it off.
    Let it sit 30 minutes or more and then run it again for 5 minutes or so and shut it off and let it sit, keep repeating.
    It will smoke more than Sammy Potkins but that is the carbon and crap burning off
    This works great and I have seen nice increases doing this method numerous times,
    I would take my bore camera and look in the cylinder for wall scars and problems on the piston.......I actually did one with my camera today.
    This sounds like a plan. Being the bottom cylinder, that is a good bet, and one at is inexpensive and simple to do. Both are great qualities I like in a possible solution. Thanks, MG
    Dum Spiro Spero

  9. #9
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    Dec 2010
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    Quote Originally Posted by Maggie Glover View Post
    This sounds like a plan. Being the bottom cylinder, that is a good bet, and one at is inexpensive and simple to do. Both are great qualities I like in a possible solution. Thanks, MG
    I wouldn't be scared of those numbers, the bottom cylinder is usually a tad lower........gets more heat as nearer the exhaust

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