Compression Question

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  • George Solana

    US Member - 1 Year
    Replies: 10
    Topics: 2
    #250732

    I came across a β€˜97 Merc 9.9 two stroke. I know it’s not an antique, but just a general question about compression that could apply to older motors.

    She runs good, starts up fine, but I ran a compression check and top cylinder was 92 and bottom cylinder was 91. Motor was warmed up 5 minutes prior to compression check. The service manual says anything below 100 psi or a differential of 15% is begging for a rebuild. My question is, how true is that? Can this motor be ran safely being 10 psi below the spec?

    Thanks in advance for all your great advise!


    dave-bernard

    US Member - 2 Years
    Replies: 1679
    Topics: 24
    #250733

    It maybe you gauge. the 2 cyl are close. if it ain’t broke don’t fix it.


    Mumbles

    Canada Member - 2 Years
    Replies: 5299
    Topics: 285
    #250740

    I’d be happy with those numbers as they are so close together. If the compression is below spec, the motor wont be able to make full power and may have trouble idling to.


    billw

    US Member - 2 Years
    Replies: 1655
    Topics: 51
    #250741

    I am pretty sure those engines have some kind of special coating on the cylinder walls….chrome or something similar. I forget. Anyway, unless the rings are just stuck, you don’t rebuild them; you just replace the power head. So, if it starts easily and makes good power, I would suspect maybe a gauge problem.

    Long live American manufacturing!


    seakaye12

    US Member - 2 Years
    Replies: 732
    Topics: 99
    #250749

    Compression testers for small engines should ideally have the pressure-release (schrader) valve close to the cylinder. Testers with the valve at the gauge end of a hose will give lower readings on small displacement engines.

    Compression-Tester

    1 user thanked author for this post.

    crosbyman

    Canada Member - 2 Years
    Replies: 2467
    Topics: 256
    #250759

    if it runs ok dont mess with it. jmho…go fishing!

    Joining AOMCI has priviledges πŸ™‚


    crosbyman

    Canada Member - 2 Years
    Replies: 2467
    Topics: 256
    #250760

    Same here…..if it runs ok dont mess with it. jmho…go fishing!

    if you are worried try some decarb stuff like Merc power tune or similar Evinrude engine cleaner and measure comp. again after a while.

    use the short meter pictured above not the long hose one and re-measure comp. you may gain a few PSI and be close enough to perfection πŸ™‚ .

    Joining AOMCI has priviledges πŸ™‚

    • This reply was modified 1 month, 3 weeks ago by crosbyman.

    George Solana

    US Member - 1 Year
    Replies: 10
    Topics: 2
    #250807

    Thanks to all!


    George Solana

    US Member - 1 Year
    Replies: 10
    Topics: 2
    #251025

    Borrowed a buddy’s compression gauge with release valve close to cylinder. My gauge is a cheapie with a foot long hose, release valve on side of gauge dial. This time, top cylinder ran 96, bottom ran 94.
    I sure as heck ain’t messing with it now. Amazing for a motor pushing 25 years!
    Thanks again guys!


    crosbyman

    Canada Member - 2 Years
    Replies: 2467
    Topics: 256
    #251027

    25 years … that is a young fellow

    my 9.5hp 9622a Evinrude is 45 years old and runs like a top with compression in the low 70s

    Joining AOMCI has priviledges πŸ™‚


    George Solana

    US Member - 1 Year
    Replies: 10
    Topics: 2
    #251133

    Wow, awesome lookin job!


    crosbyman

    Canada Member - 2 Years
    Replies: 2467
    Topics: 256
    #251165

    no really… Actually, all I did was pull the cyl head and oil it up then wacked for a week or two with a wood peg then bingo. Motor itself still has the original paper sticker for the break in period πŸ™‚

    the rest was car wax !

    but thanks anyway !

    Joining AOMCI has priviledges πŸ™‚

    • This reply was modified 1 month, 2 weeks ago by crosbyman.

    reivertom

    US Member - 2 Years
    Replies: 725
    Topics: 38
    #251235

    The worst that can happen is it might not idle really well. If you don’t use it for trolling slow, no worries.

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