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Merc 800 Compression test as follows

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  • Merc 800 Compression test as follows

    Ok got starter going and did a Compression test.
    #1 Cyl 71#s
    #2 70#s
    #3 70#s
    #4 68#s
    #5 70#s
    #6 66#s

    Still can't figure out why throttle linkage is not moving freely? Any ideas?

    Does this compression indicate that I should move forward or bailout right now?

    Whats my next step?

  • #2
    It sounds like your motor is going to go FFWWUP, FFWWUP FFWWUP :nono1:; instead of PUT, PUT, PUT.:boater1:

    My manual says 110Lbs or higher.

    Rebuild or by new I would say.
    Lovely La Rue and the Kingston Kid

    Rhapsody - 2001 Classic Craft Gentleman's Racer (FOR SALE)
    Lil' Red - 1957 Bell Boy Express 18 ft Cruiser (someday!)


    • #3

      I was wondering if you are turning it over dry. Fuel in the cylinders will boost your compression. They are so consistant, either its dry or its used up. Somebody got their moneys worth.


      • #4
        Marvel mystery oil

        Thanks for the input guys.
        I did not turn it over dry, but I didn't let the oil sit for a few days to loosen up rings. I'm gonna try squirting some oil in and letting it sit afew days and do test again. I'll let you know if that improves the compression
        thanks again


        • #5
          Compression numbers, if true, are very low.....but could be ok only if it runs and draws fuel on it` own.
          Spray Fogging oil down the carb opening and try starting.If it fires off then you may see if it idles.
          If not enough compression ,it won`t pull fuel thru the carbs and it would run with choke-only would be my guess.
          Check lower unit for water at lowest drain screw so you don`t end up with a runner that needs a lower to be useable.
          Water will settle to bottom if upright.
          Also would like to know if the flywheel is easy to turn by hand with all spark plugs finger or both hands?
          Also feel /listen for grinding or bearing noises as turning flywheel by hand,I call this "the Blind Man" test......should get equally hard on each cyl as compression builds up.No clunks or "Rod slop".
          I like to run unk.motors with 25-1 if sitting at all,basically treating it as a break-in period like when new.
          unk.year 10` Mahogeny "DragonFly"racer
          15` SAFE boat w/120 hp Johnson
          SeaRay 175BR
          Hi-Laker lapline
          14` Trailorboat


          • #6
            Thanks Tim

            I appreciate your input. I forgot to check lower unit, I'll do that today:Good one: