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70hp Mercury, knocking sound.

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  • 70hp Mercury, knocking sound.

    I have a early '80s 70hp Mercury outboard, 3 cylinders. Motor runs good at higher rpm (2000 and above), but when idling it has a knock (like valve knock on truck engines) and idles a little rough. Compression is ok (110,120,105). I am currently using motor and find no problem with it, but I'd like to sell it for a bigger one and don't want to mess up anybody. I fugured if it's a rod, it should have blown off by now, I am using motor for past month.Other question: is this motor practically half of a 6 inline 140hp ? (same pistons, rods, rings, etc.) ?Thanks.Val
    1992 Silverton 34 Convertible w. dual station
    1990 Baha Cruiser 260 Fisherman


  • #2
    Re: 70hp Mercury, knocking sound.

    Any suggestions ?
    1992 Silverton 34 Convertible w. dual station
    1990 Baha Cruiser 260 Fisherman

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    • #3
      Re: 70hp Mercury, knocking sound.

      Do a thorough decarb first.....if you are lucky, it may be carbon. May even be a reed.Try a decarb first....
      Alias "schematic"

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      • #4
        Re: 70hp Mercury, knocking sound.

        What is a good decarb product. I tried Mystery Oil and didn't have much success.Val
        1992 Silverton 34 Convertible w. dual station
        1990 Baha Cruiser 260 Fisherman

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        • #5
          Re: 70hp Mercury, knocking sound.

          Mercury and OMC both make a good decarb product. Also Berrymans B12 from Wal Mart or Seafoam from the autoparts works well.

          Comment



          • #6
            Re: 70hp Mercury, knocking sound.

            Have you changed props lately? My 2002 90 Hp went from noisy with an aluminum prop to down right clangy bangy shakey when we put the stainless prop on. I have returned the motor and have a Yamaha on the way.

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            • #7
              Re: 70hp Mercury, knocking sound.

              Bought 6 bottles of B12, filled up motor, let it sit overnight, drained everything and....it did the job. Even freed a stuck ring (lower cyl was 105 instead of 120 like other 2).Thanks guys, again.Val
              1992 Silverton 34 Convertible w. dual station
              1990 Baha Cruiser 260 Fisherman

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