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1987 Johnson 140 hp outboard, misfiring

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  • 1987 Johnson 140 hp outboard, misfiring

    I have a 1988 Mach 1 Spoiler. It's about an 18ft boat with a 1987 Johnson 140 hp VRO outboard. Last weekend I had it out on the water for two consecutive days and as the weekend went on, The engine sounded like it was misfiring, more commonly at higher speeds. eventually it became more frequent at all speeds. The misfiring resembled almost like I was hitting something with the prop like a log or something (obviously I wasn't as I was in 60ft water), where the whole outboard would shake. Any ideas? Thanks!

  • #2
    Re: 1987 Johnson 140 hp outboard, misfiring

    sparkplugs might solve it.
    If not put a timing light on each wire to see if its firing on all holes, then spark gap test it, compression test if you have a guage.
    See FAQ Files.

    Get the genuine factory service manual.


    • #3
      Re: 1987 Johnson 140 hp outboard, misfiring

      Sparkplugs looked good.
      Thanks for the feedback, I'm going to investigate a little more and let you know what happens.


      • #4
        Re: 1987 Johnson 140 hp outboard, misfiring

        If you checked all the plugs and they all look the same, Not oil soaked or totally black, no water droplets showing. All 4 plugs.

        Then check your fuel water separator for signs of water.

        If one plug is different then the rest, or two bottom or two top could be dirty carb.

        Guess what happens to 20 year old two stoke power heads with 20 years of engine use...They sometimes wear out in the rings and pistons.

        A compression test should be performed to rule this out.

        The best way to find the problem is to test every thing and narrow down the possibilities.

        Hope you are able to find the problem in a jiffy and get back on the water soon.
        Farmer's Tan....year round


        • #5
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          Re: 1987 Johnson 140 hp outboard, misfiring

          hey, in case you guys are interested. It was the lower gear case that was the problem. I'm not too boat savy, but have a friend who's a boat mechanic and he figured it out right away. One of the gears on the lower unit had teeth on it that were rounded and worn. Easy fix right? just replace it? Nope, don't make that part for this engine anymore, ended up replacing it with a whole new gear case. oh well. she runs great now. Thanks for all the help!