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1987 Johnson 120hp

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  • 1987 Johnson 120hp

    I have a 1972 Mako 19 with a 1987 Johnson 120hp. Ive been having issues with this motor and Im hoping someone can help . First I was just having idle issues. Cleaned carbs serveral times including taking them apart and replacing gaskets. I would always start the engine while giving it gas then throwing into neutral and right into drive. Compression is good. 125, 120,120,118. Cleaned all the fuel lines and replaced spark plugs. Ran good for a few rides but still had idle issues. Now I still have the idle issues but it wont run at full speed even though I have it at full throttle. Fresh gas with starTron additive. The last time it ran full speed it was in bursts. So while I had it in full throttle it would be riding at 26mph then all of a sudden it would out of nowhere pick up to 35mph. Now its stuck at 23-25mph. Please help.
    1972 Mako 19

  • #2
    Have you had a look at the fuel pump? If its still got an old diaphragm in it ethanol will make short work of it. You can try pumping the bulb while its running and see of the idle improves.
    Long Island NY [br]1985 Mako 20C[br]2003 Yamaha OX66 150[br]

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    • #3
      I have checked the diaphragm and it looked ok . No cracks . I have not tried pumping primer while running but i did push key in while in water and nothing changed .
      1972 Mako 19

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      • #4
        Definitely try pumping the bulb while at speed. My buddy had a later version of that same motor, and his would only run right above 3500 rpms if someone (me) was pumping the bulb. He doesnt believe in fixing semi-broken things, though, so I spent a lot of time laying across the rear deck of his Hewes pumping the bulb so we could run home faster after a long day in Flamingo. [!]

        Two main possibilities for that are an air leak in the fuel system or a failing fuel pump.

        The bad fuel pump probably does not explain your idle issue, since when you pump up the bulb the carbs will fill and the fuel pump isnt going to be doing anything to affect idle right away.

        Quick easy check us that idle is not set too low. Also check compression and spark. That motor will run surprisingly well and smoothly down a cylinder, although youd have lost some top rpms right away if a cylinder was down so I doubt thats it.
        Sold the 17\', moved to FL... I need a new handle.[br]1991 Jupiter 31, current project[br]2005 Everglades 243, sold[br]2012 Contender 23 Open sold[br]1989 Intrepid 246 WA--gone to Australia[br]1973 Mako 17--sold with regrets[br]1992 Mako 17--started it all[br]Pompano, FL

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        • #5
          Compression checks out good. Ive tried connecting and external tank and same thing is happening . I will double check for any air leaks and Im going to try pumping the bulb to see if that helps. Thanks
          1972 Mako 19

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