mercury 1979 70 hp 3 cylinder outboard issue

Discussion in 'Outboards' started by rd9952, Sep 22, 2013.

  1. rd9952
    Joined: Sep 2013
    Posts: 13
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    Location: Cincinnati, Ohio

    rd9952 Junior Member

    My 70 hp merc ( 1979 ) 3 cyl. is kinda hard to start and idles well once started. ( on muffs ) On the water you drop it in gear and it just bogs down and cuts off. We found a bad coil and possibly a bad wire on the # 2 cylinder. We can unhook the # 2 wire and ground it and it idles smooth. With it hooked up it idles smooth but is alot harder to start. Could a bad coil and or wire cause it to bog down and die out under load in the water but idle smooth on muffs at home???
     
  2. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Yes, obviously it can. The #2 cylinder does not contribute anything if the coil has a short, so engine power is less than 2/3 of what it should be.
     
  3. rd9952
    Joined: Sep 2013
    Posts: 13
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    Location: Cincinnati, Ohio

    rd9952 Junior Member

    When we have a good connection between the plug wire and the coil it OHMS the same as the others. I believe I have a bad wire. Changing it tomorrow. Will let you know. Thanks again.
     
  4. rd9952
    Joined: Sep 2013
    Posts: 13
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Cincinnati, Ohio

    rd9952 Junior Member

    Changed the plug wire. It idles great on muffs and with the motor trimmed up it will now ( for the first time ) acually move water behind the boat. BUT... it does not have much power with the motor down in the water. It just boggs down and stalls. I can cover the upper carb with my hand and stall out the motor. But when I do that to the lower carb nothing happens? Could the lower carb not be producing enough fuel causing no power under load?? If so would it probably be the float?? They were just replaced by me.
     
  5. Dave T
    Joined: Sep 2011
    Posts: 250
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    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    Sounds like your not getting gas to the bottom carb. I'd pull the fuel line off that carb and crank the motor over and see if if gas comes out. Could be a bad line sometimes the inside of the rubber lines deteriorate. Has the motor been run on alcohol gas? If that checks out then I would suspect the needle valve possibly a piece of rubber from the inside of a hose has plugged it. You could try spraying gas into that carb with a squirt bottle while on the water to see if it takes off then.
     
  6. liki
    Joined: Nov 2008
    Posts: 220
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    Location: Finland

    liki Senior Member

    Preferably start by checking compression and spark on each cylinder. Then move on onto fuel delivery issues.
     

  7. powerabout
    Joined: Nov 2007
    Posts: 2,903
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    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    A 2 stroke will idle great on one cylinder so you need to check all are contributing
     
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