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1994 75 HP Mercury Outboard Power Loss

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  • 1994 75 HP Mercury Outboard Power Loss

    I have a 1994 75 HP Mercury Outboard on a 17.5' Alumaweld boat. Last year the motor ran great and was getting above 5000 RPMs easy. When I took the boat out last week, the engine would run fine but would only achieve around 3500 RPMs at full throttle and just has no power. It seems to run fine until it hits around 3000 RPMs, then it seems to bogg down even though the motor is screaming. I replaced the 3 spark plugs, replaced the inline fuel filter on the motor, replaced the gas and put carb cleaner in the gas, but has not gotten better. I also made sure that the fuel tank is breathing and everything seems to be fine. I am wondering if one of the cylinders is not functioning properly. I read that you could pull the spark plug cable while the engine is running to see if there is a noticeable decline in RPM's to determine if or which cylinder may not be working but have not tried this yet. I'm no outboard mechanic but have some outboard experience and am trying to do everything I can do myself before I bring it to a mechanic. What and how should I check next?

    Any suggestions/help would be greatly appreciated.
    Thanks,
    J

  • #2
    Re: 1994 75 HP Mercury Outboard Power Loss

    The motor is screaming???? Are you saying the RPMs increase but the boat does not go faster?
    "A manual is a cheap investment"
    Fair Winds and Following Seas
    Bill
    PTC USN ret

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    • #3
      Re: 1994 75 HP Mercury Outboard Power Loss

      Originally posted by bhile View Post
      The motor is screaming???? Are you saying the RPMs increase but the boat does not go faster?
      Sorry for the confusion. The RPMs are not increasing nor is the speed.

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      • #4
        Re: 1994 75 HP Mercury Outboard Power Loss

        Start with a compression test.------Then check to see if there is spark on all 3 cylinders that will jump a gap of 7/16" or more.----------OK this may be as simple as a linkage that is off.---Do all 3 carburetors open up fully ?

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        • #5
          Re: 1994 75 HP Mercury Outboard Power Loss

          You may be running on only two cylinders. This will cause the loss of power and RPM. Start the motor and useing a pair of very heavy rubber gloves pull each plug wire off in turn and see if you get a change in motor RPM. If one doesn't change the motor rhythm and the others do then that is the cylinder that is misbehaving. The easiest thing to check then is the spark. The second is compression. The third is fuel by spraying a fuel mix, 50:1, into the throat of the offending cylinder carb. These tests will give you the direction to go in.

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          • #6
            Thanks guys. I cleaned out the carbs and found a clogged jet in one of them. Cleaned and blew out the jets and motor is running great again.

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            • #7
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              Thanks guys. I cleaned out the carbs and found a clogged jet in one of them. Cleaned and blew out the jets and motor is running great again.

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