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Mercury 25hp 2000 4-stroke, problem at 70% trottle and above

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  • Mercury 25hp 2000 4-stroke, problem at 70% trottle and above

    Hi, My 25 hp has a problem that occurs at about 70% of max rpm. The Engine sounds like it doesnt get enough fuel/misfires a bit. It starts great, it idels great and it also works fine to about this 70% of max rpm. What I have done so far is:

    -cleaned the carb twice (ultra sound cleaner),
    -changed the plugs
    -changed the spark plug wires and coil
    -replaced the fuel lines from the connector to the fuel pump and from fuel pump to carbs.
    -I used a different fuel tank to exclude broken fuel lines from the tank to motor.

    Anyone had anykind of similar problem? Could it be fuel or electrical problem? The only thing i havent changed on the fuel side is the pump. On the Electric side it could be some bad contact in some wiring or the Engine Control module, or something else. Thankful for any kind of help!
    Last edited by andr; May 16th, 2017, 05:39 AM.

  • #2


    • #3
      I finally have discovered what the issue is. There is a pressure build up in the crank case and the engine is running bad because of the amount of oil being fed into the air intake from the crank case breather tube. An leak down test revealed that the piston rings are either broken or seized on one of the 2 cylinders. This has happen most likely since the engine was not used/started for 3 years. Now i will try the easy method first, i will drain the oil and pour diesel on top of the pistons and let it soak for a cuple of weeks, then do a new leak down test. If this does not work i will have to take the power head of and open it up and repair whats broken.


      • #4
        ​ Did the diesel work?
        ​ I have a 2001 25hp Bigfoot with the same issue. Although, a local yard worked on it, and had it running fine for about a month. Now it will start and idle for a minute or two and then just stop.
        What is a leak down test?


        • #5
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          I dont know yet, im still leting it soak. It might take weeks or months to brake the rings loose from rust, if they are not broken. A leak down test: https://www.youtube.com/watch?v=ofSiTGeLXvc