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  1. #1
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    Mercury 25hp 2Stroke - occasional sputter during idle and loss of top speed

    What’s the most likely source of a sporadic idle and loss of WOT?
    This is a 1994-ish motor, SN starts with OG002.

    Both cylinders measured approx 115psi (which seems low to me).

    Digital tach shows max of about 5200 RPM and 22mph.

    I ran a mix of Seafoam in 1-1/2 gal to try cleaning out carbon. No real change in performance.

    Performance is similar with each spark plug unplugged.

  2. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #2
    What's the maintenance history- and when were the fuel filter and spark plugs last replaced (using only the OEM recommended parts)?


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    #3
    It’s garage kept and I use it nearly year round, so no winterization.
    Carb was rebuilt 1-1/2 years ago. Two new coils about the same time. New plugs this spring.

    I installed new QuickSilver reed block thinking that was a possible issue since it was likely original.

    if I go from high idle to full throttle fairly quickly, it seems to bog down slightly/miss a bit, then smooth out and takes off.

    I am starting to wonder if the float is sticking and perhaps running rich? There have been a couple of times I thought I heard fuel flowing thru carb when I squeezed the ball.

    At WOT it doesn’t seem to affect RPM if I squeeze the ball.

    I have not yet put a timing light on it to see if timing may be jumping around (due to trigger or switch box issue).

    it could be my imagination but I think it uses fuel faster than before.

  4. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #4
    Start as if this is a fresh/new problem. Confirm compression, then spark, then fuel quality/delivery.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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    #5
    Compression was first test I performed and listed above.

    I Don’t have a good way to measure the spark voltage to know what its quality is like. However, Engine performance was similar each time I disconnected one of the spark plugs

  6. Member Extremeboating's Avatar
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    #6
    How are all of your fuel hoses? If they are dry rotted, even a little bit it is entirely possible for the issue to be caused by the hoses sucking in some air upon fuel demand e.g going full throttle/ causing mild fuel leaks.

    Ensure your system is fully sealed and not leaking even if it means replacing every hose from the inlet port where the fuel hose would plug into all the way to the fuel delivery hose via the fuel pump.

    I had an old '88 Evinrude 2 stroke 15HP that did this and after replacing all of the hoses the stuttering stopped.

    Also, you need to make sure your idle air mix is properly jetted. If the engine is running too lean or too rich at the high or low jet then it will cause running issues.
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  7. Member
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    #7
    Does anyone know how the two bleed lines from the power head route to the carb/reed block plate? I haven’t found which line is supposed to connect to which port.

    Also, I have read in other places that some 2strokes have check valves in these lines. My engine appears to have simple barbed fittings that don’t include check valves, and the drawings/parts lists I’ve seen don’t include them either.

  8. Member
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    #8
    no serv manuel ???????
    .................................................. ...the scariest thing in life is the unknown ...................................

  9. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #9
    DEFINITELY need the Service Manual, and the complete Serial Number (for identification of parts, lines, fittings, etc).


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
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    #10
    S/N 0G002366

  11. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #11
    Bleed line routing is found on Page 4-17 of the Factory Service Manual.

    The UPPER bleed line connects to the front-PORT fitting, the lower bleed line connects to the front-Starboard fitting.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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    #12
    +1 ^^^^^
    .................................................. ...the scariest thing in life is the unknown ...................................

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    #13
    Anyone know why my series of the motor doesn’t have the bleed check valves and the next generation does?

  14. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #14
    Product changes/improvements.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
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    #15
    Should I consider adding them? Any idea if the performance change?

  16. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #16
    I would not try to re-engineer the bleed system on this engine. It's quite simple, and it relies on the alternating pressures of the two cylinders to function. Worked just fine.


    Dual Mercury Master Technician- for Mercury Outboards, Mercruiser and Mercury Racing at European Marine in Greenville, SC.
    Still consider myself a "Marine Apprentice" after 47 years (learn something new every day).
    Mercury Parts, Mercury Outboards, Smartcraft & Accessories, Injector Service, TDR Reeds- BBC Sponsor