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  1. #1
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    mercury optimax 150 with fluctuating rpms (with video) ot700937

    all of the simple things have been accounted for...
    good compresdion
    good fuel
    good filters
    new fuel bulb
    new plugs gapped
    yAdda yadda yadda
    now its time to look in depth...
    its either
    1. Diaphrams are worn (air, fuel, tracker)
    2. Weak compressor (i dont have the fancy guages to check air/fuel pressures)
    3. Air intake is dirty (i looked down the air intake, it IS dirty)
    I JUST WANT TO SELL THE BOAT!!! But i need a running engine that dosent sputter. What do you think this rpm fluctuation is? One of the three above? Or something else? Keep in mind i want to sell this thing. Buy a yamaha four stroke, never look back. If it is too involved, i will unbolt it and let her sink! Anyway. Watch the video please

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    #2
    got spark on all 6 cyls ????/ if air mist at air valve is fuel possible diaphragm bad
    .................................................. ...the scariest thing in life is the unknown ...................................

  3. Member
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    #3
    Yes. Spark on all six. Checked with spark indicator. Could i still have a “weak” coil? Even though it shows spark? Btw compression was good on all cyl as well

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    #4
    No fuel in the air mist. Just a bit of water. Engine needs to be running to emit the mist....if its off there is no mist...almost like the engine wont hold the air

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    #5
    gona be hard to test without gage set, might seek pro help local merc dealer
    .................................................. ...the scariest thing in life is the unknown ...................................

  6. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #6
    Several observations:

    1. You should not EVER be accelerating or adding throttle in NEUTRAL. ONLY in gear. NEVER apply throttle unless you are in gear and going somewhere (there is no good/dependable PCM logic for revving the engine in neutral!) So... this one is "User Induced".

    2. Engine Faults result in Engine GUARDIAN in most cases. So you're going to need to address any sensor problems (you referred to a "water pressure sensor"... though I suspect the actual fault is NEUTRAL OVERSPEED).

    For #1 (above), it sounds like there is no complaint when running in gear. If this is the case, stop revving in neutral.

    For #2, get PRINTOUTS of the:

    -Run History
    -Freeze Frame (Fault) History
    -Fault Seconds Counters.

    Be happy to review printouts for you if you would like.


    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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    #7
    Its definately “missing” when going on plane...it hates 1000-2000 rpm.

  8. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #8
    Quote Originally Posted by phyrefighterjay View Post
    Its definately “missing” when going on plane...it hates 1000-2000 rpm.
    1000-2000 RPM's isn't going to be going on plane- there are very, very few boats that will plane off at those RPM's.

    Perhaps I'm not understanding what you're experiencing, or attempting to do.


    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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    #9
    When i throttle the engine to get it up on plane OR when im going down my creek at 1200 rpms, it occasionally Misses. It idles fine, it runs fine at cruise. I can feel the miss when its in the 1000-2000 range. It must have a direct corralation to the video i posted

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    #10
    On plane/cruise for me is 3500 rpms. It does no miss at that rpm level

  11. Mercury 3L/4 Stroke/Verado Moderator EuropeanAM's Avatar
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    #11
    Very little effort or "preciseness" was put into the fuel delivery calibration under those conditions, as the engine will almost never run in that range for any extended period.

    Check the ROLLER on the throttle valve- make sure it's not flat-spotted or worn (this can exasperate the situation considerably).


    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