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  1. #1
    Join Date
    Apr 2017
    The Netherlands

    Question Mercruiser 120 lacks power under load

    Hi there,

    I've bought a very nice Fairline Vixen 19 feet boat with a Mercruiser MCM 120 with a pre-alpha drive.
    Engine starts and runs really good at low RPM's but won't rev much higher then 3000 rpm at WOT under load (Also won't plane as it only reaches 11mph max).

    In neutral it does rev up but only when you throttle up slowly because if you throttle up too quickly it stalls.

    I have already checked a few things...

    My first thought was a lean condition so I checked the mechanical fuel pump for diaphragm failure, but aside from some nasty stuff it looks fine so I cleaned it put it back. Next thing was the carburetor which was full of nasty stuff. Thought ooh thats the problem but after a good clean and compressed air the problem is the same...

    Next thing I did was checking and replacing the spark plugs and thats where things got weird because all 4 spark plugs were pitch black
    Click image for larger version. 

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    They don't look oily so I guess it's just carbon which tells me (If im correct) that the engine is running too rich instead of too lean. I've replaced the plugs for some new ones but the problem is still the same...

    Does anyone have any thoughts?


  2. #2
    Join Date
    Jun 2008
    Island Heights, NJ,

    Default Re: Mercruiser 120 lacks power under load

    Gone are the days when a dose of carb cleaner would clean you carb. These days ethanol can and will leave deposits that can onlybe removed mechanically, i.e. in an ultrasonic cleaner or by pushiing a fine wire through all carb jets and passages.

    1) With engine off and flame arrestor off and choke (if equipped) open...
    check to see that even the slightest movement of the throttle will cause a squirt of gas from the accelerator pump into the carb throat.
    2) Make sure the choke isn't stuck shut. ( and that it opens as engine warms.)

    It is always possible you have the wrong prop on the boat. I call it the "Boat Show Special" symdrome.
    1) In order to have a low price, manufacturer has boat at show withsmallest possible engien that will move the boat.
    2) New owner buys boat, not satisfied with speed and
    3) listens to local dock monkey expert who says.. Put a Bigger pitch prop on it!!
    4) Boat actually goes slower.
    5) Sells boat ( sometimes after putting a fin on the cav plate as well.)
    Capt Bob
    1969 23ft ChrisCraft Lancer
    Merc 5.7L 260HP/ Volvo 280

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