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  1. #1

    Default 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    Hello All,

    I have a 1988 Mercury Black Max 150, Serial Number B251261.
    The engine is running, but it is rough at low RPM (below ~ 3000 RPM),it isrunning good at high RPM and I think it has full power at top end. The roughoperation in the low range is bothering me.

    Lots done already: Carbs cleaned, link&sync, new coils, wires, plugs,stator, trigger, switchboxes, new crank seals, new cylinder head gasket, ...

    When I had the heads off, I could see some scouring in the cylinder walls, butnot sure how serious it is and if that is the reason for running rough at lowRPM.

    Here are the compression and leakdown readings.
    Note higher leakage rate on cylinder 4-6, but compression is good on allcylinders.


    Is it really worth to re-build the engine? Or is myproblem somewhere else?
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  2. #2
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    3,022

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    Scoring is not good.....makes for low crankcase pressure which effects carb signal.....
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    Thank you for the reply.
    That is a good point, the blow-by affects the suction through the carbs.

    Can somebody comment if the compression and leak-down readings I have posted support this?
    I am looking for convincing advise if I should invest the time and money to rebuild the engine...

  4. #4
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    15,480

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    Rebuilt a V-6 that had compression readings well over 100 psi.---4 of 6 were scored.----Time for this motor to come apart to decide on the path forward.----Scoring indicates that rings may be jammed in the grooves and do not seal properly.----And there are those who say a leakdown test can be done on a 2 stroke , not sure why that is !
    Last edited by racerone; 01-24-2018 at 07:23 AM.

  5. #5
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    3,022

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    What people forget is compression test is just that....if cylinder has enough compression to support combustion. It is not a good indication of mechanical condition as cylinders can be worn/damaged and still have good compression.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  6. #6
    Join Date
    Sep 2010
    Location
    ontario
    Posts
    15,480

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    If there is leakage past rings during a compression test , then there will be more leakage to the crankcase under higher pressures when motor is running.----I see many folks say---" compression is over 100 but motor won't idle "---

  7. #7
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,706

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    That's because the crankcase air/ fuel mixture is polluted by exhaust gas. If one can get the motor started and in gear--and sometimes it's not possible--it will make respectable power at high rpm since the pollution is offset by a mass of fresh air.

    Jeff

  8. #8
    Join Date
    Sep 2010
    Location
    Jersey Shore
    Posts
    4,943

    Default Re: 1988 Mercury Black Max Compression and Leakdown Readings. What now?

    You compression is fine. Your leak down is awful. Time for a rebuild.

    Once scuffing and scoring start, it is not going to get better.
    Chris
    Dockside Marine Services
    Jersey Shore.
    Mercruiser/Mercury Certified
    Let the insanity begin.

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