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  1. #1

    Default 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    Rebuilt a number of Mercury's, but this one has me baffled.

    Got a 1994 90hp 3cyl 2S that the previous owner rebuilt the powerhead.

    I replaced stator, trigger, voltage reg, switchbox, coils and plugs and plug wires with CDI

    rebuilt all 3 carbs per OEM manual, main jets .062/.064/.062

    compression is 129/130/126 cold with throttle plates WOT

    rebuilt fuel pump, new hoses, swapped fuel supply twice, no change

    adjusted idle mix screws, no change

    ck'd TDC of #1 piston with dial indicator and it matches the indicator

    DVA +coils at idle is 180

    oil injection is disconnected

    pulled reed plates off, reeds look ok and hole to fuel pump is open

    set timing to 20deg BTDC at 3000rpm in fwd gear

    tried 19p and 21p props, no change

    If I set the throttle cable for the plates to be WOT, it will bog about 5200rpm, if I set the throttle cable for the plates 3/4 open, it all runs great and maxes out at 5200rpm

    Starving for fuel or ignition??

    Any suggestions would be much appreciated.

    Much Thanks.
    Last edited by 20hpmercury; 12-13-2017 at 02:37 PM.

  2. #2
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    2,898

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    Fuel.....
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    Quote Originally Posted by faztbullet View Post
    Fuel.....

    Much thanks and had not thought of that. Running 91 no ethanol with 50/1 oil. Might try some 93 with ethanol, the best around here and see what happens. Hey tried everything else, lol's! also I think you can add octane booster, but do not know if that stuff works or not, nothing ventured nothing gained.

  4. #4
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,554

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    First thing I would do is to hook up a temporary fuel pressure gage at the carbs and see what pressure is being maintained.

    I've had brand new fuel pump check valves screw up on me. Put the old ones back in and the motor ran fine! My point is that NEW parts are not always GOOD parts, so relying on them can trick you.

    Jeff

  5. #5
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    2,898

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    This motor has the rubber/plastic disks. Rebuild the fuel pump 1st.
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  6. #6

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    Quote Originally Posted by fastjeff View Post
    First thing I would do is to hook up a temporary fuel pressure gage at the carbs and see what pressure is being maintained.

    I've had brand new fuel pump check valves screw up on me. Put the old ones back in and the motor ran fine! My point is that NEW parts are not always GOOD parts, so relying on them can trick you.

    Jeff

    Thanks for all of the suggestions, will try different fuel and install a fuel pressure gauge. I have rebuilt and installed 2 different fuel pumps with Quicksilver rebuild kits, you would think that would not be the problem, but maybe so. Thanks again and Merry Christmas and Happy New Year!

  7. #7
    Join Date
    Sep 2003
    Location
    New Tripoli, PA, USA!
    Posts
    15,554

    Default Re: 1994 90hp 3cyl 2S runs great up to 5200rpm but bogs after that with throttle plate WOT

    OB fuel pumps are simple devices that work well--except when they don't; and they don't for mystifying reasons at times. I've had brand new check valves screw up on me, yet the motor worked fine AFTER I put the old ones back in. Go figure... And how did I determine this? By using a fuel pressure gage at the carbs.

    Jeff

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