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  1. #1

    Default 2002 Mercury 90hp will not go WOT?

    Hey All,
    Hoping to get some help with my 90hp 2 stroke. Just bought the boat and did a full service. Changed mixing tube cause it was soft, changed plugs, wires and boots, new fuel filter and separator, replaced a few fuel lines that were soft, new priming bulb, impeller and a few other small things. Boat ran great for 2 weekends. Ran 3 tanks of fuel out and then Memorial Day it all of a sudden wouldn’t run over 4200rpm WOT(wide open throttle) but runs very smooth. It was running at 5000/5200 the past 2 weekends no problem. It also shutters real bad at a neutral idle and will cut off after about 5/10 seconds of idling. Any suggestions?
    Also in a side note, when the RPM’s are low the radio cuts in and out. Not sure if that’s due to the poor idle or something completely different.

  2. #2
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    4,515

    Default Re: 2002 Mercury 90hp will not go WOT?

    Compression test first then check ignition system
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  3. #3

    Default Re: 2002 Mercury 90hp will not go WOT?

    Quote Originally Posted by faztbullet View Post
    Compression test first then check ignition system

    Thanks Faztbullet.
    ive got 120 +/- 2 psi on all 3 cylinders. What specifically should I check on the ignition system. Coils are good, plugs and wires are new.

  4. #4
    Join Date
    Nov 2011
    Location
    tennessee
    Posts
    4,515

    Default Re: 2002 Mercury 90hp will not go WOT?

    Stator DVA output and CDM spark..
    Pappys Sales and Service
    Owner and Factory Certified Technician
    Former racer KDBA (Kentucky Drag Boat Association)

  5. #5

    Default Re: 2002 Mercury 90hp will not go WOT?

    Quote Originally Posted by faztbullet View Post
    Stator DVA output and CDM spark..
    Sorry for the delayed response. Do you know what the acceptable output should be?

  6. #6

    Default Re: 2002 Mercury 90hp will not go WOT?

    I have a 97 Merc 90 with CDM and am having a similar issue. Cant get over about 4,200-4,500 R's. WOT causes it to shudder and bog, but pull back a bit from WOT and it runs smooth. Pumping bulb or chock both don't improve the running. Bought boat that way thinking it was a fuel issue I could easily fix. I've cleaned carbs, run it on an external tank with new hose, changed each CDM, new stator and trigger, new plugs, checked idle and WOT timing which are on spec. All with no improvement. Changed 3 props including the factory aluminum prop and no change in max RPM. Engine also surges 300 RPM at around 2000's rpm. Idle is terrific. Fuel burn also seems to be excessively high- like 2MPG on a 17' skiff. The boat with this engine should be 40mph+ and I'm barely able to get 33 MPH. Any ideas for further trouble shooting that could be ?

  7. #7
    Join Date
    May 2013
    Location
    TX.
    Posts
    415

    Default Re: 2002 Mercury 90hp will not go WOT?

    Let's divide and conquer. Get a timing light and a buddy. Get out on the water and run at different throttle settings checking spark on all cylinders for a drop out. No drop outs, you said your compression is ok, the service manual agrees so....timing, spark, fuel (includes butterfly positioning), compression .....what's left to check?

    Timing I think would be a constant and not causing surging. If WOT isn't set to full 22*BTDC on that engine as I recall, it would limit your WOT RPMs.

    Fuel.... squeezing the bulb during unsatisfactory operation would eliminate the fuel pump,clogged fuel filter, degraded fuel lines between filter and carb can leave little black specs of rubber inside the carb which could move about and get here and there at certain RPMs. I found one in my 90 lodged in the high speed jet (bottom of carb). Cleaning didn't get it out.....accidentally found it when I blew out the bowl just before reassy. Found another several in the low speed section at the top of the carb when you take the plate off....may have come from the "fuel enrichment" hoses. Replace the fuel filter and cut the old one open. While the hoses are off, take a QTip and swab out the inside of the fuel line....any black specks replace fuel lines.

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