Omc 120 3000 Rpm At Full Load?

Discussion in 'Sterndrives' started by H180DSC, Sep 22, 2009.

  1. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Hey all,

    I have a 1986 seaswirl open bow 16.5' 120 omc (2.3l gm engine w/ 2 jet Rochester). I picked this boat up earlier this year, and have an engine issue. Here is the problem: Running in neutral, the engine revs crisply, and hits 5000 rpm (thats when I let off). When running under load (doesn't matter how many people are in the boat), the max rpm is 3000 rpm. I have a 15.5x15 pitch prop, and a 15.5x13 pitch prop. The max rpm is the same with either prop installed. I have checked compression 145-150 psi on all 4 cylinders. Did a basic tune up: cap, rotor, plugs, wires, which had nil effect. I am a CAT diesel mechanic, and if this was a diesel, it acts like lack of fuel. I am not a gas engine guy, and have been told that it is not a fuel issue b/c it would run hot? The timing was set at 14 deg. btdc. I reset it to 4 deg. as per the valve cover spec., and this had nil effect. The distributor is new, and has mechanical advance. Please Help!!
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    What do the sparkplugs look like? If it is running lean they will be very white
     
  3. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Gonzo,

    Should I check them now, or should I check them right after a full speed run? If I check them now on the trailer, the last load the engine saw was when I idled through the marina before loading. Will this change anything?
     
  4. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    I had another thought. The distributer has been replaced (not by me), and has been updated with an electronic ignition conversion kit. Is there any component in this ignition system that would limit the revs?
     
  5. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Your engine should be hitting 5,600 to 5,800 RPM if well tuned and propped at WOT under load.

    Which ignition module do you have, the CDI or the Mallory?
     
  6. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Par, maybe I misspoke. As I said before, I don't have much experience with gas engines. The distributer cap, the points have been removed and replaced with an electronic pickup. The pick-up is wired to the coil. There is an electronic component mounted to the front of the valve cover, but I don't know what it is. I can post pictures if needed.
     
  7. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If the limiter was faulty you wouldn't be able to rev it up even with no load. Check the plugs after at least twenty minutes of whatever you max speed is.
     
  8. H180DSC
    Joined: Apr 2007
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    Location: Hillsboro, Oregon

    H180DSC Junior Member

    Plugs are about as white as they can get...now what?
     
  9. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    H180DSC, I read nothing about the boat's performance, just the tacho reading.
    Where does the signal come from?
    The instrument used to get its input from the points but they have been replaced. Is it connected to the neg. terminal of the coil now?
    Could it be that just the reading is wrong when the engine is loaded?

    The electronic component on the front of the valve cover probably is the amplifier for the pickup coil signal.
     
  10. Frosty

    Frosty Previous Member

    I would be checking for full throttle on the control cable and then all fuel related stuff like filters, tank breather etc and even pop the carb off for a clean out. I would take off the pump too and check its flow rate, might as well check the internal valves while its off.

    The white plugs indicate a weak mixture for sure but unlike a deisel it would misfire --is is doing that? Sure your not loosing a cylinder over 3000 RPM.

    I would put a new set of plugs in too,--change cheap stuff first.
     
  11. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

    Diesels and gas engines are sort-of opposite in that a diesel will run fine but with limited output if the fuel is restricted and a gas engine with produce limited output (but good no-load rpm) if air is restricted.

    So would guess not a lack of fuel - it would start missing if it got very lean.

    Dirty air filter or throttle plate not opening fully makes sense. Or an exhaust restriction. Lack of ignition advance - maybe.


    PS: I wish all engines had wide band O2 sensors.
     
  12. apex1

    apex1 Guest

    Who would have thought that Diesel and Petrol engines are different?:cool:

    The ever so common nonsense when you post. You fight hard for your reputation, and successful.........:D

    CDK

    was my first concern too, after noticeing the setup was changed. Doubt the reading!
     
  13. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Hi80DSC, if the distributor has been removed (very odd), it will have to be replaced with a crank trigger (or other type of magnetic pickup sort of thing), control module and coil packs.

    It sounds like you have one of two possible issues. One is the timing isn't advancing like it should when you get to speed and the other is a plugged high speed jet in the carb.

    Considering the altered nature of your ignition system and your inability to diagnose, the obvious and most reasonable advise would be to take it to a shop where they are familiar with these types of systems. The average boat shop may not want to touch it with the modifications, but a shop that has a few race boats in the parking lot will usually have a clue what's going on and the systems involved.

    For the life of me, I don't know why they changed to a module ignition. I can understand the desire to change from points to electronic, but this takes place inside the distributor and doesn't replace anything except the points and condenser, the distributor remains to send the spark from a single coil to the plugs.

    Your plug condition suggests a lean out at speed, which is typical if your carb is in need of service. Toss a rebuild kit at it. It's an easy and inexpensive option and will probably fix your problems.
     
  14. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

    Fine point: ignition advances with speed
     

  15. Frosty

    Frosty Previous Member

    Mechanic advance is unlikely to be not working on a new distributor. There should be no vacuum advance on a marine engine.

    If your concerned about it strobe it.

    How old is the fuel? I take you have never seen this motor run 5000 loaded?
     
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