1986 OMC cobre 4.3l will not go over 3000 rpms

Discussion in 'Gas Engines' started by kstrafo, Jul 16, 2012.

  1. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    This is very frustrating! I have a 1986 OMC Cobra with a 4.3l engine. Just recently I have begun to experiance what I thought was a fuel problem. The boat would hesitate as it accelerated under load. After a few minutes it would settle down and run at full power. Now the problem has advanced to a full time problem. As I move the throtle to full power, the best I can get out of it is 3000rpms. The tach shows 3000rpm and if you advance the throtle nothing happens. It is as if the cable is not moving. The engine will speed up for a few minutes and then back down. Fuel filter replaced, fuel pump replaced, points plugs and condenser replaced, ignition coil replaced, new rebuilt carb, ESA replaced. Any help, I am losing my mind with this boat!!! By the way, not under load, engine revs fine!
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The first question is: why did you replace the parts. Did you determine any of them was bad? Also, you need to make sure the flow of fuel is proper. There may be an air leak or obstruction. Changing parts without properly troubleshooting is expensive.
     
  3. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Remove the flame arrestor from the carb and check if the 2nd barrel valve fully opens at WOT. If not, there is something wrong with the linkage; if yes, the 2nd stage jets are clogged.
     
  4. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    I replaced the parts because over the winter I removed the heads and had the valves replaced(detenation). I had assorted problems with the boat and I wanted to make sure everything would be up and running for the spring. In the process of putting engine back together the ESA unit was faulty, replaced that, then with the new carb, would not start, no fuel. Replaced fuel pump. The engine was running great up until recently when this problem developed. At WOT the engine will sporadically rev up to normal RPM for a few minutes and then bog down again. The carb is a rochester 2v. How can I tell if the 2nd barrel is opening fully? I am a bit of a novice at this so any help would be appreciated.
     
  5. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    One other bit of info, the previous owner of the boat replaced the engine with a 2000 4.3l. The engine still has points, not electronic ignition.
     
  6. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    Just look into the big holes when the flame arrestor is removed and the engine is idling.
    You'll see a disc closing the narrower part (venturi), moving the throttle lever makes this disc pivot around a thin shaft. When it is fully open it stands upright.
    The 2nd barrel remains closed until you advance the throttle further, then it gradually opens to obtain full power.

    Note that when the engine is cold, the choke valve covers the hole, so you wont see anything until the thermal device opens it.
     
  7. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    I will be working on the boat later in the week and I will check that and let you know what I found. The carb is a rebuilt just put on this year. but as the saying goes...it's a boat! I appreciate the help. I am going to try to eliminate the fuel tank and line by either using a pressure gauge or a separate fuel tank.
     
  8. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    An update: Ran boat with separate fuel suppy. I have now eliminated the fuel tanl, line, pick up screen, etc. Problem still exists. Watched carb as we went WOT. The two flaps at the bottom of the carb are on a single shaft to the linkage. Both open at the same time. Gas was spraying into carb, looked more like a mist then a steady stream. I have returned carb to Flying Fish Carbs. They said they will check carb but they do not think carb is the issue. Only thing left is fuel pump and that was replaced in spring. Is it possible float could be bad in carb? Or fuel pump not supplying enough gas????
     
  9. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    Detonation will cause power to fall off along with eventual damage.
    Does the engine overheat, what temp is it running?

    Seeing that you had that issue, what is the chance the issue is still there?

    what is the engine cylinder compression?
    Check it cold and check it hot with carb wide open.
     

  10. kstrafo
    Joined: Jul 2012
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    Location: New York

    kstrafo Junior Member

    Turns out that the points were the culprit. It was either the points or the advance springs in the distributer. Re gapped the points and lubed the springs and off she went.
     
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