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Weird rough running issue with OMC 5.7

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  • Weird rough running issue with OMC 5.7

    I have gone through internet for quite a bit and still cannot find a solution for the following:

    Symptom: Engine sounds like running on seven / running rough every time after when started, regardless of being cold or hot. The problem clears when I rev the engine once close to 4000 RPM and after it it runs very smoothly in every condition. After longer drive with low rpms it might reoccur, but systematically occurs after every start. Also when the fault is "on" the boat will not plane and engine will backfire at 2500 rpm.

    Troubleshooting/replaced parts this far:
    -Rebuilt Quadrajet Carb, with electric choke, tuned idle air/fuel with vacuum, choke works as it should
    -Distributor cap + cam
    -Pointless ignition (Ignitor I)
    -Coil
    -Sparkplugs and wires + coil wire
    -Base timing ok (one spring was missing in distributor, added another one and it runs good when the fault is cleared, will find the correct srping), distributor cleaned and the timing mechanism lubricated
    -Overruled faulty tachometer possibility as someone had found his faulty and causing backfire
    -Run into problem with a bad 50 amp fuse causing other type of misfiring, but repairing this didn't help with the original issue

    The wierdest thing is that after revving the engine one time high enough it runs good as new.

    Any ideas welcome.

    Cheers!

  • #2
    your running lean, go back to the fuel system
    Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

    1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

    Past Boats
    1970 Wooster Hellion - Merc 9.8
    2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
    1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

    What’s behind you doesn’t matter.Enzo Ferrari

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    • #3
      The symptom is the same regardless whether the engine is cold or hot. I would assume too lean not being issue when on choke condition.

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      • #4
        are you sure you are not getting water in a cyl, I had that when my head gaskets were starting to go bad. Pull the plugs before you start it, disable the ignition and crank it over. Watch if water shoots out of a cyl. Any milky oil?
        1988 Four Winns 200 Horizon
        4.3 OMC Cobra

        98 Jeep Grand Cherokee 4.0 Selectrac
        07 Jeep Grand Cherokee Limited 5.7 Quadradrive II

        "While air doesn't freeze....rust never sleeps"

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        • #5
          Originally posted by Rudinski View Post
          The symptom is the same regardless whether the engine is cold or hot. I would assume too lean not being issue when on choke condition.
          your idle circuit and internal passages in the carb do not care where the choke is. the choke is an external flap designed to stop incoming air to richen up the mix. not control the flow of fuel like the internal passages in the carb.
          Cheesehead boating the Gulf Coast of FLA 27.51° N, 82.53° W

          1988 Cruisers Rogue 2420 -VP 290 DP now powered by custom 468 - https://forums.iboats.com/forum/owner...988-rogue-2420

          Past Boats
          1970 Wooster Hellion - Merc 9.8
          2002 SeaRay 190BR - 5.0 - A1G2 - "Cheeseheads in Paradise"
          1984 Avanti 170DLI -3.0 stringer- "Ship Happens"

          What’s behind you doesn’t matter.Enzo Ferrari

          Comment


          • #6
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            The small water leak would be plausible theory as it is not tied to fuel nor ignition. Oil is normal, so it must be directly through the head gasket to cylinder leak. I will try to check this next.

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