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OMC 350/305 - RPM Stumbling Under Load Only

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  • OMC 350/305 - RPM Stumbling Under Load Only

    Hello,

    I Have a 88' OMC 350/305 (I'll know for sure which when I pull the Serial Number) that stumbles in the Approx RPM range of 1900 to 2200.

    It does not happen when when in neutral and reved into this range - only when in gear. When in gear there is no stumbling before the entering the range but once there it will continue to stumble/run rough if left in this zone. After pushing past the trouble band the engine smooths out again.

    Has anyone experienced this situation? Solution suggestions welcome.

    I just had the lower gears replaced in my Sterndrive and I would like to get eveything working 100%. I thought this mid range stumbling couln't have damage the gears but I am rethinking this after paying big $$$ to have the lower unit rebuilt.

    The carb on this is a 4barrel holly


    - Can rough running in the mid/high range cause the dog Clutch/gears to become damaged?

    - Can this be caused by a malfunctioning ESA module?


  • #2
    Re: OMC 350/305 - RPM Stumbling Under Load Only

    Has anyone experienced this situation? Solution suggestions welcome.
    Ayuh,... I'd be lookin' at the Fuel System...

    It could be as Simple as changing the Fuel Filters,...
    Or as tough as rebuilding the Carb....
    Any Grease is Better,..... Than No Grease at All.......

    Comment



    • #3
      Re: OMC 350/305 - RPM Stumbling Under Load Only

      AN engine not under load doesn't require the same fuel requirements or spark intensity. When under load the requirements depend on the load. I would suggest checking fuel (Filters and carb) and the cap and rotor. When was the last time the plugs were changed?
      1997 Larson 226LXi
      5.7EFI
      Bravo III 26p Props

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      • #4
        Re: OMC 350/305 - RPM Stumbling Under Load Only

        I changed plugs and wires last season. I was told by previous owner that the cap and rotor were replaced just before I bought it (likly true as I found the old ones in a bag under boat seat) - but I'll replace anyway to be sure.


        I think I'l replace the fuel filter / coil next. If not that I'll try to find a find a way to confirm spark intensity and fuel pump pressure.


        Can anyone rule in or out the following?

        - Can rough running in the mid/high range (1700 - 2400 - i added some buffer) cause the dog Clutch/gears to become damaged?

        - Can this possibly be caused by a malfunctioning ESA module?

        I have a pertronix ignition unit that i've been waiting to install as I didn't want to complicate the diagnosis. But as it removes the points that may solve the problem so I may go ahead and install if the above doen't work.

        Comment



        • #5
          Re: OMC 350/305 - RPM Stumbling Under Load Only

          I would be looking into the float bowls of that holley, it's too bad a marine holley doesn't have the float level plugs, could also be a bad power valve which does the fuel enrichment in conjunction with the accelerator pump shot and responds to lowered engine vacuum, when at part throttle cruise where engine vacuum would be high the powervalve should not be open at all. this is just one of many many things to eliminate and we all know troubleshooting basic are all base on the process of elimination so eliminate the easy stuff first, but for me rebuilding a holley is easy, but not as easy as say.... relacing a fuel filter. but i think if the filter was plugged you would not be able to recover from the rough spot by throttling up?
          If the esa was out of whack then the engine would pretty much fall flat and all but die.
          '90 Chaparral 2000SL
          5.8 Cobra
          '67 MFG Westfield Regal
          80hp V-4
          (No private messages)

          Comment



          • #6
            Re: OMC 350/305 - RPM Stumbling Under Load Only

            Thanks,

            I'll start with the fuel filter and go from there. I'd bet the pervious owner had the same issue and changed the distribuitor and rotor to try and eliminate the stutter - unsuccessfully.


            I just uncovered the back and got into the boat to take a look around the engine and pull son sn's.

            One supprise - the carb has a sn under the arrestor area - 6R 8025 - then I got to really looking it over and found a tag - Holley Performance - Street Avenger.

            I think one of the previous owners put on an aftermarket Car Carb on this :-( The Power Leads for the Thermostat that opens ands closes the choke is not hooked up either.

            Comment



            • #7
              Re: OMC 350/305 - RPM Stumbling Under Load Only

              Well there ya go, I believe that street avenger has manual secondaries, lots of stuff to play around with there, that carb is no good in your cruiser, unless you have the ability to tune it just right with jet changes pump cams ect. Although it very well could be a vacuum secondary carb too in the street avenger series which could be better for your engine. The number you found I think is just a generic number for a 650 cfm holley.
              There should be some more numbers on the air horn.
              '90 Chaparral 2000SL
              5.8 Cobra
              '67 MFG Westfield Regal
              80hp V-4
              (No private messages)

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              • #8
                Re: OMC 350/305 - RPM Stumbling Under Load Only

                More bad news

                I got a local engine shop job number off the engine and their records indicate that the engine was completly rebuilt in 2004 and sold off the shelf to the customer (previous owner)issue: it's an automotive engine.

                Comment



                • #9
                  Re: OMC 350/305 - RPM Stumbling Under Load Only

                  New priority:

                  what needs to be checked and possibly replaced to ensure this set_up is safe for marine use?

                  Longevity of the engine is no longer my main concern - I don't want any chance of fire or explosion

                  Carb - definatly needs to be changed

                  What else?!?!

                  Comment



                  • #10
                    Re: OMC 350/305 - RPM Stumbling Under Load Only

                    Fuel filter and other replacements didn't change anything.


                    New Carb - Problem fixed


                    Comment

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