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4.3l random acceleration at 3,000rpm

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  • 4.3l random acceleration at 3,000rpm

    1994 Wellcraft Nova Spyder with 4.3l volvol Penta. This is building on a previous post about bogging down.
    Motor starts and run perfectly every time. Will run up to about 2,800 rpm perfectly. For both of the above it has to be throttled up slowly for there to be no issues.
    If you hit the throttle for a hole shot the engine boggs down and will either die or just slow to an idle.
    If it does die it will restart with one turn of the key.
    Getting past 3,000 rpm and on plane is random. If I can get it to 2,900rpm and just let it cruise for a few minutes it will slowly gain RPM and run upto 4,200rpm and cruise for as long as I want running perfectly as long as I don't back off the throttle.
    Getting it to pass 3,000rpm is hit and miss.
    Carb has been rebuilt, fuel filter at carb and water separator have been changed. Has fresh fuel, distributor and wires have been inspected and appear to be good but not replaced.
    Anyone have an idea of what's next to look at.

  • #2
    Ayuh,...... Welcome Aboard,....... By the symptoms, I'd go back to the carb,.....

    The accelerator pump, 'n the high speed side in particular,.....


    • #3
      Carb was rebuilt. Due to prior experience working on cars and doing small engine repair carb was cleaned by first soaking in solvent, then all passages cleaned with carb cleaning pick and brushes followed by compressed air. Accelerator pump parts replaced and function verified visual by observing during operation on the boat. All new gaskets and carb filter.
      in addition to carb work a tune up and maintenance to establish current maintenance schedule has been completed.
      New Ignition coil, all new fuel lines from tank to carb, new anti siphon valve, new spark plugs, new distributor cap, new fuel pick up hose inside tank, new fuel pump, fuel tank inspected with inspection camera for debris and checked for water by emptying tank fully via fuel transfer pump and checked for water then verifying tank was empty with inspection camera, vent line checked for blockage via inspection camera and compressed air, oil change in engine and outdrive, new optima battery, new impeller.
      Timing was checked via timing light and found to randomly jump all over then settle and repeat at idle and while underload.
      Intermittently the engine will go past 2800-3000 and run perfectly. When it does it will cruise on plane until backing off the throttle. After backing off throttle from cruising speed it often will not return to cruising speed. Engine temps are good as verified with thermal laser before and during cruising speed.
      Question based on your guy's experience with Marine carbs could it still be a carb issue or should I focus on an ignition/timing issue?


      • #4
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        A way to check your fueling is to create a restriction on the carb ala the choke (can actually close the choke plate by hand, carefully, gently) while its stumbling to see if the additional fueling helps resolve the condition.

        It does sound like you have covered a lot of the bases. I would do a couple more things, replace the ignition module under the distributor cap. That's responsible for the entire spark advance curve. It has a 2 pole wiring connection that goes back to the coil (thinking the "square" shaped remote-coil.), so check that for quality. The ignition module should be coated underneath with silicone dielectric grease to aid heat transfer to the distributor base. Also the coil body...the big laminated ferrous core that sticks out, has to have a clean electrical path thru the mounting brackets to engine ground. You can wire brush and use star washers if anything is corroded, I have this Jet Lube SS-30 copper paste that I use for such connections and to help drive away corrosion.