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any ideas what this could be?

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  • #1

    any ideas what this could be?

    Was over at JP's today, messing with my car... has an "interesting" symptom. If you gently press the throttle in neutral, the engine starts to rev up but sounds like it's only running on six or seven. When you get to about 1500-1600 RPM, all of a sudden the RPMs jump up to over 2000, the vacuum increases, and the engine sounds right. Seems like a low RPM fuel delivery problem maybe? I can't see the fuel pressure gauge from the driver's seat so that makes it difficult to troubleshoot. I will put in the electric pump as soon as I find the right hose barbs for it (tomorrow if I can,) as also the noisy stock pump makes it hard to really hear the exhaust. I'm also wondering because it seems so RPM related and repeatable, is this a common failure mode of the "orange box?" I don't have another one to try...

    nate

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    55 Commander Starlight
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    55 Commander Starlight
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  • #2
    quote:Originally posted by N8N

    Was over at JP's today, messing with my car... has an "interesting" symptom. If you gently press the throttle in neutral, the engine starts to rev up but sounds like it's only running on six or seven. When you get to about 1500-1600 RPM, all of a sudden the RPMs jump up to over 2000, the vacuum increases, and the engine sounds right. Seems like a low RPM fuel delivery problem maybe? I can't see the fuel pressure gauge from the driver's seat so that makes it difficult to troubleshoot. I will put in the electric pump as soon as I find the right hose barbs for it (tomorrow if I can,) as also the noisy stock pump makes it hard to really hear the exhaust. I'm also wondering because it seems so RPM related and repeatable, is this a common failure mode of the "orange box?" I don't have another one to try...

    nate

    --
    55 Commander Starlight
    http://members.cox.net/njnagel
    N8,

    Shot in the dark here -- sticky advance plate, so the vacuum advance comes in all at once? Or sticky and/or worn centrifugal advance weights/parts?

    Paul
    Winston-Salem, NC
    Visit The NEW Studebaker Skytop Registry website at: www.studebakerskytop.com
    Paul
    Winston-Salem, NC
    Visit The Studebaker Skytop Registry website at: www.studebakerskytop.com
    Check out my YouTube channel here: https://www.youtube.com/user/r1lark

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    • #3
      Shouldn't be, it's a fairly fresh Thibeault MoPar conversion. I will check it out though.

      edit: could it be that the point at which everything feels right is when the vacuum port for the dist. is uncovered? I did hook it up to ported vacuum as I thought that that was how it was done from the factory.

      nate

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      • #4
        Easy enough to eliminate advanced plate just disconnect the vacuum line.

        You don't have to crack the plates much to expose the advanced port but I don't think you would get that running on 6 or 7 feeling just because vacuum advance wasn't kicking in right away.

        My experience with the Mopar box was hard starting when cold and/or no start at all. Never had one cause a low RPM problem but that doesn't mean it couldn't.

        Have you tried advancing the timing just to see if it makes a difference. If you were a little retarded ( timing wise ) you might get a little stumble until either centrifugal or vacuum kicked in.

        The other thing that comes to might is too much fuel. While experimenting with a brand X with an Edelbrock I richened it up enough to get a low rpm hesitation.

        Ernier



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        • #5
          It does run rich at idle according to my old Snap-On scope, and clears up a bit off idle.

          JDP/Maryland
          "I'm a great believer in luck and I find the harder I work, the more I have of it."
          Thomas Jefferson
          JDP Maryland

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          • #6
            Ernie,

            I can't advance the (static) timing any more, it's already starting to kick back slightly when the engine is warmed up. (well, I *could* but then I'd need a beefier starter.)

            I actually ended up richening the idle mix yesterday from where I set it on the scope, to get it to run better; picked up maybe 200 RPM idle speed.

            nate

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            • #7
              I cured a stumble on the Chrysler with a new base gasket on the carb and the Avanti had issues with intake leaks with the steel gaskets. In both cases the symptoms were only noticeable at idle and just off idle.

              JP is your scope measuring CO and HC?

              Ernie

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