1. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    I was makin a joke. "Grasshopper" as you used it, means someone who is young, green and inexperienced. Tony is anything but. I just got a chuckle out of it. 70aarcuda is Tony.
     
  2. Justin Crossland

    Justin Crossland Member

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    I'm looking at these thumpr cams rite now, they recommend you upgrade the springs lifters and timing chain with them, ergo you would need different sized push rods as well, so maybe the overly rough idle could be a pushrod issue, not opening long enough for its intended duration? Just a thought.
     
  3. RustyRatRod

    RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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    No need for different pushrods. The rough idle comes from a few things. Duration, LSA and overlap to name three. It's not a physical thing. It's an air flow thing. You need matching valve springs for any camshaft you choose.
     
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    • AJ/FormS

      AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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      Another thread
      back from the dead
      almost got me.
       
    • RustyRatRod

      RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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      I'm glad they keep them resuscitable. We gotta newbie here tryin to learn somethin.
       
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      • AJ/FormS

        AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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        the rough idle
        after the tune is right
        >Is mostly due to the very late closing intake, that allows the piston, at idle, to push a portion of the just-inducted intake mixture, back up into the intake manifold. This messes up the plenum AFr, but also increases the pressure in there, decreasing the vacuum, and making it difficult for the carb to meter properly at low rpm.
        >Pistons on the exhaust stroke can be connected to the intake plenum, during the overlap cycle,and the headers will pull mixture straight across the piston and out into the pipes.
        > with a single plain intake, the pistons on the intake stroke can pull exhaust gasses from another cylinder that is on the overlap cycle, which reduces vacuum yet further.

        But if the tune is bad;
        a high required idle can be the consequence of a poor T-slot sync, usually because the blades are too-far closed. because the ignition timing is too far advanced.
        In nearly all case the shake is from one of two things;
        1) a poor AFR or
        2) an excessively high pressure in the chambers, due to the high advance. At low rpm, those very strong pulses hammer the crank. The cure is as easy as retarding the timing. Back to something more normal. This will reduce the pressure, as it chases after the descending piston.
        At idle, your engine will tolerate a lot of advance, maybe in the 40 degree range. With a low pressure engine lots of Idle-Timing works. But as the cranking cylinder pressure increases, you gotta cut it back, because of the very strong power pulses. This is especially important with a manual trans, as
        the car tends to become undriveable at that low rpm. Again the cure is less Idle-Timing.
        But as others have said; the metering-rod carbs are a little unhappy at those very low manifold vacuums.
        So your first defense is retarded to normal timing, then a DUAL-Plain intake, and finally is a Holley-type PV carb.
        Of course the valves actually do have to close,lol. and the rings have to be loose enough so the pistons can go up and down without too much dragging; because as the Idle-timing is reduced, the power will be reduced.
        Just for comparison, my 11/1, 367/4-speed, idles at 500/550 in first gear, pulling itself around the parking lot, with just 5 degrees of idle-timing. But it has a hard time with any grade at all or soft spots. But it takes all of the bucking away, until just before it stalls.This with a Hughes 276/286/110 cam/61* overlap.
         
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        • RustyRatRod

          RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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          That's more like it.
           
        • Bewy

          Bewy Well-Known Member

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          AJ,
          post #56. The cure is NOT to cut back on idle timing. The cure is to use Vac Adv as I explained in the 'How to..' section. That is the beauty of VA: it is load sensitive, not rpm sensitive. As load increases, the extra timing provided by VA drops way...
           
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          • AJ/FormS

            AJ/FormS 68 B'cuda fb, Form S clone ... 367/A833/3.55s

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            The max you can get from the Mopar V-can is 22 to 24 degrees.
             
          • Bewy

            Bewy Well-Known Member

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            OK, didn't know that. I built a 440 about 16 yrs ago. I demonstrated the benefits of MVA & the owner said, 'Gotta have that!!'. Could not find any adj Mopar VA units here. But GM adj units are easy to get & they go to about 30*. I ended up up using a GM points dist that I modified to take a Chrys reluctor & pick up. Triggers a 4 pin Bosch module, which is similar to the GM HEI module. Machined an adapter to fit the dist to the 440.
            Gets some strange looks at car shows............
            Dist is still in the engine today.
             
          • Hysteric

            Hysteric Well-Known Member

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            The cure is to build engines with vacuum.
             
          • RustyRatRod

            RustyRatRod I was born on a Monday. Not last Monday. FABO Gold Member

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            If it's a street car, I wholeheartedly agree.
             
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