Secondary Stumble

Mopar Performance Issues

  1. 68 Coronet RT

    68 Coronet RT 68 GTS # 508 in Registry

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    I know this is the A Body forum but maybe someone ran into the same issue,Posting for a buddy of mine, 67 Charger with the original factory installed 440 with automatic. Solid lifter cam nothing crazy Holley street dominator intake, 850 Holley carb, stock exhaust manifolds with 3:91 gears. The car runs strong but has developed a secondary stumble with the 850. Tried bigger jetting and gets worse. Put on a Edelbrock 650 AVS-2 great throttle response but still has secondary stumble. Put the 850 back on tried different rods and springs same issue. Put an older 625 Carter on and same issue secondary stumble, tried jetting up the 625 and stumble got worse just like the 850. So obviously not carburation has to be ignition, it does have the Chrysler electronic ignition and we did try a different orange box but no change, timing is 12 degrees initial and 35 total, so where do we look from here.
     
    Last edited: Aug 10, 2020
  2. George Jets

    George Jets 1967 Dart 2 Door FABO Gold Member

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    Sounds like it is not getting enough fuel. Change the fuel filter first. 2nd put on a new Carter Mopar fuel pump, they go bad over time. Fuel pump is supposed to have 6 psi output.

    Make sure the gas tank is not full of trash. Rust and fine grit sediment accumulate in the tank plugging the pickup sreen in the tank.
     
  3. inertia

    inertia Well-Known Member

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    When was last tune-up ?
    Put in new spark plugs .
     
  4. j par

    j par Well-hung Member

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    Distributor curve? Timing initial and all in?..
     
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    • Murray

      Murray FABO Gold Member FABO Gold Member

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      Explain in more detail what you mean by "secondary stumble". What does that mean? Under what conditions, what RPM, steady throttle or when you accelerate, engine warm or cold?
       
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      • 72Dinger

        72Dinger Well-Known Member

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        My father had a similar issue on two different vehicles, a "secondary stumble" where it would fall flat when he mashed the gas. Culprit was a square bore Edelbrock intake mated to a spread bore Edelbrock carb allowing the slightest vacuum leak into the secondaries where the units didn't mate well.
         
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        • 68 Coronet RT

          68 Coronet RT 68 GTS # 508 in Registry

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          Don't believe it's the intake as it's a Holley street dominator with a Holly carb. The car ran perfect with this set up until it developed this issue.
           
        • George Jets

          George Jets 1967 Dart 2 Door FABO Gold Member

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          Is it the original gas tank in the car ? If so best to drop the tank and put in a new gas tank along with a new fuel sending unit and new plastic pickup screen that goes on the send unit in the tank.

          This will clean up your low pressure fuel supply problem. Also put on the new mopar fuel pump at the engine block and the new fuel filter too.

          If you have an electric fuel pump outside of the rear of the tank pushing the fuel forward, have seen this setup only able to deliver 3 psi fuel pressure, when it should be 6 psi up at the carb.
           
        • 68 Coronet RT

          68 Coronet RT 68 GTS # 508 in Registry

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          It has a brand new tank and lines
           
        • Treblig

          Treblig Well-Known Member

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          The Holly power valve will fix the stumble. But you didn't explain the "stumble" very well. The power valve gives the car a boost of gas when your driving along at a constant speed then mash on the gas. Did you try changing the power valve???
           
        • 68 Coronet RT

          68 Coronet RT 68 GTS # 508 in Registry

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          He can drive along at say 40 mph mash it and it will stumble then pick right up
           
        • 68 Coronet RT

          68 Coronet RT 68 GTS # 508 in Registry

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          But here's the kicker, it does the same thing with 2 other carbs even a brand new Edelbrock 650 AVS-2
           
        • mopowers

          mopowers Well-Known Member

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          Where's the timing at???????????????
           
        • Treblig

          Treblig Well-Known Member

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          That sounds exactly like a power valve problem. I had the same problem and fixed it with the power valve. The same thing (stumble) should happen and any constant speed if you mash on the gas. The power valve "senses" a drop in vacuum when you mash on the gas, when it senses the drop in vacuum it gives the engine a boost of gas giving the accelerator pump a chance to catch up with the need for gas.
           
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          • 68 Coronet RT

            68 Coronet RT 68 GTS # 508 in Registry

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            I totally agree, but it does the same thing with 2 other carbs, one being a brand new Edelbrock 650 AVS-2 that I took off my 440 that runs absolutely perfect on and it stumbled when he hit it.
             
          • Treblig

            Treblig Well-Known Member

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            I can understand why you don't think it's the power valve. I'm sure the timing is correct since the owner seems to be turning over every stone?? But no matter what carb you install....if the big motor gets starved for gas (Lean condition) when you mash on the gas you're going to get a stumble. Go ahead and check all the other stuff that other members have suggested then if none of that works you can try the power valve. I had to try about 7 different power valves before I got rid of the stumble. You get pretty close if you run the car at idle in park with a vacuum gauge on the intake. The power valve "vacuum" number should be....watch this:

            setting up holley power valve - Google Search
             
          • yellow rose

            yellow rose Overnight Sensation FABO Gold Member

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            It may be a power valve issue, and it can be explained that both carbs are lean at that spot of the fuel curve.

            FWIW, if you set the power valve opening by idle vacuum it will be wrong.

            You need to set your power valve opening by cruise vacuum. For example if you cruise at 16 inches of vacuum, you’ll use the 10.5 power valve, because that is all Holley now offers. Really, at 16 inches it should be 12.5-13.5 but you can get them.

            The “Holley” method of using idle vacuum was wrong from the beginning, and it’s still wrong.

            12many has an excellently thread where he documents tuning his power valve as outlined above. It is worth looking for that thread and reading it.
             
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            • Treblig

              Treblig Well-Known Member

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              I agree!!! I actually hooked up a vacuum gauge to the engine so I could see the vacuum drop while I was driving around. I made numerous runs at different speeds while cruising, then mash on the gas. When I finally got the right power valve the engine responded so nice. I had been fighting that stumble for almost a year!!!

              PS - the accelerator pump also gives the engine a shot of gas at lower cruise speeds. I messed with pump cams quite a bit trying to get rid of the stumble. The accelerator pump will help some with the stumble but the power valve really gives the engine an immediate boost when you need it.
               
            • krazykuda

              krazykuda Well-Known Member FABO Gold Member How-To Section Editor

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              If you watched where you were walking, then you wouldn't stumble on the secondaries when you step over them....
               
            • yellow rose

              yellow rose Overnight Sensation FABO Gold Member

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              Ahhhhhhhhhh...I see what you did there.....
               
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              • George Jets

                George Jets 1967 Dart 2 Door FABO Gold Member

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                Try an Edelbrock 1406 electric choke carb if you can borrow a good working one. They don't bog, they open the secondaries manually with the linkage and you are not waiting on the engine vacuum to open the secondaries like the AVS carbs.

                Had the 1406 on a 383, also had the 1406 on various mopar small blocks, performance and stock engines. Good all around carb, no bog on any of the engines.

                Screenshot_20200807-121328_Gallery.jpg
                 
              • Murray

                Murray FABO Gold Member FABO Gold Member

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                Please check your vacuum advance diaphragm.
                 
              • oldkimmer

                oldkimmer FABO Gold Member FABO Gold Member

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                When u put the Holley 850 back on can u elaborate how u changed different springs and rods? AFAIK Holleys don’t use them. If u go into the secondaries slowly does it bog? Did u try his carb on ur car? It may not be the secondaries at all. Or not even the carb. Most likely fuel pressure/ volume. Check the fuel pump pushrod. It’s probably worn down(shorter) Kim
                 
                Last edited: Aug 12, 2020
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                • furyus2

                  furyus2 Well-Known Member

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                  I would try advancing the timing, and see what you get. If all the carbs do the same thing, it must be somewhere else.
                   
                • Ahoey

                  Ahoey Well-Known Member

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                  If it falls flat on its face I would say check your accelerator pump massive charge of air and no fuel equals no go
                   
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