Holley stumble trouble

Fuel and Air Systems

  1. SSG_Karg

    SSG_Karg Heeeeeres Johnny!

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    I have a sneaking suspicion that my curve looks a lot like this one. I’ll get my curve mapped in the next couple days.
     
  2. SSG_Karg

    SSG_Karg Heeeeeres Johnny!

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    No reason not to try to get the most out of it, right. If I get to the point of madness dealing with this stumble then I’ll surely just disconnect the vacuum pod and continue mission.
     
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    • SSG_Karg

      SSG_Karg Heeeeeres Johnny!

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      Correct, two corner idle, front block only.
       
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      • SSG_Karg

        SSG_Karg Heeeeeres Johnny!

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        Timed port on carb
         
      • yellow rose

        yellow rose Overnight Sensation

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        A couple of taps and a drill set and you can make it adjustable. Later today I’m going to make a holley adjustable.

        Otherwise it makes tuning much harder than it needs to be.
         
      • j par

        j par Well-hung Member

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        Fix the sprinkler first! I don't need any roommates!!!...
         
      • j par

        j par Well-hung Member

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        I guess you're kind of making my point in getting the "most" out of it.. I'm pretty sure we're not building any of these performance cars for gas mileage? But good throttle response would be Paramount I believe... after that I figure the gas mileage is where it is and if I wanted more I would consider that part of the performance compromise...
         
      • mderoy340

        mderoy340 Well-Known Member

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        Very easy to adjust when the vacuum pod starts moving if using an adjustable OEM vac can. CCW- CW+ 3/32 allen wrench.
         
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        • SSG_Karg

          SSG_Karg Heeeeeres Johnny!

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          Any idea how wide the sweep is? What I mean is if the screw is backed all the way out, at what rpm does it begin to move? What if it’s cranked in the whole way? Does that disable the advance or what rpm is now required to activate it?
           
        • 512Stroker

          512Stroker We are all here because we are not all there.

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          Most that I have played with dont even start to move until they get 15 inches.
          I searched a long time before I found one that triggers at 8.5 inches
          Have not tried it yet.
           
        • 512Stroker

          512Stroker We are all here because we are not all there.

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          The operating range can be as much as 20 degrees
           
        • mderoy340

          mderoy340 Well-Known Member

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          1 turn either direction is about 1" difference in vacuum. I just get the vacuum advance max to 48* (34*+ 14*) Vacuum Adv at highway cruise. You can then adjust when the can starts to move by trial and error. If car pings street driving go CCW until it stops. Advance CW till it pings, then back off.
          @Mattax did a thread on distributor tuning but I can't find it.
           
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          • RustyRatRod

            RustyRatRod Bla de blizhibliz de blatde blizi bla bla FABO Gold Member

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            Just for kicks, put a vacuum gauge on that port and verify it's not getting vacuum at idle. That could throw your whole timing curve off if you're timing it like there's no idle vacuum.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            I guess when it was made into an 'article' the thread disappeared.
            How To Limit and Adjust Chrysler Vacuum Advance Cans
            People can still comment/contribute on it in the 'discussion'.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            Because its an OLD 4777, I'm expecting not much fiddling with the restrictions and bleeds will be needed.
            Exception may be the IAB, and next in line IFR.
            Wires are easy way to see if thats an area to investigate further.
            Maybe I'm just lazy.
             
          • D-Rocket

            D-Rocket Member

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            +1
            My 4779 750DP holley has vacuum at all times on the so called timing port.
             
          • Mattax

            Mattax Just the facts, ma'am FABO Gold Member

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            More common on factory carbs to have some vacuum at idle on the timed port.
            In general its an indicator the throttle is open too far at slow idle.
            upload_2020-4-30_19-20-39-png.png

            vs.
            upload_2020-4-30_19-16-19-png.png
             
            Last edited: Jun 8, 2020
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            • SSG_Karg

              SSG_Karg Heeeeeres Johnny!

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              Checked the timed port for vacuum at idle - nada. My t-slots are even more closed than the second picture shows.
               
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              • SSG_Karg

                SSG_Karg Heeeeeres Johnny!

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                Ok. This is turning into a dog turd, the more I mess with it the messier it’s getting.
                This is what I’ve tried so far:
                Disconnected vacuum pod and plugged port. Drove it and got it to stumble, though seemingly not as bad.
                Retarded timing all the way back to 10 initial which made my mechanical top out around 24. This lowered the idle rpm a lot so I turned the idle mix screws to get best idle. Idle vacuum was now down to 6ish from a previous 10-11. Tested. Stumble was still there but seemed to occur at a lower rpm.
                Returned home and went back to previous settings. Tested. Runs better all through the throttle range except for this damn stumble.
                Current setup:
                24 initial, 34 total with can unplugged.
                Idle rpm 800 with 10-10.5” vacuum.
                37 primary shooter, 31 secondary.
                T slot is unmeasured after doing all this fiddling but pretty sure it’s in the ballpark because the idle mix screws are very responsive and with that much initial timing I had to turn the idle screw way out.
                I don’t know where the hell to go from here.... try a small wire in the IABs?
                I will map my timing curve tonight, still suspect it’s too fast.
                 
              • 512Stroker

                512Stroker We are all here because we are not all there.

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                Curious, way did you take the timing "all the way back to 10 initial" ?
                That's a huge jump.
                 
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                • SSG_Karg

                  SSG_Karg Heeeeeres Johnny!

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                  I figured retarding timing is the same as adding fuel. Therefore if the stumble disappeared with less timing the same would be true with more fuel added to the previous timing setting. Is that nuts?
                   
                • 512Stroker

                  512Stroker We are all here because we are not all there.

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                  No it not nuts. It is a balance between the two , fuel and timing.
                  10 degrees is a bit extreme, I would be more prone to be in the 14 - 18 range of initial timing.
                   
                • Mattax

                  Mattax Just the facts, ma'am FABO Gold Member

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                  Too many changes at once. laugh2-gif.gif
                  But sometimes you just have to try things, so its all good as long as you learn something.
                  Eventually you'll get a sense of when big jumps are OK and when they are not. You can generally do that with field artillery and naval guns but not always with engines. Once outside of a working range fluid dynamics of the fuel and air start doing unpredictable things.

                  Going back to the baseline as you are is the logical move in my book too.
                  Then make one move from there. I'd say the .035" t-slot is a good one. And since you've done that, try 18 and or 20* and see if you can get it to idle at same rpm and vacuum without opening the t-slots.

                  Come back to that decision after reviewing some of what has been learned.
                  That's interesting! While its possible that timing may related to the stumble, removing vacuum advance from the equation did not completely resolve it. 102859-8cc002171b669003e07a6c5dc808fc62-gif.gif That's definately something to think on. I've used the Direct Connection Race distributor, which is super fast advance (no vacuum advance) and no stumbles.

                  I used to think something similar. Its not nuts, but its not correct. Or maybe it is nuts, but then I'd be admitting I'm crazy too.

                  First of all, timing's relationship with mixture density (which is more or less the same as how rich the mixture is in the combustion chamber) is tied together. In a lean running or other slow burning condition, more time is needed for the burn to build maximum pressure as the piston is going down.
                  upload_2020-1-19_15-40-20-png.png

                  Second, with a high overlap cam the idle vacuum is already somewhat low. If you recall the 'idle circuit' uses the manifold vacuum to move fuel from the bowl to the idle and transfer ports. So when the
                  both air flow and fuel flow was reduced. Opening the throttles brought in more air, but not a proportional amount of fuel.

                  Take the carb off, measure t-slot openings for 1/4 turn increments of the adjustment screw. Start from just touches to .045 or .050 open.

                  If your chicken scratch is getting impossible to reference, copy it over neatly into a spreadsheet or a fresh sheet of paper or whatever works for you.
                  My carb notes (almost) always goes onto a spread sheet later that day or week. I also print some stuff and copy things onto 8.5 by 11. Most now goes into file folders which I can grab easily. A few things still go into 3 ring notebook - which was my initial method or organizing.

                  Returning to the next steps:
                  Going back to the baseline as you are is the logical move in my book too.
                  Then make one move from there. I'd say the .035" t-slot is a good one. And since you've done that, try 18 and or 20* and see if you can get it to idle at same rpm and vacuum without opening the t-slots.


                  I don't know what will happen when you try a little less initial with .035. My guess is that there will not be enough air flow. That's probably why the car was running with .045" of the slots showing. Opening up the throttles let more air in.
                  So I would plan on it not idling as strongly with only 18 or 20* initial and .035 to .040 t-slots. In which case let more air in at idle.
                  One method would be to crack the secondary throttles just a little further open. Measure the throttle to base distance before you do this. Or if you have a wire gage you could measure clearance to the wall. There is no other reference for the factory setting! Then give the screw a 1/4 turn. Give it try.

                  A more easily adjustible method is to put a T in the PCV line. Plug the base of the t and drill an 1/8" hole in the plug. If that helps try 3/16.
                  (Later you can drill the throttle plates or crack the secondaries.)

                  See if that brings the idle speed up. It should.

                  Next, see what the idle mix screws are doing. They should still have an effect on the mix, especially with the t-slots further closed.
                  But the mixture may need more enrichment. If so, experiment with wires in the primary idle air bleeds.

                  Once an idle is established that is at least as strong as the baseline (800 rpm 10.5 "Hg). Then try driving it.
                  If the dead spot is still be there, increase the wires in the IFR.

                  Cheat sheet for a few IAB sizes.
                  upload_2020-6-9_16-22-52.png

                  upload_2020-6-9_16-23-16.png

                  upload_2020-6-9_16-24-3.png

                  So if the 4777 has .076 IABs, and .025" wire is placed in each one, they will be equivallent of .072 IABs.
                  Run the wire under the aircleaner gasket to secure it. I usually use one wire for both bleeds even though it takes longer to bend up jsut because its easier to keep track of. Two individual wires works just as well.
                  Typical IABs will be in the .072 to .076 range.
                  With the air bleeds, a jump of .002 or .004" should noticibly effect the idle through transition fuel.
                   
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                  • furyus2

                    furyus2 Well-Known Member

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                    I'm wondering about the original post says it's got a 10.5 power valve. At 10 inches vacuum.
                     
                  • yellow rose

                    yellow rose Overnight Sensation

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                    And 20 inches at cruise. Idle vacuum doesn’t matter. Really, if you could buy a 15.0 power valve that’s what I’d run with it.
                     
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