Choke doesn't seem to work as it should

Discussion in 'Fuel and Air Systems' started by gzig5, Oct 25, 2018.

  1. gzig5

    gzig5 FABO Gold Member FABO Gold Member

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    I'm just starting to be able to drive the car after working on it for five months. I know I need to optimize the carb tune at idle. When started cold with no heat in the motor and below 50 degrees outside, it really doesn't want to idle. It fires, runs ok for a few seconds and then starts to surge and die and occasionally backfire. I would assume that I need to apply the choke to keep the rpm up, but that just seems to make it worse and it dies almost immediately. So I leave the choke off and feather it at about 1200 rpm for a while until it will idle on its own at 800-900 rpm. When fully hot the idle is up to 1100 but if I set it lower it will die in gear. When it is up to temp it idles/drives good except for a bog at initial tip in from lower rpm. I'm going to pull the carb off and start from zero but I'm wondering if this issue with the choke is pointing to something specific?? Seems like it already has too much fuel and choke just makes it worse until it heats up. I did initially go through the process of initializing the transition slot coverage and air bleed screws but I don't know where I am now.
    Its a 71 340 with headers, stock-ish cam, automatic trans, 13 degrees initial, and a 650 Holley DP, that has been rebuilt by me.
     
  2. 69FBCuda

    69FBCuda Life in the Fast Lane

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    Not idling at low RPM would be the idle jet screws, no acceleration would be the accelerator pump(obvious explanations) Have you checked the fuel pressure at low rpm and while accelerating? Sound like the primary fuel delivery is not working correctly
     
  3. gzig5

    gzig5 FABO Gold Member FABO Gold Member

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    I'm pretty sure the bog is due to the acc pump cam or shooter and just needs to be tweaked.
    I've got a gauge on the carb an pressure is 7-8psi when cold and seem to drop to 3-4 when hot. I have wondered whether it was overpowering the float valve after start up. I have a regulator I can put on it. I don't know why the pressure changes over time but it is a cheap import pump but only a couple months old.
     
  4. TrailBeast

    TrailBeast Slightly Twisted Member

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    Note where the choke butterfly is on the carb when dead cold and unstarted.
    Push the throttle to the floor once, then check it.
    It should be most of the way closed and just a little spring resistance to it.

    You can also start the engine and open/close the butterfly by hand to see what it wants while it's running.
    Pretty easy to tell if it wants more or less choke doing this.
     
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    • gzig5

      gzig5 FABO Gold Member FABO Gold Member

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      It's a manual choke so I control whether it is open or closed, should have made that more clear in the first post. The more the plate is closed once you get to ~1/4 travel, the worse it runs and will quickly die out even if I feather the throttle. I've just been leaving it completely open and babysit it for a few minutes until it warms up enough to get along on its own.
       
    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      Does the carb have a fast idle cam on it?
       
    • Murray

      Murray FABO Gold Member FABO Gold Member

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      Carb has two idle circuits. When cold, choke partially open, the linkage swings up into place a 'fast idle cam" with its own adjuster screw. Adjust cold idle here, maybe 900-1000 on the last step. When warm this cam swings out of the way and you can adjust the "curb" idle. If you have a stumble when it is warm, that is another issue.
       
    • toolmanmike

      toolmanmike FABO Staff Staff Member FABO Gold Member

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      The thread title is deceiving. If a manual choke doesn't work as it should it's more operator error and not a choke issue. If it were a automatic choke the choke being out of adjustment will cause all sorts of issues.
       
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      • TrailBeast

        TrailBeast Slightly Twisted Member

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        Now it sounds like an overchoked situation with a misadjusted or misunderstood fast idle.

        To the OP.
        You should be able to press the throttle to the floor while pulling the choke out all the way.
        Then as soon as it fires off push your choke back in just until the engines smooth's out but with a high idle.
        You MUST press the throttle while pulling the choke or the high idle will not set.

        At this point it should be running smoothly around 1,500 RPM's
        Once it starts warming up the idle will continue to climb, and at this point you should be able to take the choke off and tap the throttle to bring the idle down closer to normal.

        I run and electric choke that is adjusted slightly on the lean side, as I would rather baby it a little at first than have it overchoke.
        Point being, having to baby it a little by feathering the throttle for a minuet or two is preferable to a black smoke, rich as hell plug fouling condition.
         
        Last edited: Oct 26, 2018
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        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          Yes, what Trailbeast said! ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
           
        • gzig5

          gzig5 FABO Gold Member FABO Gold Member

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          Sorry for the delay in getting back to this. I went at it again this weekend. Initial timing is at 15 degrees with the vacuum advance disconnected and plugged. I pulled the carb and reset the primary curb idle so the transfer slot was just exposed. I had taken it well past where it should have been in order to increase idle speed to keep it from dying. Float level seems to be OK. I reset the idle air screws to 2 turns out and then set the base idle speed with the screw on the secondary. Now the manual choke seems to function correctly. I may have screwed up the start up process once or twice but typically I do pump it once before starting and have only recently needed to use the choke.
          The car is also more responsive at throttle tip in, doesn't hesitate like it was before so I made some progress. Now, I have an issue (it seems) with the idle air screws. Using a vacuum gauge I see very little change when adjusting them. I have ~14/15" of vacuum at 900rpm in gear. Adjusting the screws doesn't seem to affect much going one turn either way from start. What I see now is with the idle set so it just barely doesn't die in gear at 900rpm, it is about 1300rpm in park. This is fully warmed up and choke all the way open. I know I should be able to get the idle lower with this fairly mild/stock cam. I'll admit, I haven't touched a carburetor in 30+ years and wasn't that good a tuner back then either, so I appreciate your help and patience. Running out of time to get it better because it goes to storage later this week so any other words of wisdom are appreciated. It drives pretty good, just doesn't seem to want to idle real well in gear.
           
        • toolmanmike

          toolmanmike FABO Staff Staff Member FABO Gold Member

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          With that cam (apparently mild) it should idle around maybe 700 in drive and 8-850 in park. 1300 is way too fast.
           
        • gzig5

          gzig5 FABO Gold Member FABO Gold Member

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          Yeah, it definitely isn't right. Just haven't had much time to play with it. It shouldn't be this hard, there is only a couple screws to tweak for the idle settings. Maybe I missed something in the rebuild.

          Backstory....
          When I got it, the car didn't run. The carb that was originally on the car was completely gunked/dried up and wouldn't run so the seller threw in this 4150 I'm working with now. It was screwed up too and was just dumping fuel into the engine so I rebuilt it, but maybe missed a blockage or something? I'm going to rebuild the "original" holley 4165 List 6711 that is on the shelf now over the winter. It is a correct Thermoquad replacement for the '71 340 and I like the idea of the spreadbore for the street. Car goes to the storage facility for the winter on Wednesday so unless I come up with a non-invasive magic wand, I'm going to live with it for now. I just need to get it into storage and I'll deal with it in the spring. At some point I'm going EFI, so I hesitate to drop ~$400 on a new carb that will just get replaced.