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Technical Flathead Dual Carb Questions

Discussion in 'The Hokey Ass Message Board' started by Ziggster, Nov 13, 2025.

  1. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,963

    Ziggster
    Member

    So, trying to figure out how to block off the idle mixture circuit. Seems if you turn the idle mixture screws in all the way, you will block off the main idle circuit and not the off-idle or transfer holes which are above the throttle plate and idle circuit holes (see diagram below). I guess you could also add some solder to the hole in the idle tube to block the idle circuit as well. Not sure if that’s a good idea considering the solder could come loose and ****** up other circuits or even somehow get into the intake.
    Not sure how important it would be to have the transfer holes/circuit functioning on a progressive linkage setup considering the second carb would only kick in around the 65% full throttle position of the main carb. I’m guessing that by turning the idle mixture adjustment screws in all the way, the idle circuit is effectively blocked off, but the idle transfer circuit/p***ages would still be open. Soldering shut the idle tube would I ***ume block off both circuits. I’m surprised I can’t find more info on this subject on the interweebs. Going to strip down the remaining carbs today and tomorrow, and check to see how well the throttle plates seal against the Venturi body. If there isn’t a good seal, I’ll then probably bit the bottle and order the new secondary carb base that is guaranteed to seal.

    Idle tube that could be soldered shut to block off both? circuits.

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  2. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,963

    Ziggster
    Member

  3. flatout51
    Joined: Jul 26, 2006
    Posts: 1,307

    flatout51
    Member

    Straight linkage and save yourself a headache and find some 97s. You will be better off. Ive tried progressive linkage with 94s and straight linkage with 94s. Blocked off power valves, differnt sized power valves. Finally found some 97s and its a world of difference from setup to driving.
     
  4. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,963

    Ziggster
    Member

    Interesting. I’ll give this a try first. I’m trying to be budget minded, and would love some 97s, but the cost is typically $$$. If I can get a good seal on the throttle plates with any of the 94s I have, then that’s the direction I’ll go. I need a rebuild kit, a couple of extended length throttle shafts, and some throttle shaft arms, and I should be good to go for the most part, although I’ll be ordering some other stuff.
    One carb base is practically new after bead blasting a few years back. There is no play in the throttle shaft, but plates need a good cleaning. One of the other bases is pretty crusty, and shaft is seized. The other 94 is working as it should. Nothing seized. I won’t go into detail in any rebuilds in this thread, but might create another thread of the rebuild. I’ll likely rebuild all three of my extra carbs.

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    Last edited: Dec 6, 2025 at 10:02 AM
  5. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,963

    Ziggster
    Member

    Can see light past both plates where the shaft p***es through on the one non-seized carb. Got the other bases soaking in vinegar, and the seized base now has full movement on the the throttle shaft. Haven’t tested it yet for sealing, but came across this HAMB thread on lapping the throttle plates, but not much help in the “how to” dept. Seems thicker throttle plates are required to seal 100%, so a new base specifically built for a secondary carb used in a progressive linkage setup migt be the only way.

    https://www.jalopyjournal.com/forum/threads/throttle-plate-lapping-compound.953538/

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  6. Ziggster
    Joined: Aug 27, 2018
    Posts: 2,963

    Ziggster
    Member

    Looking at my current single carb stk intake and fuel pump, I started wondering how the secondary carb was going to fit. Decided to mock up a carb and a spare full pump/vent ***y on the Edmunds intake just to check. It looks like the second carb will barely clear the pump. I see they rotated the mounting studs on the intake for the pump/vent ***y. Ahhh…

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