Thermoquad Rich Idle

Discussion in 'Fuel and Air Systems' started by wish4hemi, Aug 10, 2018.

  1. wish4hemi

    wish4hemi Well-Known Member

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    The Thermoquad on my bone stock 1973 440 (motorhome) is about to get tossed in favor of a Holley, but I thought I'd come here first. Basically, I pulled it off two weeks ago because it had a jet well leaking. Got that fixed with Seal All and rebuilt the carb, but can't get rid of a rich idle condition. Fuel looks to be dribbling out of the boosters and one shot of the accelerator pump seems to dump in a 1/4 gallon of fuel. At idle, the primary throttle blades are drenched in fuel.

    - Floats were the first check. I tried anywhere from 29/32" to 1-1/4" (with gasket on to the top of the float when the air horn is upside down). No change.
    - Tried two different sets of brass floats to make sure nothing was funky with one set or the other. No change.
    - Needles are new and clean.
    - Metering rod tree is working as it should. Metering rods are not bent.
    - Very little throttle shaft play.
    - All vacuum ports are plugged for tuning purposes (although I also tried it with them hooked up to the corresponding components).
    - Sprayed ether everywhere to check for vacuum leaks. No signs of one.
    - I've tried both kinds of primary well seals; the x-rings and the o-rings. Thoroughly unimpressed with the fit of both. The ID of them seems to almost be too big to functionally seal.
    - Carb to manifold gasket is the thick one, as the engine came with.
    - Had a spare, non-leaking phenolic bowl so I swapped it in to make sure warpage wasn't an issue. No change.
    - The rebuild kit was a Hygrade 657C.

    A 600 cfm Holley 1850-3 is on deck. Between the janky jet well setup and the hokey primary well seal on the Thermoquad, I don't trust this thing at all.
     
  2. wish4hemi

    wish4hemi Well-Known Member

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    Here is a picture of the primary side after I pulled it back off today. Completely black from fuel overload. Idle mixtures screws were almost turned all the way in. The Holley is on now.

    1E255A6D-A9FD-4829-89A1-DD3FD5C7087C.jpeg
     
  3. AJ/FormS

    AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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    Is the check plug under the pump nozzle?

    I would do a pressure test on the floatvalves see when the blow off and if they are able to seal.

    I love TQs
     
  4. wish4hemi

    wish4hemi Well-Known Member

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    Check plug is the new one that came with the kit. Needles are new, but I guess I can pressure test them.

    I liked TQ’s too before the wells started leaking on this one...
     
  5. AJ/FormS

    AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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    I used to work on metric motorcycles, most of which, you might know, have one carb for each cylinder,lol
    I learned real quick, that if even just one of six carbs can't even stand a 2ft head, the whole shooting match goes down the toilet. Every carb got tested,one atta time. You know how long it takes to rebuild a 6 cylinder any-brand, motorcycle? It makes the owners cringe.
    But Hey who left the gasohol in there unstabilized over the winter? And who told you not to do that? Didn't we have this conversation about a year ago.
    Yeah that's what I thought. And no, I can't give you the DA discount; maybe next year.......

    Yeah so the point is I never send a carb out without a pressure test.
     
    Last edited: Aug 14, 2018
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    • Vamisk

      Vamisk Well-Known Member

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      CBX? I saw one for 10 grand on Cragslist not long ago.
       
    • mderoy340

      mderoy340 Well-Known Member

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      make sure the primary and idle air bleeds are clean. blocked bleeds lead to more fuel flow. they are next to left and right metering rods.
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      Up here, they were a tough sell from the get-go, and there were a quite a few years where you could hardly give the away.
       
    • Jadaharabi

      Jadaharabi FABO Gold Member FABO Gold Member

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      .
      How about to much fuel pump pressure?
       
    • AJ/FormS

      AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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      Well very few motorcycles have pumps and the ones that do are low-pressure, usually no more than 3 or 4 psi.

      But Ima thinking your question is in regards to automobiles.

      Yeah that can be an issue, the factory 2bbl pumps are good to 4.5psi, and the 340s are about 6.5 maybe a tad more. But the TQs have always worked for me up to whatever the P4007040 puts out, (advertised at 80gph@8.7psi), in your application. The TQ cover, inverted,with the brass floats, will usually blow a seal at around 5psi. If it holds at 4psi for 5 minutes or so,, I've never had a comeback.
      If it doesn't, I'm looking for the reason.
      'course the floats actually have to float........ in gasoline, and not accumulate liquid inside..
       
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      • Vamisk

        Vamisk Well-Known Member

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        If only i had a time machine. I know they're collectible now but i would put some serious miles on a CBX
         
      • RustyRatRod

        RustyRatRod Just another dumbass. Technical Editor

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        You said the primary piston for the metering rods is working as it should. That's pretty vague. Is it adjusted correctly? It is adjustable and here's how it's done.

        Depress the metering rod primary piston until it lightly bottoms in the bore. Hold it there. With a small screwdriver, turn the screw counter clockwise until the primary piston stops going down. Then, slowly turn the screw clockwise taking note where the primary piston starts to move up. From the point it begins to move up, turn the screw 1 1/2 turns clockwise. This is a starting point. From there you can fine tune it by turning the screw clockwise to richen the mixture and counter clockwise to lean it. Good luck!
         
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        • AJ/FormS

          AJ/FormS 367 FormS clone 3.09-1.92-1.40-1.09-.78od 3.55s

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          They were pretty good in their day, but you couldn't give me one now.