340 fuel/ignition issue?

Fuel and Air Systems

  1. midnight340

    midnight340 FABO Gold Member FABO Gold Member

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    340 with Thermoquad and MSD distr. with 6A box, recent plugs, good wires.

    In my last Thermoquad thread I was diagnosing fuel leaking into the intake. Turned out to be a leaky brass float which I got fixed. This was a #6518s that I had rebuilt not long ago. ('74 440 "Industrial".) Also I had this really dialed in for my 340, great A/F #'s, good plug readings, etc.

    Call me crazy, but I looked at my parts carbs and see I also had a #6322 core ('73 Truck 440) in decent shape, so I decided to build up a backup carb. So I set it up with a new kit, similar jetting/rods, same settings, etc. The only problem with it was worn at the primary shaft, so I ordered one of those cool reaming tools with brass bushings from Carb Junkys on eBay. (That thing is so slick, and the process was easy and it now feels like a new carb.)

    So I decided to stick it on to test it. Started up, warmed it a bit, and went for a drive. Like the other one, I was getting 13.5 with light throttle, mid 14's at steady cruise, 15 or so on a bit of hill. All good, great TQ throttle response, etc.

    I get out of town, and with the very first WOT in 2nd, all of a sudden it craps out, starts missing badly, will hardly run, after a bit it started running a little better and I made it home. But on the return trip my A/F was now reading 10:1 and power was down. I pull the plugs and all are heavily black and sooty.

    Any ideas what happened???? With it being the first run on the carb I suspected it, but don't know how it could cause the motor to instantly crap out and be missing like that. But it doesn't seem like ignition would all of a sudden drop the A/F down to 10's.
    Thoughts???
     
  2. 67Dart273

    67Dart273 FABO Gold Member FABO Gold Member

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    My thought would be some junk in the needle seat has caused fuel to flood the carb

    Does it sit there and idle perfect at the present?
     
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    • Rmoore

      Rmoore Well-Known Member

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      I was going to say the same thing as 67dart273 ! you have way to much fuel for some reason ! most likely trash
       
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      • midnight340

        midnight340 FABO Gold Member FABO Gold Member

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        OK, THANKS!! I appreciate it, will pull it apart and clean it out. Damn, I'm getting good at dismantling Thermoquads!! HaHa!
        I'm going to re-bush the other carb as well, guess I'll just relax into some bench time. :)
         
      • Bewy

        Bewy Well-Known Member

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        [1] Make sure the base gasket seals all the passages in the carb base. A leak here causes strange things.
        [2] Make sure the choke blade is vertical, not flapping around, engine running.
        [3] Make sure the sec air valve dashpot is working; should pull in with engine idling. With engine idling, make sure the connecting rod from dashpot to AV moves freely.
        [4] Engine off. Push in dashpot plunger & release it. Compare to the other carb. If it is slow, the diaphragm has gone hard, causes a rich mixture.
        [5] Make sure the AV moves freely. Should be at 2 turns as a starting point.

        Keep us posted!
         
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        • midnight340

          midnight340 FABO Gold Member FABO Gold Member

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          Thanks, Bewy,
          Re: #3, To clarify, this sets the AV drop, and you are saying to check that there is a bit of wiggle/looseness so there is some drop set?? (It was set at maybe .020" but I increased it a bit.)

          [1] Haven't dismantled it to check the base gasket yet, or to check for debris in needle seats, but will soon.
          [2] No choke
          [3] Both dashpots are new. Activates easily with MityVac at only 5 in/hg. It does release just a wee bit slower than the one on the previous 6518, but does seem to release fine, smoothly.
          [5] AV does move freely, was set at 1-1/2, and I was not getting any bog in the bit of driving before the problem.
           
        • Dartswinger70

          Dartswinger70 make it 3 yards and well have us a automobile race

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          I have found with the Edelbrock Performer carbs that Edelbrock recommends it set to "best lean" idle" and that backing the screws out untill the engine "sounds good" may lead to fouled plugs. I would suspect the TQ to be similar in carb tuning and setup? How is the mixture at idledoes the exhaust stink like rich? any black smoke?
           
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          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            Apart, all seems good. I may have had something in one of the needle seats, not sure, but cleaned everything out again, and will reassemble.
            Checking my base gasket, all gaskets in both rebuild kits seem to cover the same holes, and be open in the same places, the only difference is in the center, and I'm not sure what difference this would make. Like this:
            TQ base gasket.jpeg
             
            Last edited: Sep 15, 2021
          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            Always smells a little rich to me even when the A/F meter reads 14.5 or so. No black smoke, though there might have been when the A/F all of a sudden went from 14's to 10:1.
             
          • Bewy

            Bewy Well-Known Member

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            Midnight,
            [1] Gasket in post #8. there some differences there & from memory if it is wrong, it can cause an air leak to the AV dashpot vacuum passage. If you hold the black plastic body upside, you will bee a recess/groove that follows the curvature of the secondary bore; this at the rear of the sec opening, passenger side. It connects with a passage in the alum base plate. That passage has a small hole in it that is drilled into the sec throttle bore. When the sec blades are closed, this hole is covered; so you have to open the sec blades to see the hole. Trace this hole/passage/recess back to the gasket & make sure that nothing is exposed because the gasket is wrong.
            [2] The base gkt I was referring to in post 5 is the one between the carb & manifold. The passage referred to above can be uncovered with some TQs
             
          • Bewy

            Bewy Well-Known Member

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            Sorry, pushed wrong key.
            Continuing..
            so make sure your base gasket covers all openings in the carb base. Any leaks in [1] or [2] cause the strange problem that you are having.
            [3] Flooding. Run engine & shut off. If it is flooding, the boosters will drip.
            [4] AV linkage rod. I should have explained it better. Make sure the AV moves freely from closed to open, no binding or catching.
            [5] AV 'kick'. I like 0.060 - 0.100" for performance engines. Gently push in the dashpot plunger in with a flat blade screwdriver. The back of the AV rests against a projection on the air horn; it should move the distance listed. Bend the rod to get the measurement.
             
          • Dartswinger70

            Dartswinger70 make it 3 yards and well have us a automobile race

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            yeah i suppose what i think may be going on is, if the engine is rich at idle the plugs may be fouling then eventually they just get to where the car stops running al together. Bets to set best l;elan idle, then if they plugs are fouling iit isnt because of the idle circuit, actually the idlke circuit is only in play at idle from what I understahd,but ive had engines foul out plugs idleing..it isnt fast but over time the car will miss and run bad and youll think bad ignition and it is, but its fouled plugs.
             
          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            Discovering some issues, will update as I am more certain as to what happened.
             
          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            OK, update details on the 6322, (even though I'm still not sure exactly how some of this happened.)
            First, the tiny hole hidden by closed secondary blade was clogged with carbon, so cleared that.
            I re-cleaned the fuel inlet passage to make sure there was nothing else to get in the seats. Blew out passages with air.
            Set AV spring at 2 turns, changed the drop to about .060".
            Rechecked float levels at 1-1/16"
            Checked functioning of the dashpot.
            Checked all settings, checked fuel wells were still secure, etc.
            Installed it, and it idles with a 11.5:1 A/F ratio, screwed in idle adjustment almost all the way, no change. (even though it originally idled at 14.5 prior to the test run where things went wrong.)

            I am baffled!!! As soon as I get my original 6518 re-bushed and new nitrophyl floats installed, I'll put it back on. It had never given me problems. But this "back up TQ" has really driven me crazy!!
             
          • Bewy

            Bewy Well-Known Member

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            Is the flooding/secondary operation fixed now?
             
          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            Thanks for checking in, Bewy!
            I think secondary operation will be OK now, but two issues:
            1) It doesn't want to drop the last little bit down to proper idle. Will close down seemingly smoothly, but it hangs up, so motor races a bit. Returns just fine with pump linkage removed. Maybe just more than the double springs I have now, or stronger ones.

            2) As I've said earlier, when first installed it metered beautifully. Then going into secondaries at WOT it apparently really flooded out. There may have been an issue with secondaries, but they seem to operate properly now. The flooding appears to be elsewhere.
            After I've pulled it apart, cleaned and double checked everything and made no changes to rods/jets, yesterday it was giving me 11.5 A/F (instead of previous 14.5) at warmed idle, still smelling rich, sooty plugs ...and the idle screws were making little difference.

            I can't locate what's causing this. Bewy, you mentioned checking the base gasket but it's the same one I used when things were working well. (The thick factory style.) I did find the tiny hole under secondary blade clogged, but cleared that.
             
            Last edited: Sep 20, 2021
          • midnight340

            midnight340 FABO Gold Member FABO Gold Member

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            I know the float levels are good, and they are new nitrophyl floats. I'll pull it apart again, check the needle seats are tight in the cover, do a pressure test, double check the fuel wells, make sure the o-rings are in their proper place, check the air bleeds are clear, and lower the fuel pressure a bit.

            This will be the second time apart after it started running so rich. I don't know what else there could be though. The weird thing is it seemed to have happened all of a sudden.
             
          • Bewy

            Bewy Well-Known Member

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            Midnight,
            The base gasket [ if incorrect ] usually causes a problem with sec operation. It will affect AV dashpot operation. If secs are working ok now, that is good, gasket ok.

            Make sure you have the correct air horn gasket. There were at least two types, difference was the divider between the pri & sec section of of the gasket.

            Rich idle. Are the little pods under the pri jets sealed properly to the black body? Leaking here will cause richness. Are the T-slots correct: 0.040" or less below the blades. The TQ has 3 air bleeds for the idle system, various names used for them, but they bleed air into the idle fuel. Make sure none are blocked. Only other thing I can think of is corrosion of the alum. If this carb has corrosion inside the alum & the corrosion is blocking passages, it might cause strange problems like you are having.

            Another thing you could try: you have a smaller 800 TQ, correct that works ok? As a last resort, use the 800 air horn on the 6322 body.
             
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            • midnight340

              midnight340 FABO Gold Member FABO Gold Member

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              Finally up and running strong! New brass bushings for the primary shaft, new nitrophyl floats, and for anyone who's interested:
              .101/.149 jets (no choke)
              Rods are 67/52/42
              .035 nozzles
              Floats set at 1"
              2turns on the AV spring
              1-1/2 to rods screw
              .060" drop on AV
              Open 17/32" from choke tower
              Idle screws 1-5/8 turns out
              Timing set at 18/38

              Settings based on the old Mopar Bible combined with recommendations from "Demonsizzler" (Lord of the Thermoquad) whose old posts I've collected from this forum and elsewhere.
              Thank you all !!!
               
              Last edited: Sep 24, 2021 at 11:46 PM
            • Bewy

              Bewy Well-Known Member

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              So what was the problem?
               
            • midnight340

              midnight340 FABO Gold Member FABO Gold Member

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              As I first said, I've been working with two carbs. My original 6518 was working well until a brass float sank, and the fact that the throttle shaft was getting fairly worn. So I decided to build a 6322 that I had. I re-bushed the throttle shaft on that one, did a rebuild kit, new nitrophyl floats, new dashpot, drilled jets to match my original, all the same settings. Installed it worked great for a test run, then started giving me the screwy symptoms, mostly in the end I couldn't get it to idle lean enough, or return to idle as it should.

              In the meantime, I replaced the bad brass floats with nitrophyl in the 6518, and re-bushed the throttle shaft it it also. Once that was done I reinstalled it (old faithful now in like new condition!) Thermoquads are such great carbs! Love the throttle response, the steady metering, good mileage, and the WOT power. Like fuel injection with no electronics!! :thumbsup:
               
              Last edited: Sep 27, 2021 at 9:47 PM
            • midnight340

              midnight340 FABO Gold Member FABO Gold Member

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              With my 6322 I'm starting to wonder if I simply had too much spring in the rod piston with the fairly big cam I have in the 340. (Solid lifter with .235/.243 @ .050) This might be the reason the 6322 had the inconsistent idle A/F #'s, going way rich at times at idle.
               
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