Thermoquad low vacuum readings

Fuel and Air Systems

  1. midnight340

    midnight340 FABO Gold Member FABO Gold Member

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    UPDATE: Vacuum Leak??? (all details in my thread "Thermoquad help questions.")

    Thought it best to start a new thread here.
    In my last post I was getting very low vacuum with this freshly installed 850 cfm Thermoquad 6518S I rebuilt, using a Walker kit and new pull off from Mike's. I used a Mr. Gasket adapter to my unmodified LD340 intgake. Driving with vacuum gauge inside, I was getting 5-6 in/hg with gauge T'd off the rear pull-off port.

    Forum member lemondana saw that my Mr. Gasket unit was for a Quadrajet and had less material at the back that can cause a vacuum leak. He was helpful and very kind in sending my a nice older solid aluminum adapter actually made for a Thermoquad. It is shaped to fit a TQ much better. I'm very glad to be using the right one, but in this case it didn't help my vacuum.

    I am still getting a 5-6" reading. I have listened through a small hose to all connections, and around the base of carb, etc. Sprayed carb cleaner all around to see if it affects idle. I have pinched off the vac line to my power brakes, to my PCV, to the pull-off itself, even the vac advance, everything I could find. None of these made a change.

    I get the same vacuum readings at any rpm.
    I have checked my vacuum gauge with a Mighty Vac and it is spot on.
    I AM supposed to be getting full vacuum at the rear port aren't I???? If so, what is going on here? My A/F readings:
    At idle I am adjusted to 13.5 to 14 or so.
    At 2000 rpm on the highway A/F is 16.5
    Any part throttle acceleration goes to 17:1
    This is a '73 440 carb that has the same jets and rods as were used in the '73 340's.
    I shouldn't be this lean. ANY IDEAS???

    Edit: Also, I do have the choke flap zip tied open and the link to the secondary air valve removed. Seemed the best way to disable the choke.
     
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    • Dana

      Dana FABO Vendor FABO Vendor

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      Don't be scared of 16 or 17: 1 with a TQ or Q-jet. Remember atomizing the fuel is the name of the game. And those 2 carbs do it so well, they can get away with those numbers. If it starts to studder up around 17 on tip in, stretch your metering tree spring just a little. As far as the vacuum... I got nothin.
       
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      • midnight340

        midnight340 FABO Gold Member FABO Gold Member

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        Forgot to add that with this cam and the Edelbrock 750 before my vacuum was around 11 in/hg.
         
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        • midnight340

          midnight340 FABO Gold Member FABO Gold Member

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          That surprises me! But I appreciate the info and advice. I had the 340 running the best it ever has with the 750 after much tuning. So when something is perfect that’s a call to mess with it right?!! :D

          I’ll figure this out, but thought if anyone knows something I don’t, it’ll sure help.
          (This forum is a great resource and helps so many of us!!)
           
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          • yellow rose

            yellow rose Doctor of Thinkology.

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            As long as the ignition will fire that lean A/F ratio there is no reason not to get it that lean.
             
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            • yellow rose

              yellow rose Doctor of Thinkology.

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              You had 11 inches of vacuum at a cruse or at idle??
               
            • rumblefish360

              rumblefish360 So close, yet so far away

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              Can I get a picture of each of the 4 sides of the carb please.

              Spraying carb cleaner around the base yielded no results?
               
            • midnight340

              midnight340 FABO Gold Member FABO Gold Member

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              11” at idle. (Of course with a little WOT I could get it down to 1-1/2”! :D)

              No results spraying carb cleaner. I may try again later to see if I missed anything but I don’t think so. And I’ll get some photos a little later.
               
            • rumblefish360

              rumblefish360 So close, yet so far away

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              Thanks. I’m having trouble wrapping my head around this problem.
               
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              • pishta

                pishta I know I'm right....

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                question: Your tapping vacuum off the choke pull off, you stated the vacuum reading is the same at any rpm? (I found it is manifold vacuum *)
                2. Your idle AFR is too rich. it should be on the leaner side and your part throttle should be closer to stoich but that may be because if your vacuum 'leak' (your AFB was getting better idle numbers)
                EDIT>> choke pull off vacuum is manifold vacuum, straight shot to plenum but though a restriction!<<<

                * Lots of info about this port and its restrictions and its sources at 4:30

                listen at 1:40 for leaner running on a TQ.

                 
                Last edited: Nov 10, 2019
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                • midnight340

                  midnight340 FABO Gold Member FABO Gold Member

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                  Gotcha. I suspected I needed to get vacuum readings elsewhere. The new adapter has an extra port so I’ll find a fitting to pull off that.

                  Secondly, do I have to run a choke of some kind for all this to work right? If not (and I’m ok with not having one) then what’s the best way to disable the choke?
                   
                • yellow rose

                  yellow rose Doctor of Thinkology.

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                  Me too. I can't tell if he's talking idle vacuum, cruise vacuum or WOT vacuum.
                   
                • pishta

                  pishta I know I'm right....

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                  The choke hurts nothing and there are a grip of linkage parts attached to the secondary door buffer (choke pull off) so I'd run the choke even if its never 'applied'.
                   
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                  • rustycowll69

                    rustycowll69 Well-Known Member

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                    cool video. that TQ in the video is so cool. It looks like a jewel, it is so clean and shiny.
                    to the OP, keep looking for vacuum leaks. Maybe the primary throttle valves are not closed enough, and/or maybe the secondary throttle is not fully seated. It's kind of a long shot, but maybe the throttle shafts are too worn.
                    send some pics, if you can figure out how to post them.
                     
                  • midnight340

                    midnight340 FABO Gold Member FABO Gold Member

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                    Currently I have the choke tied open like this..

                    F31A246A-069C-4B08-973E-BBAB19D50857.jpeg
                     
                  • midnight340

                    midnight340 FABO Gold Member FABO Gold Member

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                    If I need to run a choke how do I do that? With the LD340 I can’t use a stock type manifold choke.

                    I’m going to pull a reading off manifold. Maybe there’s no problem.
                     
                  • pishta

                    pishta I know I'm right....

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                    Hmm. i dont know if that is defeating anything related to your issue. Does it not work or is it not propped open during hot idle? OK, just read your post. If the TQ requires a rod to the OEM choke mechanism to apply, then it should not require a strap to keep it open, just a lack of 'pull' from a divorced choke. I dont have one on the wall anymore so I cant tell if the choke is sprung on carb. TQ guys: does the divorced choke push/prop the choke open when hot? If so, then wire that rod up.
                     
                    Last edited: Nov 10, 2019
                  • midnight340

                    midnight340 FABO Gold Member FABO Gold Member

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                    OK, with gauge pulled off the front of the adapter I now have my 11” idle vacuum as is normal with this cam. Sorry for any confusion, and thanks for the help.

                    Once I get the choke disabled the correct way I’ll be getting somewhere!!
                     
                  • midnight340

                    midnight340 FABO Gold Member FABO Gold Member

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                    Idle now in the 14.5 to 15 range.
                    11” idle vac, around 16” at steady cruise. Tweaked the rods screw a bit, so cruise A/F now just a bit richer, at 15.8-16 or so. Getting there!

                    Still goes lean (17.5-18) giving it some mid rpm throttle. This motor really liked Edelbrock’s strongest springs. Maybe here is where I stretch the spring a bit like dana suggested?
                     
                  • rumblefish360

                    rumblefish360 So close, yet so far away

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                    Move the tree upwards via the center screw to enrichen the mix. What your doing there is pulling the rods upwards out of the jet.

                    You may simply needs bigger jets up front.
                    Or thinner rods to hang on the tree.
                     
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                    • midnight340

                      midnight340 FABO Gold Member FABO Gold Member

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                      I couldn’t get the jets loose when I rebuilt it. Didn’t want to break anything. I’ll check my other parts carbs and see what rods I’ve got. (I’ve got lots of Edelbrock rods if I have to go that route.)

                      Probably done for a couple of days with Arctic front coming in. 61 now, changing to highs of 35 with lows around 13 degrees. Brrr...
                       
                    • rumblefish360

                      rumblefish360 So close, yet so far away

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                      We are getting the same front two days. Cold is right!
                       
                    • midnight340

                      midnight340 FABO Gold Member FABO Gold Member

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                      Low vacuum issue solved (didn't know whether to start a new thread or not....) So here is where I am now.
                      I have my 800 rpm idle A/F at 14.8-15 ratio
                      Cruise when steady at 15.8-16
                      WOT right at 12-12.5

                      So that is all good. but giving it any throttle mid rpm at it drops to 17-18 or leaner.
                      I have the rods raised 2 1/2 turns up
                      I did try the nozzles I drilled to .031" and it seemed to give it a bit more umph off idle.
                      Pretty sure my air valve spring is at 1-1/2 turns according to one set of instructions as to a good starting place.

                      My only concern is that it goes so lean with mid rpm throttle application. What next? Any ideas???
                       
                    • rumblefish360

                      rumblefish360 So close, yet so far away

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                      Different rods and/or jets.
                       
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                      • midnight340

                        midnight340 FABO Gold Member FABO Gold Member

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                        8A6A3EA5-2251-4536-993F-9DAB40E5D6FF.jpeg OK Thanks. Just checking. Wanted make sure I’m on the right track. I did just get these. Mine are .098 so they are one step bigger.
                         
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