Vacuum Secondaries

Discussion in 'Small Block Mopar Engine' started by DadsNewToy, May 13, 2018.

  1. DadsNewToy

    DadsNewToy Member

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    I've got a 340 with a Holley 750. I was disappointed with the kick when I punched the throttle. I discovered with the help of a bobbie pin that the vacuum secondaries are not opening. I adjusted the throttle cable and gained more petal travel. The difference was amazing but the secondaries are still not opening. I've looked at Holley videos but still not 100% sure the vacuum lines are correct. The PCV valve hooks to the btm rear of the carb? What do I need for vacuum pressure to actuate? Is the 750 too big. Any suggestions are appreciated.
     
  2. rumblefish360

    rumblefish360 so close yet so far away

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    For what it’s worth, and a static test has the car seats and you hover over the top of the carburetor opening throttle wide, the secondaries would be challenged to be opened. As you’re traveling down the road the vacuum secondary carburetor should be a smooth opening of the secondary side. The kick! That you mention comes from the downshift of the transmission.

    The vacuum pod contains a spring which you can swap out for a lighter or heavier spring.
     
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    • DadsNewToy

      DadsNewToy Member

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      I used the trick where you place a paper clip or bobbie pin on the shaft coming from the vacuum pod. If the secondaries are working the clip will be shoved down the shaft as the secondaries open. Took the car for a drive with a couple dead stop launches but the clip never moved. Are the springs colour coded?
       
    • yellow rose

      yellow rose Doctor of Thinkology.

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      This is why I don't like those carbs. I've been on the dyno with them and they won't open the secondaries until way past 7k. That was a big block Chevy. Most of the time they won't open until it's way too late, even with the lightest spring in them.

      I call them boat anchors and catch hell for it.
       
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      • xLURKxDOGx

        xLURKxDOGx "An angel fat, at satan's feast"

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        Here you go.

        Jake

        [FOR SALE] - carter afb competition carb 750 cfm
         
      • DadsNewToy

        DadsNewToy Member

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        I've got a Holley 650 double pumper with mechanical secondaries from a previous 340 many years ago. Worked great in the 1973 Duster it was in. A rebuild kit is about 80 bucks. Option number 2.
         
      • DesertRat

        DesertRat I don't care it's all psychobabble rap to me. FABO Gold Member

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        They sell a quick change secondary spring adaptor and spring kit. I got one for mine and when you follow the instructions the secondary opening is seamless.
        The vacuum required to open is internal on the carburetor, I suspect you have a compromised secondary diaphragm. Without the QC kit, they are a mother to
        get to but I find the carburetor as a whole to be easily tunable. Good luck!
         
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        • Mattax

          Mattax Just the facts, ma'am FABO Gold Member

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          They are only supposed to open when the airflow in the venturi is moving fast enough. Your engine has to be consuming enough air to do that. Stomp on it at 45-55 mph and take it to passing speed.

          The signal can be improved by using a tube into the venturi like Holley did at least into the '70s for the 700 cfm and larger carbs.
          see here: Vacuum Secondary Diaphragm Signal Tube
           
          Last edited: May 14, 2018
        • rumblefish360

          rumblefish360 so close yet so far away

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          FWIW, the Carter/Edelbrock is a mechanical secondary carb but above it is a air velocity valve designed to open along engine demand. The secondaries always open unless the choke is on or not fully opened. The velocity valve can cause a big if you stomp on it to early and if the engine speed isn’t up to consuming it all.

          While they are as super tunable as a Holley, they can perform very well. 750 cfm is fine for the 340.

          Springs for the Holley are color coded.
           
        • 70aarcuda

          70aarcuda Master Hoader of SBM FABO Gold Member

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        • 70 DUSTER CRATE

          70 DUSTER CRATE Well-Known Member

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          IIRC, If this is not a new carb. and someone has taken the secondary spring housing off to change springs, they may have lost the small cork gasket seal. It looks like a cheerio. Without the gasket you haven't any vacuum to operate the unit.
          To prevent this problem get a quick change spring unit, with a new gasket, like DesertRat said or what 70aarcuda suggests. Hope this helps.
           
          Last edited: May 14, 2018
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          • krazykuda

            krazykuda FABO Gold Member FABO Gold Member Technical Editor

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            Get the holley vacuum secondary spring assortment and install the short yellow spring.... It will allow the secondaries to open much sooner...
             
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            • nm9stheham

              nm9stheham Well-Known Member

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              As noted above, the external vacuum lines have nothing to do with it; the vacuum passage for this is internal. The above small ring gasket could be lost or cracked, plus the vacuum diaphragm inside the vacuum pod could be punctured/torn, or installed wrong....as mentioned.

              There is a rod from the primary throttle shaft that links from a cam on one end of that shaft and links to a cam on the secondary shaft; this is the lock-out that makes sure the secondaries are closed at idle and is on the left side IIRC. Make sure that linkage is all in place properly and not binding; you can rotate the primary by hand and see where it releases the secondary so that it can move IF enough vacuum signal in the primary throats is present. While checking this, open the primaries fully by hand and then turn the secondary shaft lightly by hand; you ought to feel a bit of resistance (that is the vacuum pod's airflow restriction fighting you) but the secondary shaft should turn smoothly almost 90 degrees; when you let got, it should close by itself.

              These will not typically open at low RPM's but only at high RPM's. So dead stop WOT without running the revs way up will not open them, especially on a larger one.
               
              Last edited: May 14, 2018
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              • Mattax

                Mattax Just the facts, ma'am FABO Gold Member

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                To see if a vacuum secondary system is working at all (without driving or even running engine). Use shop air.
                Open the primary throttle.
                Shoot air down the primary venturi closest to the vacuum pod.
                 
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                • roccodart440

                  roccodart440 Well-Known Member

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                  This /\ /\ /\

                  The750 isn't too big. There is no reason to ever run vacuum secondary carbs. Once you get over that metal hurdle of the old school "vacuum sec for street, mech sec for strip" you will be much happier.
                   
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                  • rumblefish360

                    rumblefish360 so close yet so far away

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                    Vacuum secondary = General Purpose
                    :lol:
                     
                  • Hot Metal

                    Hot Metal '69 440 Cuda

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                    Yeah...general purpose and SLOW:(:D......we ran a 750DP on my mild .484 cammed '71 340 on the street and strip...was 2/10ths slower than an 850DP when we stepped things up some. Tried a 750VS just the once to see the difference and it was total rubbish compared, we even came across a 1050 Holley 3bbl which we ran on the street for a while but even that wasn't up to it. Can't beat a well tuned DP in 800>850 or even a proper 950 depending on how race oriented your combo is for general purpose use.....my2c's
                     
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                    • furrystump

                      furrystump Well-Known Member

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                      Just as an side note I was curious at what rpm the air door on my 800 thunder series started to open and how long it took to open fully. I rigged two leds and some phone wire, had the wire just touching tge door when it was close and the second led wire would just touch the door when open full. I was very suprised to see it starts to open at 2500 and is fully open at 3000 rpm. Taking a fraction of a second. I video taped the lights next to the tach. MUCH earlier and took so much less time to open fully! 500 rpm!
                       
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                      • furrystump

                        furrystump Well-Known Member

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                        I launch the car at 2500rpm so it’s fully open before I past the tree! Probably why I didn’t lose any going from mechanic secondary to the AVS.
                         
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                        • krazykuda

                          krazykuda FABO Gold Member FABO Gold Member Technical Editor

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                          600 vacuum secondaries....

                           
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                          • furrystump

                            furrystump Well-Known Member

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                            Go up a 1/2 to a full turn on the right front torsion bar. See if the rear doesn’t settle to the right as much. Could be the road or steering imput.
                             
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                            • roccodart440

                              roccodart440 Well-Known Member

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                              Am I missing the point here?
                               
                            • General Disarray

                              General Disarray Well-Known Member

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                              Maybe you do need a touch lighter spring for the secondaries to open at just the right time.
                               
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                              • inertia

                                inertia Well-Known Member

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                                There are some exce!llent vids at Holley on all aspects of tuning including proper secondary opening. As with most things, familiarity, trial and error, lotsa experience, the vacuum secondary can be very well tuned.
                                I also recommend starting with the short yellow spring. Be aware of the small ring gskt.
                                Be prepared to grin.
                                 
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                                • Mattax

                                  Mattax Just the facts, ma'am FABO Gold Member

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                                  The bigger venturis of the 750 and up often can use the tube extension for stronger earlier signal.
                                  Holley isn't going to do a video of that. lol :rolleyes:
                                   
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