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New edelbrock AVS2 vacuum port is question

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  • Fuel System: New edelbrock AVS2 vacuum port is question

    Installed a new 500 CFM edelbrock 1901 on my 289. Running well but there are 2 vacuum ports. A timed port that has vacuum at higher RPM and a port with constant vacuum. The mechanic hooked it up to the constant vacuum. However from what I had read that may be wrong depending on your distributor. What advise is there for a stock 289?

  • #2
    From the Stude factory, "full" vacuum was used.
    At the time of the original Studebakering, there was "no" such thing as ported vacuum locations on carburetors. That came to be during the "gasoline" woe's of the late 70's.

    If you need, drop the idle speed to your liking.
    The bad thing about "full" vacuum...is that you do not get the advantage of the full (vacuum) advance in the distributor...when you need it..!
    During full vacuum, when you hit the throttle, the vacuum "drops" very quickly and slowly comes back up to a point.

    With "ported" vacuum, when you hit the throttle, the distributor (vacuum) advance "rises" to provide a little more power. Remember to a point, ignition advance...is power and mileage. When you let off the throttle to cruise, the vacuum drops a little to "around" the same location as with the full vacuum .

    While under normal circumstance, ported is an overall better way to go, it's best to experiment a little. This WILL require a little initial advance (moving the distributor) manipulation to get the most out of each port experiment.

    Mike

    P.s. - Buying the new AVS over the others was a good decision.

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    • #3
      Not sure instructions are correct. I had mine hooked up to the wrong one: it runs much better hooked up to the left one as you face it. I'll double check and if I got it backwards (again!) I'll let you know.

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      • #4
        Above the butterfly center line, "ported" vacuum.
        Below the butterfly center line, "full" vacuum.

        Mike

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        • #5
          Thank you all--I appreciate the help.

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          • #6
            If it's the same as the 1406 on the table in front of me. The left is ported (above throttle plate), the right is unported (full vacuum).
            sigpic

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            • #7
              The honorable Mike V wrote - "At the time of the original Studebakering, there was "no" such thing as ported vacuum locations on carburetors. That came to be during the "gasoline" woe's of the late 70's."

              That idea seems to have started by posts by John Z, ( John Hinckley) Corvette enthusiast and a GM (manufacturing) engineer. As best I can tell, his original statements are just that black-and-white. Pre emissions = NO ported vacuum advance.

              Attached is the cover of an Audels manual from the 50s or maybe even 40s. Hardly the emission era. Also attached are two pages from the section discussing "automatic spark control."
              Two points -
              1 Their description of carburetor side and engine side refers to where the vac port is relative to the throttle plate when the throttle is completely closed. When they say "above" or "below" the throttle plate, they seem to be are referring to updraft carburetors, NOT to downdraft carburetors which i think will be the carburetor style in just about any discussion on this forum. "Engine side" would be full manifold vacuum all the time. "Carburetor side" would be ported vacuum, which is atmospheric pressure ( not manifold vacuum) until the throttle plate/butterfly is opened just a little bit to expose the port to manifold vacuum.
              2, Note in Fig 6, the stated "most popular application of the vacuum principal" is the "carburetor side" or ported vacuum signal.

              Many factory shop manuals of the 50s and 60s provide similar info. Some manufacturers used ported vac , while some used manifold vacuum.

              Attached Files

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              • #8
                I've been following this discussion in several threads now and I really don't know where the idea comes from that ported vacuum didn't start until the 70's. Every car I've ever worked on all the way back to the 40's has used ported vacuum for the vacuum advance. The first and only time that I ran into a full vacuum to the advance was in the early 60's when I was working on a big block Chevrolet. It kept kicking back against the starter when trying to start it and as a result wouldn't start. Can't remember what I did to resolve that problem. The problem was that as soon as it would try to start the distributor would pull to full advance and it would kick back. All I know is that at least on Studebakers you always use the ported vacuum to the vacuum advance.

                Just reread the first reply in this thread. While it's probably true that early up draft carburetors didn't have ported vacuum ports the cars did have manual spark control levers in the steering wheel. The oldest Studebaker I've worked on is 1942 and it has ported vacuum to the advance.
                Roger List
                Last edited by Roger L.; 09-22-2019, 04:32 PM.
                Roger W. List
                Proud Studebaker Owner

                sigpic

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                • #9
                  I've been running my 289 (R1, over bored) with the vacuum spark modifier of the HEI distributor connected to full vacuum for years and it runs great. My total advance is set at 45 degrees.
                  sigpic

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                  • #10
                    I will need to double check, but the Avanti R1 has indents in the throttle plates, so has vacuum at idle. The WCFB does not. I am not sure about non R1 AFBs. I can check in the morning.

                    BTW, it is not ported vacuum, it is timed vacuum. Ported vacuum is something else entirely and makes use of the Bernoulli effect.
                    78 Avanti RQB 2792
                    64 Avanti R1 R5408
                    63 Avanti R1 R4551
                    63 Avanti R1 R2281
                    62 GT Hawk V15949
                    56 GH 6032504
                    56 GH 6032588
                    55 Speedster 7160047
                    55 Speedster 7165279

                    Comment


                    • #11
                      Just checked 3540 and 3507. Both have the same indents in the butterfly valves. So there should be at least some vacuum at idle.

                      in any case it really does not make much difference. If you use the timed port, there will be vacuum just off idle.
                      78 Avanti RQB 2792
                      64 Avanti R1 R5408
                      63 Avanti R1 R4551
                      63 Avanti R1 R2281
                      62 GT Hawk V15949
                      56 GH 6032504
                      56 GH 6032588
                      55 Speedster 7160047
                      55 Speedster 7165279

                      Comment

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