No vacuum coming from "manifold vacuum" port on QJet

Discussion in 'The Venerable Q-Jet' started by Smartin, Dec 22, 2018.

  1. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    I've never encountered this before...the top right port (about 1/4" round) is usually where I find full vacuum at idle. This carb has NONE coming from it. I'm concerned it is causing it to run poorly...and I don't have a known good carb to swap and test. Prior to running the car, I pulled the carb apart and cleaned it up really well...and installed new gaskets. They were exactly the same as the ones that came out of it.

    It's a 72 Stage1 carb on a 455 that sounds like it has a bit of a choppy cam...but nothing huge. There was no spec provided with the car, so it's all a guessing game at this point on what has been done to it. Supposedly Rick Henderson built the engine, so he may know. I don't know how to get a hold of him anymore.
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Strange. The only thing I can think of is the carburetor to manifold gasket may be blocking something. Have a look at the bottom of the carburetor and see if you can spot something obvious.
     
  3. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    It's wide open with no restrictions at the base gasket where it feeds.
     
  4. techg8

    techg8 The BS GS

    is the nipple clogged?

    theres not much to that port and its passages.
     
  5. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    I'll take a look at that tomorrow. I agree...seems perplexing.
     
  6. Cliff R

    Cliff R Well-Known Member

    "Top right port" above the vacuum supply to the choke pull-off? This would be the upper vacuum fitting in the main casting, left side as viewed from the front? I'm assuming that is the port in question as it's in the same passage as the vacuum supply to the primary pull-off. If the lower port has vacuum the upper should as well, provided something isn't blocking it......Cliff
     
  7. BYoung

    BYoung Stage me

    Incorrect main body to base gasket possibly?
     
  8. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Possibly...I'll look at it again when I get back to the car.

    Yes, Cliff, the top right port (the large one).
     
  9. carmantx

    carmantx Never Surrender

    Make sure it is not in too far in the body. It can bottom out against the back wall. The port is only about twice as deep as the markings on this vacuum port. The area does not extend all the way to the primary like it would look on the outside. So if your port is shorter out of the body than in the picture below, try pulling it out some.

    2018-12-23 11.11.01.jpg 2018-12-23 11.08.07.jpg
     
  10. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Thanks Mark
     
  11. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    Here are some photos of the carb in question. Looks like doesn’t have a way to get manifold vacuum via this particular setup. I’m thinking wrong base?
     

    Attached Files:

  12. BadBrad

    BadBrad Got 4-speed?

    Maybe you have a full race cam; switch over to a three quarter race cam to get back 25 percent of your original vacuum.
     
  13. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    I just swapped the gasket to one that looks like it matches the carb a bit better. And it has a straight shot to full vacuum to that port. Will test when I get the rear back in the car.

    Brad, I can’t say I didn’t think about the cam:D
     
    BadBrad and techg8 like this.
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Hmmn, just as I suggested back in post #2:)
     
  15. techg8

    techg8 The BS GS

    From the photos I'd say you're correct. The fuel bowl gasket with the big hole spread more to the center of the carb is the better choice.
     
  16. Smartin

    Smartin antiqueautomotiveservice.com Staff Member

    And that is where I started looking! I figured it had to be something simple. I've never done that before...but all I did was replace the one that was in the carb with the exact same one. Figures. Who am I to judge a pro rebuild?
     
  17. carmantx

    carmantx Never Surrender

    Here is gasket that should be correct for a 72 Stage 1 carb.
    https://quadrajetpower.com/throttle-body-gasket-tb-818/

    The base has the rear vacuum port for rear auxiliary vacuum brake that Buick had. The primary throttle blades didn't have holes like these, so those must have been added by someone. Those were common on the Oldsmobile quadrajets.
     

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