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Rear vacuum port, what goes there?

Discussion in 'The Venerable Q-Jet' started by GoldBoattail455, Aug 15, 2010.

  1. GoldBoattail455

    GoldBoattail455 462 -> TH400 -> Posi

    I'm trying to sort out some issues with my carburetor and don't know where to begin. I thought it would be best to make sure all the vacuum lines are intact and are routed correctly to and from the carburetor . It is a stock '71 Q-jet I just rebuilt including a "rebush" of the throttle shaft. There is a vacuum port on the rear of the carb, what goes there? Also, there is a small square opening on the rear of the carb, does anything need to go there. The service manual isn't much help. Any help is greatly appreciated. :beer
     
    GoldBoattail455, Aug 15, 2010
    #1
  2. Steve Craig

    Steve Craig Gold Level Contributor

    Rear port was usually used as a vacuum source for the vacuum storage tank. The tank holds a vacuum to operate heat/AC controls after the ignition is OFF. It's mounted on the firewall just to the right of the regulator.
    It's shown on Fig. 13-185, p. 13-162 of the '71 Chassis Manual.
    You get into a mess with the internal vacuum lines let me know. The Chassis Manual drawings are incorrect. I have the Factory Service Bulletin with the correct revision.

    The hole/opening on the rear of the carb. is not used. Not sure what it's for, but not used in the Buick for '71.
     
    Steve Craig, Aug 15, 2010
    #2
  3. Schurkey

    Schurkey Silver Level contributor

    There is often a 1/4" NPT port at the rear of the throttle body, centered between the throttles on a Q-jet; typically used for power brakes; or power brakes and other accessories. There may or may not be other vacuum nipples on the rear of the carb.

    If I understand what you mean by a square opening--the one I'm thinking about is more rectangular, and with a screw hole top 'n' bottom--that'd be the fast idle compensator. Not used in most carbs; and the hole is not drilled all the way through. Therefore non-functional.

    Carbs that do have a functional compensator use a thermostatic vacuum device within the rectangular opening; the port is plugged until the carburetor (and therefore the thermostatic device) heat up enough, at which point the passage is unblocked and additional air flows into the manifold, raising the idle speed--and therefore the water pump and fan speed.
     
    Schurkey, Aug 15, 2010
    #3
  4. GoldBoattail455

    GoldBoattail455 462 -> TH400 -> Posi

    Steve,
    I thought it might be the climate control but wasn't sure. I have the full set of Factory Service Bulletins (Full as far as I know, we'll always find new ones) I will look there for the update. Thanks for the heads up. I know the ACC isn't working quite right and suspect a few vacuum lines are mis-routed.

    Schurkey,
    Thanks for the explanation, you made it very clear. :beers2: I couldn't find any information on it but was still worried that something was missing.
     
    GoldBoattail455, Aug 15, 2010
    #4
  5. drspencer

    drspencer Well-Known Member

    Hi Steve-Would you happen to have a diagram of the vacuum hose routing for the Ram Air cleaner on my '71 GS350?

    All I really need to know is where the Air Cleaner vacuum hose plugs into the carb.

    Thanks
     
    drspencer, Jul 1, 2011
    #5
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    The air cleaner needs full manifold vacuum, not ported vacuum. Very simple to distinguish the 2 from one another. Start the engine and let it idle. There are two nipples on the front of the Q-jet, up high. Pull the hoses off one at a time. When you pull the hose off, one of two things will happen. If nothing happens, that nipple is ported vacuum. If you hear a hiss, and the engine runs rougher, that is manifold vacuum. You want a manifold vacuum source. You can tee into the line with a vacuum tee (available at any auto parts store), if there is a vacuum hose there already.
     
    LARRY70GS, Jul 2, 2011
    #6
  7. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    That port on the back of the Q-jet carburetor is for the vacuum storage tank on the firewall. On all the OEM Buicks (at least 70 and up), that I've seen, the power brake booster has it's own port off the intake manifold, at the rear of the intake.
     
    LARRY70GS, Jul 2, 2011
    #7

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