1. ofb383

    ofb383 Member

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    Hi guys, carrying on from my PCV question the other day, this is my Holley and manifold - front and rear views.
    On the rear I have been using the full manifold vacuum for the booster, the one blanked off, should this be for the PCV ?
    On the front I have been using the small bottom port for the Vacuum advance on the dizzy, is this correct ?And on the front there is a port unused with a black bung on it, what is this for and would it be ported or full vacuum ? Carb rear.jpg Carb front.jpg
     
  2. traintech55

    traintech55 Senior Member FCBO Gold Member

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    Picture # 1, You have your vacuum hose for the power booster in the correct place, the small port is for your heater/ AC. The blocked off port should be the one to use to hook up the PCV system. Also could you take and post a picture of your throttle linkage, it looks like it could be at a bad angle in this picture. It could cause you the damage your transmission.
    Picture # 2, Any vacuum port above the throttle blades should be ported, and correct for use on the vacuum advance to the distributor. You are using the lower port, you need to hook up a gauge to this port and see if you have vacuum at idle. If you do move it to the blocked off port.
     
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  3. detmatt

    detmatt Old Man with a Hat FCBO Gold Member

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    I don’t think the intake is installed Yet in these pics.
     
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  4. ofb383

    ofb383 Member

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    Correct, it's not on the engine yet :lol:
     
  5. CBODY67

    CBODY67 Senior Member

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    In general, you can look a the size of the vacuum hose and then determine where the various ports might need to go.

    The larger ones are for the pcv or power brake booster. GM usually ran their power brake boosters directly from the rear of the carb, whereas Chrysler generally used one of the rear intake manifold runners. The runners might be a better location as it might better insulate the brake booster from various hot soak fuel vapors . . . just a theory of mine. IF the rear "big" port is screwed in, Holley usually supplied a pipe plug to block that port.

    The port on the side of the primary metering block, on a Holley, is usually for the ported vacuum advance. Which means slight to no vacuum pull at hot base idle, only when the throttle is opened should hit have a significant amount of vacuum present. On the front bottom, there usually is a large pcv-sized port there. The smaller ones can be for ported (one) and full manifold vacuum (one) for various other uses. The manifold vacuum port can be used to run the air cleaner shorkle valves and/or provide vacuum to the emissions charcoal canister.

    On some cars, usually GM/Chevy V-8s, they'd use one of the rear intake manifold runners for their main vacuum source and then plug a "vacuum tree" into that fitting. That would supply vacuum for the hvac system, cruise, air cleaner, trans vacuum modulator, etc. I don't recall Chrysler doing that back then, though, as their vacuum hose maps were not that complicated.

    When you have a multi-fit carb, as you do, you usually end up with more ports that are needed. Which is why some might be blocked off. Using a pipe plug for the rear, large port can be more reliable than a rubber hose (and look better, too). The smaller ones can be capped, as pictured, but using a better rubber plug can be important (rather than the less expensive vinylized compounds. The better ones are usually in the install kits which Holley supplies with their carbs, plus probably separately from their online store.

    The vacuum level differences in the ported and full manifold vacuum (smaller) ports will be quite apparent when the engine is at hot base idle. When we got the evaporative emissions systems, plus some others, on the mid-later '70s cars, things got more complicated, but they all ran of either ported or manifold vacuum signals, some with a pressure bias between them (as the purge valve on the top of the emissions canister. Add in the thermo-vacuum controls (for EGR) and it got more complicated. But once you understand the hose diameters are usually specific of where they go and which (ported or vacuum) they run off of, it can become more simple. I mention this as a point of reference as I suspect your application has none of these items present.

    Enjoy!
    CBODY67
     
  6. 70bigblockdodge

    70bigblockdodge Old Man with a Hat FCBO Gold Member

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    Put the one on the rear of the carb to PCV that way any oil vapor can be mixed will fuel and distributed evenly.
    Booster to fitting in runner as it is clean "air"
    Lower front nipple is where vacuum guage goes to tune the engine.
    The small one on passenger side of the metering block is for vacuum advance to increase your MPG by more complete combustion at part throttle, and thereby saving excess fuel wash on your Piston rings increasing the life expectancy of your engine.
     
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