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Old 06-25-2012, 10:44 PM
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help with carb vacuum ports

Can anyone tell me what these vacuum ports are for? I only know the big one is for the PCV and thats it. Any help is appreciated. Carb is a quadrajet 4mv

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Old 06-25-2012, 10:54 PM
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Bottom small one is manifold vacuum.
Small one on the right upper is ported vacuum.
Smallest one is vent to charcoal filter IIRC.
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Old 06-26-2012, 12:59 PM
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great thanks. Which one would be better for a distributor? manifold or ported? there's also another small port by the choke.
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Old 06-26-2012, 01:09 PM
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Manifold or ported is an ongoing debate.
Depends on your motor. You'll have to experiment.
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Old 06-26-2012, 03:13 PM
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Quote:
Originally Posted by gsoleski
great thanks. Which one would be better for a distributor? manifold or ported? there's also another small port by the choke.
What you need to watch out for is the port used for the EGR valve. It's not suited for use as a source of vacuum to the distributor vacuum advance.

The port locations and whether they're ported or manifiold vacuum changed through the years. What I do is use a can of carb spray to shoot into the vacuum ports to see whether the vacuum pick up point inside the throttle bore is above (ported) or below (manifold) the throttle blades. If you find any that have pick up points above AND below the plates, do not use it, that's an EGR port in all likehood.

Obviously, plug any ports not being used. And the location of the ports on the outside of the carb cannot be relied on for whether the pick up point in the throttle bore is above or below the throttle blades (or both). In other words, the port above the port to the choke pull off (passenger side of the carb) is likely manifold vacuum, yet it is located (externally) above the throttle blades.

The large vacuum port in the rear of the carb (if you have one there) is usually used for the power brake booster.

More on this here. Good luck.

Last edited by cobalt327; 06-26-2012 at 03:20 PM. Reason: Add link.
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Old 06-26-2012, 04:53 PM
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On my carb and others I've looked at the small port above the fuel fitting (by the choke) is capped. I think it may be for EGR, which I don't have.

The small port on the right upper is where I get manifold vacuum. The two lower ports are PCV valve (larger fitting) and vapor recovery canister.

Bruce
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Old 06-26-2012, 04:57 PM
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Quote:
Originally Posted by 75gmck25
On my carb and others I've looked at the small port above the fuel fitting (by the choke) is capped. I think it may be for EGR, which I don't have.

The small port on the right upper is where I get manifold vacuum. The two lower ports are PCV valve (larger fitting) and vapor recovery canister.

Bruce
Traditionally, when we say "right" and "left", it as though you were sitting in the driver seat. So when you say "the small port on the right upper", to me I'd think it was the small port above the fuel fitting. But I don't think that is what you meant. Or is it?

It's often clearer to just say "passenger" or "driver" side so there's no question to what is meant by L or R.
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