Carb vacuum port ID

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by: Cobalt327
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[edit] Overview

This page will help to identify the different vacuum ports that are on most if not all carbs, excepting marine applications.

[edit] Types of vacuum ports

There are basically three types of vacuum as it applies to an internal combustion gas engine. You will want to be able to identify a ported vacuum source from a manifold vacuum source from an EGR vacuum source.

Ports are also supplied on most carbs for the PCV system and for a power brake booster.

[edit] Ported vacuum

A ported vacuum source will register vacuum only when the throttle blades are opened past idle.

At idle there should be no vacuum or very nearly so, depending on how far the primary throttle blades are opened to allow the engine to idle. Engines having big cams and a low vacuum signal at idle will often have the primary throttle blades opened too far, causing a vacuum signal at the ported vacuum port(s). This can cause "nozzle drip", a harsh odor of exhaust, and a poor idle quality. This can be fixed by supplying the engine with bypass idle air sufficient to allow the throttle blades to be returned to a normal position. More on this [link to carb article].

[edit] Vacuum advance

One use is for the distributor vacuum advance. There is always debate on which type of vacuum for the vacuum advance is "best". The simple truth of the matter is there are reasons for either to be used, depending on the situation. While ported vacuum for the vacuum advance is often thought to be for "emissions" applications, in certain cases there are sound reasons for a performance engine to use ported vacuum.

A brief Crankcase Coalition article on vacuum advance can be found here. A thread at on ported vs. manifold vacuum for the vacuum advance is here

[edit] Manifold vacuum

A manifold vacuum source will be there all the time- at idle and when the throttle blades are opened.

[edit] PCV vacuum ports

EGR port on Q-jet from a Pontiac

The PCV is often connected to the large front vacuum port in the carb throttle body. Its position there allows the crankcase vapors that are pulled from the engine by the PCV system to be shared equally between both primary throttle bores. The primary side is used instead off the secondary because the primary side is almost always flowing a good quantity of air so the vapors will be well mixed before entering the combustion chambers to be burned.

[edit] Power brake booster vacuum port

The large rear vacuum port on the throttle body (often threaded with pipe threads for a female fitting to connect a steel line) is used for the power brakes. Or a vacuum port located on the intake manifold can be used.

[edit] Transmission modulator vacuum port

If you have an automatic transmission that uses a vacuum modulator, this also uses a manifold vacuum source. Often the factory used a fitting with a restricted orifice that was fitted to the intake manifold for this but you can use a port on the carb.

[edit] EGR vacuum

The EGR port will not have as much vacuum at idle as a manifold port- but it will have enough vacuum to register on the gauge, unlike a ported vacuum port that will show less or no vacuum at idle (unless the primaries are open too far due to a low vacuum cam, or from bad adjustments). Because of where the EGR port picks up its vacuum signal (from the venturi as opposed to just above or just below the throttle blades), an EGR vacuum port may tend to show more vacuum on the gauge as the engine speed/throttle angle increases.

Another way to ID the EGR port is to blow smoke or carb cleaner spray (or WD-40, etc. if the carb is apart and can be easily cleaned) through the vacuum ports until you find the one that has the discharge coming from an orifice that's in the venturi area of the carb.

[edit] Unused vacuum ports

Once everything that needs vacuum is plumbed, plug any remaining vacuum ports w/good quality caps- the parts store "HELP" aisle caps are junk for the most part, try to find either OEM plugs or good quality/silicone plugs.

[edit] Carburetors

[edit] Rochester Quadrajet

It should be noted that Q-jet carbs prior to the introduction of the vacuum operated EGR system obviously will not have an EGR vacuum port. These carbs having an EGR vacuum port are mainly going to be from about ’71 or so and newer.

There is one vacuum port on many Q-jet carbs that was used for the EGR valve. It is not suitable for use as a manifold or ported vacuum source. One problem in locating the EGR vacuum port on a Q-jet is, the EGR vacuum port is not always in the same location on all Q-jet carbs. It is often the port on the front passenger side of the carb, in the throttle body nearest to the idle mixture screw. If you don't have a port there, or you want to check it, one way to tell is to use a vacuum gauge- which you should be using anyway, come time to set the idle mixture screws, etc. The EGR port will sometimes have a larger diameter than the other vacuum ports (except the very large PCV and power brake ports), so it's a good practice to test those larger-sized ports for being an EGR port first.

[edit] Quadrajet carb numbers that will likely have an EGR port:

• 1971- 7041xxx

• 1972- 7042xxx

• 1973- 7043xxx

• 1974- 7044xxx

• 1975- 7045xxx

• 1976-’79- 1705xxxx

• 1980-up- 1708xxxx

[edit] Edelbrock carbs

Manifold vacuum is on the driver side bottom, ported on the passenger side bottom.

[edit] Holley carbs

Many Holley 4150/4160 carbs will have a ported vacuum port on the side of the metering block. The other vacuum ports will usually be manifold vacuum.

[edit] Engine diagnosis using a vacuum gauge

[edit] Table of diagnostic vacuum readings

Smooth and steady idle
(800 to 1200 RPM)
Between 17 to 21 inches Engine is in good condition, but perform next test to be sure.
Open and close throttle quickly Jumps from 2 to about 25 inches Engine is in good condition.
Smooth and steady idle Steady, but lower than normal reading Worn rings, but perform next test to be sure.
Open and close throttle quickly Jumps from 0 to 22 inches Confirms worn rings.
Steady idle Intermittent dropping back 3 to 5 inches and returns to normal Sticky valves. If injection of penetrating oil into intake manifold temporarily stops pointer from dropping back, it's certain the valves are sticking.
Steady 3000 RPM Pointer fluctuates rapidly, increasing engine speed causes more pointer swing Weak valve springs.
Steady idle Fast fluctuation between 14 to 19 inches Worn intake valve stem guides. Excessive pointer vibration at all speeds indicates a leaky head gasket.
Steady idle Constant drop Burnt valve or insufficient tappet clearance holding valve partly open or a spark plug occasionally misfiring.
Steady idle Steady 8 to 14 inches Incorrect valve timing. It must also be remembered that vacuum leaks and/or poor compression can result in a low vacuum reading.
Steady idle Steady 14 to 16 inches Incorrect ignition timing.
Steady idle Drifting from 14 to 16 inches Plug gaps too close or points not adjusted correctly. Damaged plug or coil wire

[edit] Resources

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