Dan S

Holley Power Valve Tuning

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How to ensure your Holley carburetor is using the right power valve for your engine.

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Holley carburetors have a power enrichment system that provides fuel to the main power circuit during heavy loads or under full throttle situations. The vacuum operated power enrichment system is controlled by a Power Valve that times the operation to your engine's specific needs.

How it works:
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the main power circuit. The valve itself is a small rubber diaphragm with a small coil spring. When opened, it allows fuel to flow through a calibrated opening in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.


The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.

1) Check the manufacture date of your Holley carb.
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Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.

2) Test it using the idle mixture screws
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If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown.

High Performance Engine Power Valve Selection:
High performance engines with modified cylinder heads, long duration camshafts and single plane intake manifolds may require a change to the power valve. To find out which power valve your high-performance engine needs, you can perform the following procedure:

1) Hook a vacuum gauge to an intake manifold vacuum port.
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2) Warm up the engine and note the vacuum reading at idle. Automatic transmission vehicles need to be in the Drive position, while manual transmission vehicles can be in Neutral.

3) Divide the vacuum reading in half. The number will determine the correct power valve.
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Each power valve is stamped with a number that indicates the correct vacuum opening point. For example a power valve with the number #65 stamped on it, will open at 6.5 inches of engine vacuum. As an example, a vacuum reading at idle of 13-inches, is divided by two and results in a 6.5 inches of vacuum. Therefore, you should have a #65 Holley Power Valve installed in the carburetor.
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If you divide the vacuum reading and it falls on an even number, you should select he next lowest power valve number. For example a vacuum reading of 8-inches, divided by 2 and you come up with a number of 4. In this case you would use a #35 power valve.

Finally, if your engine produces 13 inches of vacuum or more, the stock power valve that the carburetor is equipped with from the factory, is sufficient.

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Holley power valves come in a range of orifice sizes. The higher the number the more fuel is added.

Text by: DS Media Relations
Photos taken from Holley Carburetor Installation And Tuning DVD

A Look Inside Your Holley Carburetor: Tuning Tips- PDF
Carburetor Installation And Tuning DVD

Updated 03-27-2013 at 05:10 PM by Dan S

Holley Carburetor Tuning Tips


  1. brian eiland's Avatar
    Hi Dan, are you still there?
    I'm rebuilding my 4180 carb and having trouble with EXCESSIVE fuel at idle. It appears as though my 'power valve' may be at fault. The new one that came in the GP Sorensen Kit appears a little different from the one I took out. And neither of then look like the photos you posted above.

    Plus the new one does not appear to have any ID markings on it, and the old one has 2 markings that are 'strange' (254 & 2C2).

    I posted some photos and some descriptions over on this Ford forum

    Any help/suggestions you might have would be GREATLY appreciated as I've been at this rebuild job for over a week now,...very frustrated with these holley carbs.
  2. dlrenshaw's Avatar
    Most all the Holley books and reference material I have recommend a power valve that is 1" - 1-1/2" Hg below the idle vacuum. My 302 '69 Camaro Z28 idle vacuum is 9"-10" Hg. Using the recommendation to half the idle would indicate a 4.5 to 5.0 PV. Would that not leave a hole or dead spot until the PV would tip in?

    Thanks for your input.
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