How does the power valve work in a Holley?

The Holley power valve is a simple diaphragm with a tapered seat-valve that is exposed on the carb body side to intake manifold vacuum. When the power valve opens, it directs fuel from the float bowl into the main well in the metering block.

How do I know if my Holley power valve is bad?

Problems: 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.

How many power valves does a Holley carb have?

For this example, we’re going to use a mechanical secondary 650 cfm Holley. The Holley 650 (P/N: 0-4777-9) carb is factory jetted on the primary side with 67 main jets and a power valve with a 0.038-inch PVCR.

How do I choose a Holley power valve?

The divided number will determine the correct power valve that you need. As an example, a vacuum reading at idle of 13-inches needs to be divided by two, which results in a number of 6.5. Therefore, you should have a number 65 Holley power valve installed in the carburetor.

Do I need a power valve?

The function of a power valve is to enable the vehicle to have proper fuel mixture at idle and low to mid throttle operation. If your engine has a large cam, very little vacuum at idle and low speed, and/or you leave the line at high RPM, then you won’t need the power valve.

What causes power valve failure?

The little rubber diaphragm has a tendency to tear or just plain fail at usually the most inopportune times. It is especially vulnerable to engine backfires. When the engine sneezes back through the intake manifold, it generally will kill the power valve.

What happens if my power valve is too small?

If you run too low of a power valve value, the symptoms will be a lean transition as the power valve opens “late”. You usually have a lean popping and possibly even a backfire when you punch the accelerator. It will have poor acceleration.

What causes a blown power valve?

When the engine sneezes back through the intake manifold, it generally will kill the power valve. When this happens, the torn rubber diaphragm allows engine manifold vacuum to pull fuel through the valve, making the engine run extremely rich. Fixing a blown power valve is easy.

What does a power valve delete do?

The advantages of the power valve delete are: No more cables to adjust or break, better clearance for pipes, more simplistic. Its not that it is a bad thing, just not the best thing. We sell a lot of the block off kits and most guys are happy.

Does a Holley double pumper have two power valves?

Standard double pumpers come with two power valves.

How does the vacuum work on a Holley power valve?

Engine vacuum is what actually operates a Holley power valve. The spring that is part of the power valve is the resistance to the diaphragm that only allows it to open at a certain vacuum reading. Spring pressure is what changes the actual operating range of the power valve.

What kind of system does a Holley carburetor use?

Holley carburetors utilize a vacuum operated power enrichment system and a selection of power valves is available to “time” this sys- tem’s operation to your specific needs. Each Holley power valve is stamped with a number to indicate the vacuum opening point.

What does the number 65 mean on Holley power valve?

Each Holley power valve is stamped with a number to indicate the vacuum opening point. For example, the number “65” indicates that the power valve will open when the engine vacuum drops to 6.5″ or below. An accurate vacuum gauge, such as Holley P/N 26-501, should be used when determining the correct power valve to use.

What should intake manifold vacuum be for Holley engine?

Intake manifold vacuum for carbureted engines is generally expressed in inches of mercury, or “Hg. For example, the most commonly used Holley power valve is rated at 6.5 “Hg, which means the valve will open when manifold vacuum drops to 6.5 “Hg or lower.