MOTORCRAFT/AUTOLITE & HOLLEY

These are examples of typically Holley and Motorcraft powervalves with their corresponded gaskets. Pay close attention to the style on the far right. The gasket needs to be an open circle that sits around the outer edge of the mating surface. If you use one of the gaskets on the two left power valves, you won’t get a proper seal. While you can use the right gasket on the two left powervalves, it’s extremely difficult to insure the gasket is perfectly centered to prevent leaks.

Power valves used to blow out anytime an engine back fired, but today's power valves are designed to last.

Under normal circumstances the power valve size in our carburetor kits are the correct size.

Under not so normal circumstances such as a built up engine, multiple carburetors the vacuum will be reduced and will require a power valve that opens up quicker.

The power valve stays closed until it reaches it's rated HG. In the example above the power valve won't open until the vacuum drops to 8.5HG.

High performance and engines with multiple carburetors will have a lower vacuum reading thus requiring a lower power valve.

WHAT POWER VALVE SIZE DO YOU NEED?

SINGLE STAGE POWER VALVES

Determine your power valve size by connecting a vacuum gauge to the manifold and read the HG under power (hold the break on and power up) and divide by 2 and round to the next .5 For example a reading of 15 HG would require a power valve size of 7.5 HG.

TWO STAGE POWER VALVES

Two stage power valves are not adjustable. The screws in the ends are there for the factory to set them. You tune a power valve by replacing the existing power valve with one of higher or lower hg rated valves.

When the engine is at idle the vacuum is at the highest and the power valve (enrichment valve) is closed. As the engine speeds up vacuum drops and the power valve opens allowing more fuel into the carburetor.

On a 2 stage power valve such as these, the 1st stage opens up at a certain vacuum, i.e. 7,0 HG. This is at partial power. At close to full power the vacuum drops to very low HG, i.e. .5. This is where the 2nd stage opens allowing even more fuel to get to the main circuit.

TIPS AND GENERAL INFO

• Changing from a 1 stage power valve on a carburetor that requires a 1 stage, to a 2 stage power valve is not recommended. If you aren't sure of what you are doing, you could cause too much fuel to be dumped into the carburetor.
• It’s very common to find that rebuilders have swapped the OE 2-stage powervalve for a 1-stage powervalve. We have not found there to be any real problem with this.
• The power valve covers are two different heights on the Motorcraft carbs depending on the power valve, 2-stage powervalves obviously require a much deeper cover. Keep this in mind especially when you’re purchasing a rebuild kit as the kit will contain whatever was original to the carburetor.
• Be sure the gasket is centered.