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.
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.
Troubleshooting Your Power Valve
The incorrectly sized power valve, or a blown-out power valve can cause problems like 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.
Check the manufacture date of your Holley carb.
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.- Test it using the idle mixture screws.
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.
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.
Each power valve is stamped with a number indicating 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.
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 eight inches divided by two give you the number four. In this case you would use a #35 power valve.
Holley power valves come in a range of orifice sizes. The higher the number, the more fuel is added. 
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.
Check out the video below (courtesy of Holley) for a step-by-step look at how to tune and select the right power valve.
Does a power operate on manifold vacuum drop or the manifold vacuum at that point in time,i.e., if your manifold vacuum is at 15 cruising and you’re using a “5” valve will it start to open at 10 inches or 5 inches?
Great ready would love to have a chart if you have one .