Carburetor tuning has always seemed like an art-form back in the 60's and 70's, and was often the subject when muscle car owners got together. Back then, guys talked about blowing out the power-valves in their Holley four-barrel carbs, or changing the power-valves to add more fuel at higher engine rpms. Once you know how power-valves work in a four-barrel carburetor, like the Holley 4150, it's easy to select, troubleshoot and install the right one for your application. Here's some helpful tips compiled from Holley Performance's website that show a variety of tuning procedures and tests to make sure your carburetor's power-valve are set up correctly.
How it works:
The power valve opens at low vacuum, such as at wide open throttle, and directs more fuel into the carburetor's 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:
- 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.
How To Select The Perfect Power Valve:
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:
- Hook a vacuum gauge to an intake manifold vacuum port.
- 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 the Neutral position.
- Divide the vacuum reading in half. The number will determine the correct power valve.
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.
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. Holley power valves come in a range of orifice sizes. The higher the number the more fuel is added. Additional resources can be found on www.holley.com.