See if this description sounds familiar: Your Holley-carbureted street car idles terribly, has taken to fouling spark plugs every week or so, the gas mileage absolutely sucks, and theres an ominous black cloud swirling from your exhaust. If any of these descriptions fit your Holley carbequipped street or race car, you probably have a blown power valve.
While there is no more popular carburetor in the performance world than the Holley four barrel, it does suffer its share of shortcomings. One of its weaker points is the power valve. When this little valve fails, it can cause all of the above driveability problems. The power valve is not hard to replace, but it does require the carburetor be partially disassembled. The good news is that fixing a blown power valve is easy and it can be done without removing the carburetor. Lets take a look at what the power valve is, how to identify a bad one, and a couple of Holley power valvetuning tricks.
What It Does
Most carburetors employ what is generally called a power valve circuit. This circuit enriches the air-fuel mixture when the carburetor goes to wide-open throttle (WOT). At WOT, intake manifold vacuum drops to almost zero. When this occurs, the power valve opens and directs more fuel into the main power circuit, in addition to fuel delivered by the main jets. The Holley power valve employs a small rubber diaphragm that is opened by a small coil spring. The valve is held closed whenever sufficient engine vacuum is present. At WOT, engine vacuum disappears and the power valvespring opens the valve, directing fuel through a small, precise orifice in the metering block called the power valve channel restrictor. This restrictor determines the amount of additional fuel delivered to the engine.
Power valves are used most frequently on the primary side of a Holley carburetor. They allow the carburetor to operate with much leaner main circuit jetting for part-throttle fuel economy. Then, when the throttle is slammed open, the power valve adds additional fuel, creating the rich air-fuel ratio needed for WOT operation. Most Holley power valve circuits are designed to add the equivalent of 8 to 10 jet sizes of additional fuel. Holley does offer a power valve block-off part that closes the power valve circuit, but this means the jet size must be increased in order to compensate for the lost power valve circuit fuel. Imagine how bad your fuel mileage would be if you had to add 10 jet sizes to the primary side of your carburetor!
The problem with the Holley power valve is that it is the most sensitive circuit in the carburetor. 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. 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. At most, it requires removing the primary float bowl and metering block. Once these are apart, its a simple job to remove and replace the valve. Lets take a look at what it takes to replace a blown Holley power valve as well as some other useful power valvetuning secrets.