Step By Step
Most Holleys utilize power valves in the primary side of the carburetor. If you are not sure if your Holley has a secondary power valve, remove the secondary float bowl to find out for sure. The first step is removing all four bowl screws. We placed this Holley on the workbench, but this work can easily be done on the engine. Use a small metal or plastic cup to drain the gas from the float bowl when removing the screws, so as not to spill gasoline all over the intake manifold.
With the float bowl off, the power valve is the round brass fitting (arrow) that sits just above the two main circuit jets in the metering block. To get to the power valve, you have to remove the primary metering block.
Once the old gasket has been removed, you can easily loosen the power valve from the metering block with a 1-inch box-end wrench. There should also be a small round gasket between the power valve and the metering block. If there is no gasket, this could be the reason your power valve is leaking.
The biggest hassle with working with Holley carburetors is that the gaskets stick and usually tear when the carburetor is disassembled. As you can see, when we pried the metering block off the main body, the gasket stuck to both the metering block and the carb body. The best way to remove these gaskets is with carburetor cleaning spray and a scraper or screwdriver. Be gentle here; dont gouge the gasket surfaces.
Moroso makes this more elaborate aluminum test fixture (PN 62295) that places the power valve in a chamber sealed with an O-ring. On top of the chamber is a small nipple that can be connected to a vacuum source.
Note that you can see the spring loaded valve (arrow) in the cut-out portion of the tester. This way, you can use a running engine as a vacuum source. If the spring-loaded valve stays retracted (up), the valve is good. If the valve stays down, the power valve is bad and should be discarded. Weve discovered that even new power valves are sometimes bad, so be sure to test your new valve as well. When using any tester, make sure it works properly. If you test 10 valves and theyre all bad, suspect that your tester has a leak.
The best way to test a Holley power valve is to apply vacuum to the valve and see if the valve remains closed. If it holds vacuum, the valve is good. If vacuum will not pull the valve closed, the valve is bad and must be replaced. The simplest and easiest tool weve found to test Holley power valves is the Mity-Vac vacuum pump and the small rubber suction cup thats included with it. Wet the cup with saliva, place the power valve on the suction cup, and hook the Mity-Vac to the suction cup. If the valve is good, the Mity-Vacs little gauge will read and hold vacuum (generally around 15 to 20 inches of vacuum). If the valve is bad, no vacuum will be generated.
There are two different styles of Holley power valves as well as two different gaskets. The open window design (left) requires the use of the standard round gasket. The power valve on the right uses a series of small holes, as opposed to the open-window style. The valve on the left requires a gasket with inside diameter points, which locate it on the valve. Through experience, weve discovered that the best plan is to use quality power valves from a company like Holley, or another reputable carburetor supplier. Stay away from the bargain-basement units. These cheaper power valves tend to fail much more often.
When it was time to reassemble the carburetor, we used Barry Grants non-stick Carburetor Gasket Spray to prevent the gasket from sticking and tearing the next time the carb is disassembled. Weve also used WD-40 silicone sprayand even ChapStick lip balm in a pinchand these worked almost as well. They just save having to scrape gaskets every time the carburetor is disassembled.
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.