Holley Power-Valve Blowout ProtectionAdding power-valve protection is cheap and easy
Holley carburetors have been the mainstay of high-performance engines for more than 40 years. In fact, they're still standard equipment for Winston Cup cars as well as for the most powerful carbureted engines of all, NHRA Pro Stock. In spite of the continuous improvement that attends race programs, the Holley carburetor has escaped major change. It seems that Holley designers got it right the first time, and now only minor refinements are being incorporated. If you've worked on or tuned Holley's 2300 (two-barrel), 4150s, or 4500s, you'll be right at home making this simple modification outlined below.
One of the minor but important changes Holley has made to most of its new performance carburetors is a feature called power-valve blowout protection. It's a small ball/spring check valve assembly that closes the vacuum passage to the carburetor when there is a backfire in the intake manifold. Holley power valves are diaphragm/spring assemblies that add fuel enrichment for increased power at specific manifold vacuums. (A 6.5 power valve allows more fuel to flow when manifold vacuum drops below 6.5 inches.)
It doesn't take much to rupture these valves; just barely a perceptible cough or sneeze in the intake manifold will do. When ruptured, the power valves are essentially open all of the time, making the engine extremely rich at idle and part throttle. The easiest way to check for blown-out power valves is to gently turn the idle-mixture screws all of the way in. If the engine starts and idles without stalling, it's getting fuel from somewhere else-most likely a leaking, blown-out power valve.
Even if the power valves in your Holley are not currently ruptured, it's a good idea to add this inexpensive protection the next time you have your carburetor apart. The carburetor used for this demonstration is an 850cfm, vacuum-secondary, marine carburetor that has both primary and secondary power valves. We ordered a Holley Power Valve Check Ball Kit No. 125-500. It has check-ball assemblies for both ends of the carb and includes a stop drill for installing them. Installation is relatively simple: remove the baseplate from the main carb body, then wield an electric drill with a steady hand.
The vacuum passages to the power valves are easily identified on the main carb body-to-baseplate surface. They are the holes right next to the center screws that hold the baseplate to the main carb body. The first step is to drill this passage out with the provided bit, after you've set the drill stop-collar for a depth of .300 inches. Drill carefully to keep the hole straight, and let the drill rotate for a few extra turns after the stop has bottomed. This will remove any burrs from the bottom of the hole, which is now where the ball seats in case of a backfire. Blow out the hole to ensure that no chips are blocking the passage and drop the check ball supplied in the kit into the hole. Next, slide the tapered coil spring into the passage with the tapered end facing the ball. The last step is to tap the retaining washer into the passage with a small hammer and the appropriately sized punch. It needs to seat below the carb-body gasket surface.
This simple little modification will take less than 30 minutes once the carb is off the vehicle. Don't forget to purchase a new carb body-to-baseplate gasket before you start. Finding the correct gasket at your local parts store may require a special order and a couple of days' wait.
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