[chopper]

Not intending to start a big argument here, but I disagree somewhat with what 6T5 Cbra describes as the function of the power valve. The power valve is not there to “eliminate fuel starvation when you jump on the throttle.”. That’s the function of the accelerator pump. The power valve is there to compensate for increased load demand at a given rpm. Envision your car in two different modes, the first in steady level cruise at 2800 rpm on a level interstate highway; the second mode is at 2800 rpm climbing a 6% grade. Obviously, the engine must put out more power (produce more pound-feet of “work” in the same amount of time) in the second scenario. The power valve in the Holley senses the increased need for more power at constant rpm through a comparison of manifold pressure (or, as it’s more commonly referred to, manifold vacuum) to a predetermined reference (the spring opposing the movement of the power valve). When the manifold vacuum falls below the level necessary to overcome the opposing spring force, the valve opens and admits more fuel to the air flow, effectively enriching the mixture and providing more chemical energy to produce the required work. Holley states that opening the power valve is equivalent to increasing the main jets by 8 to 10 sizes. If the power valve diaphragm is blown, it will remain open and allow the carb to function as if you had increased the main jets by that much. This may be too rich to start, especially if you have fouled the plugs. I do agree with the statement by kountzecobra, that if you have a newer Holley (say, built in the last four years or so) your carb probably has the ball check valve “blowout protection” and this should minimize the risk of blowing the valve. Note I couched that because the valve is not a 100% certain preventative measure, and it is still possible to blow the valve during a backfire if the check ball is sticky. If the valve does blow, it will rarely fail closed because the manifold vacuum is what’s holding it closed. Loss of the manifold vacuum seal will allow the spring to open the valve. Additionally, fuel will leak through the ruptured valve diaphragm directly into the manifold below the carb.. Failure of the valve almost always leads to an overly rich condition.

All that having been said, the power valve does, in fact, have some minor influence on the carb mixture during rapid throttle opening, but it's much less than that provided by the accelerator pump. If you are having problem when you floor the pedal, I'd recommend looking at the accelerator pump before the power valve.

There are essentially two ways to check the power valves. First, replace the valve and see if it fixes the problem. Not too difficult, relatively cheap and no more work than removing and checking the valve. Second, get a power valve tester and a small vacuum pump, remove the valve and test it to see that it opens at the appropriate setting. I prefer the second method because I’ve had power valves that opened at values as much as 7 inches off of the rating stamped on the valve. I have two in my garage right now that are over five inches off the stamped rating. I use the tester to check that new ones open at their advertised number. Summit sells a Willy’s brand power valve tester (which is the one I use, for what that’s worth) in their catalog for $34.95. I got one at a swap meet some years ago and it’s proven invaluable in troubleshooting carb problems. It allows you to answer positively whether or not the valve is functioning correctly so you can eliminate that as a possible problem source.

I do agree with 6T5 Cbra’s exception with the rule of thumb that the valve should be rated at half of the manifold vacuum at idle. This will get you reasonably close for a normally cammed street engine. With the type of high lift cams we tend to use in these cars, that rule may not provide a properly rated valve. Holley recommends a more accurate method, which is to hook a vacuum gauge to your manifold, mount it in the cockpit where you can see it and drive the car at various stabilized rpms in all gears. The power valve you use should open at 1.5 to 2 inches below the lowest steady state manifold vacuum you observe during this test. In the case of my car, the lowest manifold vacuum was observed at idle, at 10.5 to 11 inches. Based on the “rule of thumb”, I initially installed 5.5 power valves (dual carbs) and experienced a wicked bog during slow steady acceleration (a mode mimicking the steady climb up a grade). I removed the 5.5 valves and installed 8.5 valves and the problem disappeared. Hope this helps.