Bad power valves can be easily identified by the way they hang out on street corners, smoking cigerettes and wearing leather jackets.
However, the really bad ones (in your carb) will cause your car to idle very rough, emitting large clouds of black exhaust. What's happening is that your carb is letting way too much fuel into the engine, causing it to run too rich. When you step on the gas, the engine smooths out (because it now has the right amount of fuel/air mixture) The only way to drive a car with a blown power valve is to constantly blip the throttle, otherwise the engine will stall.
Replacing a power valve involves removing the metering plate from the carb, which can be done while the carb is still on the engine but it's much easier to take the carb off. You then take a 1" open wrench and un-screw the power valve. Be sure to center up the round power valve gasket when re-installing the new valve. You will also probably need to replace your spark plugs as well since you most likely fouled them.
The first day I drove my Cobra, I fouled the plugs (due to main jets that were too large) and almost didn't make it home. Ever since, I always had a set of new plugs in the trunk of my car.
You really shouldn't have any problems with a power valve unless you have an older Holley carb. Newer carbs from Holley don't suffer (as much) from blown power valves because the carbs come with a little check valve that prevents backfires from reaching the power valve. However, power valves can still fail due to dried out seals.
Cheers
Jim
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