From the Holley web site: "Problems:
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.
1) Check the manufacture date of your Holley carb.
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Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.
2) Test it using the idle mixture screws
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If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown."
The idle circuit never quits. The idle and power circuits are both fed fuel by the main jet. The power valve circuit is in parallel with the main jet. If the power valve is open at idle, additional fuel is feed before the emulsion tubes causing an overly rich mixture at idle. Chuck