Well I would really doubt that it is a power valve, because Edelbrock carbs don't use power valves. I know my 600 cfm one doesn't anyway, and I have never seen or heard of an Edelbrock carb that does. In fact thats one of Edelbrock's sales pitches is that they have "no power valves to fail".

Power valves are for Holleys.

I would check the choke. Is it electric or manual? If its an electric one, it may have died and can be causing the carb to run to rich. You may (or may not) notice it getting worse once the engine is warmed up.

Lotsa Luck,
Firechicken