several things that i know of can cause this. i'm not sure which one is the most common. if someone knows i'm sure they'll chime in with their advice.
what i know from my own experience with my quadrjet, is that you can have a number of problems, including:
-a problem with the spring and latch that attaches to the blades
-a problem with the condition of the carb, could there be dirt or grim keeping the valves from rotating easily?
-not enough vaccuum being created by ur engine, with enough vaccuum your 3 and 4 barrels should open up despite any other problems
i'm sure there are more possibilities that other people will mention, but these are the ones i know of or had to experience.
my advice is that you replace the carb if it is the original. autozone and o'reiley's offer affordable rochester replacements, both rebuilt and brand new. that's what i ended up doing. now she roars like a lion when you put ur foot down.
sam