things on quadrajets I've seen:
-leaking welch plugs on the bottom of the main body casting. These are notorious for leaking, and most rebuilders put a special sealer over the plugs when the base is off. This also causes the bowls to be empty in a day or so, if you don't drive it much.
-Internal vacuum leak caused by warping. This usually makes the car almost seem like it's got a slight "cam", by the way it idles, and adjusting the mixture does still afffect idle quality, but can't seem to get it stable. Just went through this with a late 70s vette. It had a new kit and new gaskets did not help.