[Coley]

What were the results of doing a complete check for a vacuum leak?
Remember, this isn't a quick 'visual' test or cursory check.
You have to identify everything on the top of the engine that can 'leak' air...from the intake manifold, to the carb insulator gasket, to the small ports on the carb, to the power brake vacuum line, every line....cracked hose?...vacuum tube pulled out?..leaking main brake booster?
Sometimes a pair or pliers will allow you check each and every hose on the upper part of the engine. You have to clamp every one of them and hold them for 5-10 seconds to see if there is change in engine speed or the way it runs.
How old is the Quadrajet?....have you ever had it off and cleaned it thoroughly with carb cleaner?....or run a full can through it?
I like Quadrajets but the best thing you can for them every couple of years is to fully remove them from the vehicle and vigorously clean them on the work bench with carb cleaner and compressed air.

...my two bits.
Coley