Power rods that are in the carb could be stuck in the "down" position, restricting fuel through the jets.
This is not an uncommon problem with todays gas and the quadrajet.
The garbage left behind from today's fuel creats a build up around the piston and it can hang up in the bottom of the bore therefore not lifting up to supply the fuel through the jet as needed.
Vacuum controls the sliding up and down motion when things are right. But if it's stuck in the bore, it will do exactly as you described.
If this is your problem, disassembly, a good soaking, along with attention to the power piston will cure it.