The original Quadrajet for my 67 400 was in the trunk when i bought the car about four years ago in pretty rough shape. I've sent it off to two different quadrajet resto specialists (Quadrajet Pwr & Carolina Carb Specialists) and still encountering same problem that i'm looking for solution to. The carb is stumbling on the primary side upon initial acceleration. It hesitates upon normal acceleration and takes a second or two to 'catch-up'. Trying to determine if any one has experienced this issue and knows of a recommended fix..or if this a known issue these early gen quadrajets.
Check the fuel pump to make sure you get enough pressure and it's in good shape and not leaking. Check around the airhorn and entire carb to make sure you don't have any cracks. Ensure your vacuum hoses are connected properly and your timing is correct. Connect a vacuum gauge and and set the mixture screws to about 2 1/2 turns out from fully seated. Watch this video: https://www.youtube.com/watch?v=HKGqKZsf3cc
Do a search and then you can ask a question if you don't find what you are looking for.
I did a quick search and found a couple of posts with similar symptoms to yours and the recommendation was to open up or resize the down channel restrictors (DCR)
"Down Channel Restrictions are part of the idle circuit in the main body. The size of this orifice will restrict fuel flow during idle. When increasing compression and cam on an engine, these should be enlarged to allow more fuel to flow with less vacuum."