Are they an original set that has been rebuilt? Or are they brand new? I've seen some sets of 1969 originals that had the "kill bleed" holes drilled oversize in the outboard carbs which prevents them from opening with vacuum.
I don't see where you answered this question.
Since you said the center carb doesn't close all the way. If they are other than brand new, on the center carb try to loosen the screws that hold the throttle blades to the shaft so that the blade can move slightly. The screws are staked when manufactured and won't back out very far easily. While applying pressure to close the throttle blades tighten the screws back up. Do they close further now ? You might also try to plug the small holes in the throttle plates and see what happens.
