That type carb has history with bad accelerator pumps-------- are your metering rods free and in good shape?
have you worked each carb individually to see which one might be the culprit?
work carbs , rear one first to about where seconaries start to open and the front so both reach wot at same time
next do it with front carb first then rear-----

good squirt of fuel from both carbs pump?

do you have those air flapper valves over secondary butterflies? free and smooth?

set timing total with vacume disconnected

check dist for free movements of the weights and advance meckanism on dist upper shaft(most of the older Delco type dists that I have worked on had sticky, seized up advance mech)