Still running stock ignition? For some reason, I would check the vac advance unit on the distributor. Not really sure, but that's just what came to mind first.
Did you bypass the ECM when you changed carbs? If not, a simple bypass will allow you to remove 90% of the ridiculous unneeded vac system. Research Nutter bypass, or take it one stop further and replace the distributor with an HEI diz. Thunderchicken has new ready to install units for a great price. Mine was under $80 shipped.
Admin of The DV Sanctuary, a non profit resource center helping survivors of violence.