340 sputter on initial acceleration

Condensing fuel or just a cold combustion chamber, not being able to properly fire a cool intake mix that is otherwise the right mixture. EFI can be used to compensate; the computer uses human observation to incorporate corrections but the only equivalent on a carb is a choke.... which you don't have. The idle may be rich too, since it fires off OK without a choke..or you are good at using the accelerator pump to compensate!

A hotter spark MIGHT be able to fire off this cool mixture better... but maybe not. What do you have for an ignition system?