340 popping through carb at low RPM.

Eddy carb...You are short on pump shot or need stiffer step up spring to enrichen more if it's off the line.

....but cruise would intake valve cracked or open....exh lobe flat.... very rare to have the timing that far advanced to emulate that...but you never know.
If an exh valve were hung open it would pop ...pop... pop.. out the tail pipe.