72 dart 318 problem please help

Sounds to me like your dashpot is sticking. It looks like a small vacuum canister that is screwed to the carb and has a plunger that contacts the throttle linkage. It's purpose is to ease the throttle back to idle and theoretically reduce unburned hydrocarbons. I would remove it and see if that fixes the problem (it's held on with a couple of screws). Then you have to decide if you want to replace it or leave it off. Hope this helps.