Have you looked at the throttle return spring or possibly binding throttle linkage?

 

Hello thank you for the reply, yes I checked it out and they seem like they are fine, here is a picture of the springs

 

When was the last time you checked the weights in the distributor? Are they rusted and sticking? Sticky vacuum advance? That's the next place I'd look.

 

Too lean or vacuum leak can cause slow to return to idle.

 

Double check your return spring, looks funky. Better to get a dual return spring kit.

 

your linkage does not look parallel to the carb. stock linkage had a offset in it

 

If your throttle snaps shut without the linkage, then most definitely the straight rod from the carb back is the
issue. Shorten the return spring mount and flip the rod over to enter the carb from the inside. Might help.

 

Maybe I’m interpreting the OP incorrectly. I read it as if you continue to give it throttle but the rpms drop back down to idle. Or, are you actually releasing the throttle, and it takes a while for the rpms to drop back down?