[jhv48]

Here’s a way to test it.
When everything is good and hot, park the car. With the engine running, open the hood and slowly add throttle. Then slowly let it off. If the rpm’s stay high, close the carburetor throttle manually to see if it is hanging up and not returning to the stop. If it’s not closing the throttle completely, then try a stiffer return spring. You can take the air cleaner off and look at the throttle blades to see if they are closing all the way.
If everything works as it’s supposed to, then you might have a hanging distributor weight that won’t allow the distributor to return to idle setting. Look for corrosion in the distributor.