Hey Mark,
Three thoughts occur to me. One already mentioned above. In order of ease of diagnositcs:
1) Distributor springs are old and no longer strong enough to return it to the idle setting. You can check this by using a strobe light while the engine is running. At idle note the ignition timing on the flywheel through the inspection port on the top of the bell housing. Now rev the engine a few times, maybe even hold it at 2,000-3,000 RPM for a few seconds. The ignition timing should advance, release the throttle and the ignition timing should go back to where it was before at idle. If not I would strongly suspect the return springs in the base of the dissy.
2) Leaks around throttle spindles, if you have an vacuum leak there it can speed up the engine and cause all sorts of other issues. Try adding a little grease around the throttle spindles, if you do that and all is good on the running side of things I would say your spindle bushes need replacing.
3) Butterfly valves. I've had a couple of cars with HS8 carbs have their butterfly valves shift on their shafts slightly. In that case they don't quite shut all the way when released back to idle. Unfortunately the easiest way to see if this is the case is to pull the carbs, remove the piston and look through the venturi when the throttle is fully closed (you'll need to back off the idle screw all the way). If you see a sliver of daylight around one side of the valve I would bet that's the case. I found the best way to reseat them was to loosen the two screws securing them to their shafts a turn, and snap the throttle open a couple of times to seat the butterfly correctly. Look again, if you see no light snug up the screws again.
Hope that's some help,
Steven
