Dash Riprocker,
My first thought would be a vacuum leak in the system. Under high vacuum, as when the throttle closes, air is entering the system and causing the engine to speed up for a moment.
The power valve in the carburetor reacts to this change, but in reverse to what you are seeing.
The vacuum advance is also on this circuit, but should not see this high vacuum if the throttle plate is closed.
I would check the vacuum advance diaphragm for leaks, and would make sure the carburetor base gasket is good.
If you poked around with the engine running at a idle, and shot carb cleaner in suspicious places, you might find a leak.
Sticking throttle linkage could be a possibility, but I'm sure you have checked that, right!
Stuart
