[Captain Obvious]

I've gone around and around with this, and cannot find what is causing my hanging idle.

I just replaced the distributor, so it's not that.

Car has a Quadrajet rebuilt by Quadrajet power. I just put the Progression Distributor on it, loaded what I think is a pretty conservative curve, set the idle mixture as best I'm able, and took it for a drive. It sounds good, no pinging or bogs. But it still does something that it has done since the restoration was finished - once you drive it around and it's good and warm, the idle will hang up about 150-200rpm above the set point. When you turn off the car and start it up, it will go right back where it belongs. Choke is properly adjusted as best I can tell, the throttle lever appears to be seated against the stop. There is a yellow throttle return spring installed.

This cannot be replicated by holding the throttle open or blipping it in the driveway. It always comes back to the set point under those conditions.

Almost seems vacuum related since it resets to the set idle when the car is shut down and restarted.

Anyone have any ideas here?