Odd engine behavior after first shutdown of the day

[Sledge]

I tried searching, could not find the right keywords to get meaningful results.

Engine—carburated Dart/Windsor 427, Quickfuel 750 cfm carb, Victor Jr. manifold, AFR/Smeding heads.

Previous engine was a 351W, 385 hp, originally with a Street Avenger carb, then a Pro Systems carb (no choke), Performer RPM heads and Air Gap manifold.

Both engines act(ed) very nearly the same, including a slight harmonic at about 2050 to 2150 rpm.

First start of the day or hot but after a very short shut down, it runs great. Nice idle, not too rich. Throttle tip-in from idle is crisp, as is the exhaust note.

But if it has been fully warmed up and then shut down for more than 5 to 10 minutes, it starts okay but then runs as if the mixture has become too rich, but there is no visible sign of being too rich. Off-idle throttle response is a bit hesitant and I need to rev the engine higher to get a smooth getaway from stopped. Nowhere near as crisp as before being shut down, and I also need to keep blipping the throttle to get it to run its smoothest at or near idle.

After a while--5 to 10 minutes of running--engine response returns to the way it was after the first start.

I have shut the engine down by shutting off the fuel pump and running the bowls dry. Makes no difference. I have disconnected the choke after the engine is up to temperature. Again, makes no difference

Recently, when getting off the throttle—like when stopping at a stop light—engine idle rpm is initially about 1100 rpm and then slowly reduces to about 950. As if the throttle cable needs lubrication, or if I need stronger return springs. I’ve even backed off the curb idle screw at least a quarter of a turn, with no change in response. Initially idle was set to 800 to 850 rpm. Perhaps the engine has loosened up some since new?

There is insufficient underhood room for a decent spacer between the carb and the manifold. I did install a .3 in thick Holley base gasket yesterday but no difference noted. The fuel system does not have a return to tank or regulator but does have an Aeromotive fuel pump set to about 7 psi according to product info. The car is at sea level.

I can live with this but it does not seem right. All help appreciated.