Here we go again. Another thread about the Rochester Quadrajet carburetor.

Yet, I am far from understanding everything about it.

This is about the original carburetor that came on my 1971 LS5, just rebuilt. It works fine (full power, nice idle, as good as it gets gas mileage, etc...) BUT:

It has a hard time getting back to idle. Tends to stick to 1,500 rpm until I gently tap the gas pedal, even when hot. I thought the cable was not sliding well enough, but no. This is not the problem.

Today, I had a fairly long drive and the outside temperature was 92F. The engine would not go back below 2,000 rpm even when tapping the gas pedal. I checked: The cable was not holding it. I even disconnected the solenoid to make sure it won't prevent it to go back to idle.

I removed the air filter. The secondary air valves were closed. If I pushed them open, the rpm dropped to normal. It looked like the choke was not working properly.

I shut the engine off and left it cool about half-an-hour. Started it again. It was working normally... until it got hot again.

Any suggestion or advice?

When it's hot and won't go back to fast idle, remove the air cleaner and check to see that the choke linkage is dropping the fast idle cam completely off.

The only other thing I can think of would be the throttle shaft bores are sloppy and letting the butterflies hang up or the butterflies themselves are not aligned perfectly.

I was in Q-jet idle hell for many years after my original choke spring broke.  I tried two aftermarket springs over 3 yrs that did not work well.  I was rotating them up to 90 degrees to get them to release.... sometimes.   One day I figured I needed an OEM.  Found one on eBay and it has been flawless out of the box for years now.

That is what I did. And the butterflies where nicely closed. However, if I pushed them open, then the rpm would drop to idle,  coming back up to 2,000 when I release them. That puzzles me.
The cooling system works fine, maintaining the engine around 200 per the gauge at any time. However this would happen only when I drove for a while and it is quite hot outside (yesterday, I had driven over an hour and the temperature was 92).

Are you talking about the carburetor return spring? The one that pulls the cable to the gas pedal back up when you release the gas pedal?

The only other thing I can think of to check would be the idle mixture screws. It seems counterintuitive that when you add air by opening the throttle slightly the idle goes down.

OK. I did not take the time to do anything about it. Time is a scarce commodity.

I drove it yesterday and nothing changed. When cold and for the first 10 minutes, the idle sticks to 1,500 rpm but I can make it go down to 1,100-1,000 with a light tap on the gas pedal. After that, when it gets real hot, the idel sticks at 2,000 rpm and the best I get with the gentle tap is to get down to 1,500 rpm.

Today, I decided to test drive it without the air filter in order to have a clear view of the carburetor without the need to remove the air filter first (hot). I could drive for about 30 minutes - well into normal hot temperature - without the 2000/1500 thing, only the 1500/1100. Go figure????

My air filter is one of these K&N washable air filter. I had it for a while without problem. I washed it a few times per the K&N instructions and product. Last time I washed it was a couple months ago. Do you think the air filter could cause the problem by preventing enough air to come in? At idle while I can get all the power I want at higher rpm?

I just don't get it.

With the air cleaner off and the engine at the higher idle speed, have you verified that the fast idle cam is completely off?

It sounds to me like the AFTER MARKET air cleaner housing is hanging up on the linkage.

Bingo!   I have that issue with my stock air cleaner if the tube hooked to it is not pushed out of the way.

Check your choke on the passenger side of the carb.  You may need to adjust it.