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.

1973 L-82 4 spd

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.

1973 L-82 4 spd

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?

1973 L-82 4 spd

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

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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.

No, the air cleaner housing is not after market but original. Only the filter is after market. There is no contact between the air cleaner housing and the carburetor other than normal.

Well, that is not clear to me. Hard to see.

When cold and working properly, the high idle is at 1,500 rpm while the low idle is at 1,000 rpm. The issue I have is that it goes to 2,000 rpm and sticks there when hot. Sometime I managed to make it drop down to 1,500 rpm by taping the gas pedal. It is as if it was adding 500 rpm at both low and high idle, when hot.

From the description of the issue, it sounds like the throttle blade shaft(s), and/or bores for them in the baseplate might be worn. I've also seen the base gasket hang the throttle blades if it becomes distorted/damaged, or is not properly cut when it is made. The gasket between the baseplate and the carb fuel bowl can also be distorted and hang the throttle blades.  With the air cleaner completely off, and the car idling hot, see if you can manually push the throttle linkage(right where the cable is attached) forward to lower the idle speed. Wiggle up/down the linkage(on the carb itself) and see if the idle changes as you wiggle. This would indicate a worn shaft or bore. 

dam spelchek

Joel Adams
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I have replaced a lot of throttle body plates due to the wear problem.  In addition to what Joel has said, with the engine off open the throttle a few degrees and then try to lift the throttle link up and down and front to rear.  A SLIGHT movement is okay but anything else is an indication of a worn shaft and or bushings.   The plate can be rebuilt, but it may be hard to find someone competent to do the rebuild/repair.  You may need to just replace the throttle plate.  Make sure it's an exact match or  you will be installing other problems you don't want.

Here's a good place to get it rebuilt or parts for it.  https://cliffshighperformance.com/

1973 L-82 4 spd