I am having an idle issue on my 1970.  The original engine was rebuilt and installed this past summer and I have since put about 1000 miles on it.  The only glitch now is that there seems to be no cold - fast idle, when fully warmed the enging will usually hold idle.  Also, occasionally when warmed up the engine will lose idle when the clutch is depressed.  If I turn off the car and restart it starts with no problem and holds idle.  Otherwise the car runs fine.  Then yesterday I started the car and it idled at 1500-1700 rpms for about 5 minutes then idle went down to operating temp idle.  It is a 350/350 with a Rochester Quadrajet carb.  Choke was replaced and the carb was rebuilt with a rebuild gasket kit.   

Use a Holley!

Does it seem like it's loading up with too much fuel? Is there any black smoke from the exhaust?

I want the car as stock as possible and since it came with a Rochester carb, and I believe the one that is there is original if I am interpreting the casting number correctly, I would like to keep the current carb.  Then again I want to enjoy driving the car also. 

If it has the original type carb on it, then it should also have an idle stop solenoid on the driver's side, front of the carb. If this solenoid is losing power(12v) for some reason, it will drop the idle down dramatically. When you turn the key off, and back on, it would re-energize the solenoid, and then hold the idle up to a normal rpm. It really sounds like the solenoid is the problem. There is also a possibility of the TCS(Transmission Controlled Spark)system malfuncioning. This could raise/lower the rpms intermittantly.
Pushing in on the clutch pedal may be moving/pinching a wire under the dash to one of these systems, causing a problem.

I will check the solenoid.  It does have the TCS system, which I replaced for looks but did not hoop up.  Thanks, maybe the solenoid is the culprit.

BTW, If you are getting whitish smoke at start up, could be valve seals, not sure when yours were replaced last but....they get pretty brittle..not too hard to fix if you're handy.

 Just thought I'd mention that.

There are several idle adjustments on the Q-Jet.

All are made with the engine at full operating temp.   The first is the base idle done with the screw on the left side of the carb, and contacting the throttle linkage.  This is set in gear with an automatic, or in neutral with a stick.  If there is not solenoid that is the only warm idle adjustment.

The solenoid, if equipped comes next.  The solenoid can/has been used in two different methods.  The first is an antidieseling adjustment.    In this case the base idle is set lower, and the main idle is set using the solenoid.  The solenoid is on whenever the key is turned on.  It will no usually set until the gas pedal is tapped just a bit, then it holds the idle up a bit, at normal settings.   When the key is turned off it dropps the idle to prevent dieseling.

The second use of the solenoid is a kicker used to compensate for load from the air conditioner.  It raised the idle when the A/C is on to help keep it from stalling, then drops off to normal idle with the A/C off.    This prevents stalling from additional load with the A/C ( or defrosters ) on.

The third idle is the fast idle.  It is set using a screw on the lower right side of the carb.  It can be difficult to find.  Befoe adjusting this screw, the fast idle cam must be set.  This is a cam on the right side that kicks up in stages or steps as the choke plate is closed.   It is often set on the first or second step, and adjusted to the correct RPM with the engine warm, not cold.    Later when it's cold the rpm is correct.

The next adjustment is the choke pull off.  If it is not operating the carb can run too rich when first started.  This is a vacuum device on the right side of the carb. 

The last adjustment is the choke spring or coil, controlling when the choke opens and closes.

Look at www.vetteprojects.com under Ken's projects to see the locations and procedures for these adjustments.  That's my carb and photos when I rebuild my 75 Q-Jet.

It seems as though my idle issue is solved.  The carberator was missing the choke assembly.   We had installed a new choke when the engine was rebuilt but did not realize that there were several other parts missing.  The choke flap in the carberator was not hooked up to be regulated by the choke.  So when starting the car cold the choke flap was closed.  The engine was getting to much fuel and no air.  Also causing the engine to die at times when clutch was depressed.  The car runs phenomenal now.  I think it has 30-40% more power.  One more thing solved. 

It's always the simple things that kick our bo-bos, eh?
Glad ya got 'er goin!