33 year old choke

Have you adjusted the screw for fast idle speed? I can see the one part that comes up with fast idle, there is a second part also. Surprised that choke coil works with the rust.

the choke idle screw was also my first thought.

The cold coil lifts the green arm which lifts the fast idle/choke cam into position and pulls the choke flap closed. From the pics it looks like its working.

If the choke idle screw is set too far out, it will not be able to engage the fast idle cam step and give you the cold start fast idle.

This can also occur if the curb idle screw is set too far in...

Think of a scenario where you take a smaller engine's carb and put it on a larger engine. the carb needs more throttle angle at idle due to CID, cam, idle circuit malfunction, or whatever. So the curb (hot) idle screw is turned in some more.

Now the fast idle screw needs to be longer because it has been rotated further away from the fast idle cam.

EDIT: for the big rpm drop - at a cold start you are probably getting good fast idle cam action, giving you 2000 rpm. When the engine is running the primary pulloff draws in and pulls the green choke arm with it. If the pulloff pulls it too far in, OR again if the screws arent quite right, the fast idle cam will be released to drop too far or the fast idle screw will no longer engage it.

I would go back to the right length fast idle screw and have a look.

I think the interplay of the two idle rpm screws could be your cause?

I have this site bookmarked for my own reference. It was helpful when I swapped the qjet back on my car. For your car I would check the choke tension and the choke pull off. It sounds to me as if your choke is not set tight enough to fully engage the fast idle cam ( although in the pics its fully upright and engaged ) and with a weak setting it will not hold the choke for as long as may be needed allowing the choke pull off to pull it off fast idle too soon. Is the primary choke pull off working smoothly ?



http://www.florida4x4.com/tech/quadrajet/index.php

I was having problems with my original tube heated choke coil on my 78 El Camino so I switched out the Choke coil with a electric one.
AIRTEX / WELLS Part # 2C1032 www.rockauto has it for $13 it is listed for a 79 Cadillac Deville with a 425.
It solved my problem. It starts with one squirt of the accelerator now .

I wonder if the middle normal step for fast idle speed is worn on the linkage. That arm is moving as it should. I have actually bent the linkage, pretty sure it acted like that. I too always convert hot air chokes to electric.