Warm Idle Problem

Bit of a long story here to explain a current problem that's baffling me.

1968 Vette with a mild 350 (my apologies to the purists). Stock Quadrajet, stock intake.

The car developed a hot restart problem some time ago. Acted like it was flooded. I was due for a carb rebuild, so I took it to a good carb shop. No change. I installed a 5/8” spacer, and that solved the problem. My old car still uses the bi-metallic coil choke, so I made a new (longer) choke rod to account for the spacer. The choke sets correctly, the car starts easy, and fast-idles well when cold. The choke releases correctly, but when it gets warm, the car stumbles, won’t idle below 1000, and dies.

The attached pic shows the carb warmed up – choke released. Note that the choke plate on top of the carb is entirely vertical. If I close the choke plate to something less than vertical when it’s warm, the idle improves, but I have to close it a lot. I thought this might suggest a mixture problem, but I couldn’t resolve this issue with a mixture screw adjustment.

I can’t find any vacuum leaks, timing looks correct, and the plugs look good. I’m out of ideas. I’m no expert, so I suspect other ideas exist.

Please let me know what you think. Convertible weather is on the way, and I want to be ready!

Thanks

 

The Rochesters and Carters (now Edelbrock) get leaner when the mixtures are opened and richer when closed so you might try turning the screws in a turn or so. Or about 1-1/2 to 2 turns from bottomed out.