Dies at Idle

[Classics Fan]

Good NFL Sunday! I have a 1971 Jimmy with 35,000 miles on the rebuilt original engine. What I assume to be the original Quadrajet is on the truck. It doesn’t have a choke but does have factory manual throttle that I use for cold start warm-up. It’s been converted to HEI ignition.

My problem is with idle. The vast majority of the time, like 9 times out of 10, even after warm-up it will idle nicely at around 850 RPM (IIRC) for around a minute then slowly develop a stumble, and if there is no addition throttle input it will eventually die. It also has a slight off-idle stumble on acceleration. I have replaced the fuel filter, checked and set timing, replaced HT leads and olugs, and gone over the engine a number of times and am confident there aren’t any vacuum leaks in the plumbing. I sprayed the throttle shaft and carb base plate with Berrymans, but there was no noticeable increase in RPM to indicate vacuum leaks in those areas. The only possible thing that I can think of is a problem in the carb idle circuit. I have never worked on any type of carburetor beyond setting idle mixture, which, by the way I have done on this vehicle as well. Does anybody have any thoughts or ideas on where the problem could lie besides what I’m thinking? TIA for any input!
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