I could be way off and have not heard any of the details just a very common symptom of flat cam that can go away in 5 min. or less on start up. Probably wrong if nothing in pre-filter or no noisey valve train, all pushrods spinning on idle and compression test shows great with no variation from cyl. to cyl.
Way too rich on carb, pull vac hose to brake booster for huge leak installed, if rpm goes up by about 2/300 then it will lean it out to confirm. Other check would be to look at ignition for this kinda problem. is the color from a plug stuck in a removed wire neon blue spark? Should be neon blue.
If it is a yellow or red/orange then the fault is ignition=== yellow is condensor or saturation fault and orange is coil...... All the wires do not go bad at once so not a wire. MSD or the like will be on/off problem many times. Let me know