Man, that's a good one! My first move would be to pull the carb and look at 3 things.
First would be throttle plate alignment. Has the screw loosened, or is something else going on that won't allow the carb to close completely?
Second, I'd pull the idle adjust screw and squirt some carb cleaner in there while watching the tiny fuel ports by the throttle plate. You should see at least some carb cleaner coming through. I'd also check the idle screw itself, looking for a damaged tip.
From there, I'd look for something that might be causing low fuel pressure on the idle. Is it possible the vacuum port is partially blocked by the carb gasket? A gob of gasket sealer blocking the port somewhere?
Nice job on your documentation....
Best of luck -Al