I had a similar problem with the 348 in my 1957 NAPCO. It ended up being one of the end carbs needing to be rebuilt. I determined that was the problem by blocking off the two end carbs and disconnecting them from the linkage. You could stuff a wet/damp rag in the air horn of the potentially offending carbs. I would just do that for a "quick test." At idle you normally run on the center carb anyway and it should go to normal idle. Anyway, that was what my problem was with that symptom..
don
