I thought I'd posted that earlier. Anyway, there's no question in my mind that the problem was the foam tape coming loose. The "pie pan" filter assembly seems to be too small for the older pre-s/n-400 air box assembly, so there's a bunch of foam tape used to keep it from sliding around. This tape had worked loose, and the filter had slid off-center, allowing the loose tape to get into the carb throat. Replacement of the "pie pan" filter with the newer K&N Challenger filter (which fits much better and needs no foam tape) fixed the problem. I strongly recommend that anyone with the pre-s/n-400 air box make this replacement to prevent this problem from developing.