One more thought.
There is a "tube" to the left of the carb. It is actually a "muffler" for the intake air. Go here:
Then, go to SWINGARM @ AIR CLEANER @ MUFFLER, it is item number 4.
Anyway, I just found that one of the three holes in my tube was half closed by a spider nest/web thing. I would have never thought to check this for plugging, but maybe it is worth your while to check it.
The downside - you may have to remove the carb (again) to get access to it. I've pulled one out with the carb in, but haven't ever tried to put it back in unless the bike was apart.