A couple of things to check:
Look for a split in the air box, depending on where it is, it can introduce unfiltered air into the carbbies. The most common place for a split is on the front, at the bottom. Coincidentally, that spot will allow unfiltered air to carbs.
Is the little 90* hose at the bottom of the air box attached to the airbox AND the crankcase? This is a crankcase vent, and dumps its air into the airstream after the air filter.
Once things are cleaned up, avoid using gasoline that is not Top Tier.
http://www.toptiergas.com/Good Luck.