Hmm. Now I'm just grasping at straws, but you could also double-check that the airbox/crankcase vent hose is still attached -- I accidentally knocked mine off once while reinstalling the carburetors, which made it very hard to get the bike started and to keep it running. The vent hose is the S-shaped hose that attaches to the bottom of your airbox (pictured
here without the airbox, and with an aftermarket white plastic breather attached to it). And you've already probably done this, but it would be a good idea to double check that the carburetor/airbox boots are securely fastened, with the boot springs in the right place.