Start by cleaning the carbs. Idle is controlled by the pilot jet circuit. The air and fuel passages to the pilot jet are very small and are easily gummed up as gasoline evaporates from the carb. Keihin carbs are very similar to Mikunis in the way they work, so you can get a rough idea on how to clean them by looking at the "how to" section of the forum. There are several really good posts on the Mikuni round slide carbs.
How fresh is the fuel?
edited... bob beat me to it