I always drag this one out when reading of these symptoms.
First thing I would check would be the balance of the carbs. Especially if everything seems okay when you rev the engine, ie, the cooler cylinder chimes in and the exhaust gas warms up just by cracking the throttle open slightly off idle. Also if it revs cleanly when blipping the throttle, then it could just be that the throttle slides are slightly out of synchronisation. This can cause the problem you are seeing when the bike sits on idle.
The adjuster is the spring loaded screw that sits inbetween the carbs on the throttle mechanism.
I used a temporary tank to let me get at it to set them up. I also ended up doing it by ear as I got fed up trying to do it by eye!
The idea is to try and get the slides to open together at the same time.
You may want to clean/blow out the pilot jets, etc while you are at it anyway. Being as you are already in there.