I am working on a '12 EX250J that has a super bad bog right off of idle. If you snap the throttle the bike dies completely. If you gently baby the throttle off idle (idle is about 900 right now) it will bog between idle and about 2-3000. Once you get past 3000 rpm it picks up and seems fine. I've removed and completely cleaned the carbs (removed everything, ultrasonic clean, air blasted all orifices, checked jets, checked hoses, etc.) and there is no change. As far as I know everything is stock. The floats are set at 17mm. I've checked for vacuum leaks around all the boots and vacuum hoses. The only thing that raised any concern during the rebuild is that the air cut off valve did not completely cut off air, there was very minor amount of flow in the closed position.
I rebuild a lot of carbs and have looked for all the usual suspects. The pilot jet is good and clear, idle air mix is set at 2 turns out, the carbs are very close in sync on the bench (couldn't find a port to sync on the bike).
Thought and suggestions?