You won't believe this at first, but I bet almost anything it is your air filter. Not that I would ever do this (this is a lie, I did this), but I have a "friend" that put oil on a paper-element air filter, which completely stopped it up. With a basically plugged air filter, the motor just isn't breathing, so it is pretty much impossible to lean it out. So, either replace your air filter, or clean it really well, and be very careful - richen it up or you will blow out your motor once it starts breathing again (again, I have a friend that may have had to replace my, i mean his RB worlds because of this very reason).
Post up if this is the problem - I chased this issue for months, or maybe even a year and it drove me crazy! Good luck.