If you are popping on decel, air fuel screw too far out-too rich...maybe? Yfz may be a bit different. Are they Mikuni carbs?
Have you had it apart recently? Or: Is this problem out of the blue? If you havent had it apart recently, and it has been running fine, take apart and clean it. If you atre just now jetting for mods, I am not sure I can help much. Good luck.