Could you give me close guess on my 04 450r. It back fires on deceleration bad shots flames out the tail pipe.^^^ Ghini beat me to it, lol. ^^^
Try the HRC needle, and scrap the "jet kit" idea and just get some factory keihin jets.
Also, list all of your engine mods and complete intake and airbox configuration, i.e. lid on or off, kind of filter, stock or aftermarket pipe, etc. and we can get ya close on your jetting.