just to clarify, are you talking about the pilot screw?(under the base of carb) or mixture screw? (on side of carb) it sound to me like the bike is lean, popping on decelaration is a good indication of this (if you have ruled out intake and exhaust leaks first) I would dropping your needle clip one position and retest.
05 YZF450 White/Red no mods yet
03 WR450F, grey wire mod,James Dean blue needle,4th clip position,170 main 48Pilot,70PAJ (stock) ,1-3/4 turns on Pilot screw, Throttle stopcut, Powernow, FMF powercore4, 1500ft alt.