look,if you are sure that the engine is running in right temperature and it's not lean at hls.then you are lean or rich the low screw and it still running out.then you have an airleak.first change the backplate oring and try it again.if it still running out go and buy some orings for the carb.Take the olds with you and buy the next bigger number for the thin one that goes in the crankcase and the next smaller number for the last big oring that goes around the carb.then buy the next bigger number for the 2 littles that are in the secure system.
always use grease when you install them and go easy to avoid damage[since they are a bit bigger.

but first of all,try a orion plug,[i don't think that this is your problem-but...]
try them and tell us the results.