the mix screw is adjusted by turning down the idle speed screw on the left side of the carb as low as possible without stalling and with the motor up to temperature ,then turning the mix screw to obtain the highest idle speed.  I f at that point the screw is less than 1 turn out your idle jet is to big ,if more than 21/2 turns open it is to small  , three turns open, it has a good chance of falling out as you run down the road. If the bike still doesn't run right I''d pull the carb and clean it, which sounds like you should do anyway. the bike is 6 yrs old and hasn't even enough miles on it to say the rings are even fully seated (1500) that tells me it has  been  sitting (with old gas in the carb)  most of it life.