hi , Motocross action , Dirt bike and the lesser known Motocross , magazines from 1984 all recommended the following for the YZ490L
stock is 440 main needle Q8 slide 2.5 pilot 50 air screw 1 3/4 turns out
recommended 440 main needle Q4 on 3rd clip slide 3.0 pilot 35 air 2 1/4 turns out , if it pings go richer on needle pos. g.