help?

hi on my carb there is a screw with a spring which when u turn it in make the bike rev more nd wen u turn it out revs lower. there is also a screw on the airbox/carb if i screw in/out will this improve the bike performance? do these screws have anything to do with the mixture? aslo i had a black spark so if these have anythink do do with the mixture can sum 1 please tell me how to adjust thanks