Injectors

Does anyone know what the screw on top of the injectors is for I cant find it in my books. Ive pull the injectors and blew them with air and flushed them and blew them again and they look clean but I need to know about the screw, is it like an air /fuel mixture screw and how do I adjust it? When I took the injectors out the screws were about half way in, do I tighten them down and could that be the reason im smoking so bad too much fuel cause they wernt in all the way