Hi, I have a Homelite gas trimer... try to unscrew the head of the line... it has kind of round black knob and a screw inside of it. I try to unscrew everything together but it's so tight. I woner if I have to get to that screw somehow? It looks like it's part of the knob... the knob itself is cut out to fit the screw in. If I push the knob in to get to the screw it doesn't go enough to reach the screw. The screw stays in the knob. Help please!