opalstar

Hello,
I have an old Free Westinghouse sewing machine. It works and I need some help. I have attached pictures. The motor starts but the small back wheel doesn't connect with the large metal wheel unless I push the half circle thing in the back that is attached to the small wheel covered at the end with a piece of black rubber. This half circle thing moves up and down a little bit. I don't know if that is supposed to happen or not. If I move it and hold it up a little it connects and all is well. Of course it is hard to sew while holding it up. Any advice will be greatly appreciated.
Thanks in advance,
Jackie