Generally speaking, tight stitches up top and a bird's nest underneath is an upper tension problem, and it sounds as though your upper tension unit needs to be taken apart and cleaned up anyway.
Regarding the cam problem, with the machine in forward, and a cam in place, if you turn it by hand will it move cloth in the correct direction? I would try this with no thread, upper or lower. And I would try this with the top off so I could see what's moving, or not moving up top.