The left to right adjustment of bobbin tension is for uneven feeding of the bobbin, for example if there is more thread on one side of the bobbin than on the other.
I have a bobbin winder tension that is missing one of the pieces, so no tension on that one. But if you have all the pieces, bkay is right to adjust the screw on the front.
Something else that may help, besides weight of thread spool, would be to put a spool pin felt on the spool pin on the bed of the machine.
It seems that some of the 15s have a notch on the front side of the nose plate, so one could run thread from the top spool pin to the front notch of the nose plate, down to the tension disc and then up to the bobbin winder.
Janey - Neat people never make the exciting discoveries I do.