How can I increase the friction in this shifter. A tightening of the screw does not solve the problem. That is, if I tighten it too much, I can't move the shifter. If I loosen it so the shifter with shift, the load on the cable is enough to move the shifter up, especially while riding - caused by the riding vibrations I'm sure. The black washer under that screw has disintegrated (see black fragments on the left of the last pic).

This is a 1972 Schwinn but my '80's Tommasini/Campy NR has the same affliction.

Any help?

Thanks!