Windscreen washer jet angle

[Turnip]

OK. There's that little screwdriver adjustment on the washer jet that controls where the jet hits the screen. The adjusters on mine are a bit 'easy', so is there a way I can stiffen them up so they won't make their own decisions as to which way to point?
My first thought is rubber washers. Is there a proper way of doing this that I've missed in my naivety?

Thanks, Simon.

[Hugo]

Screw them in another turn - they're on a tapered thread ;-)