Bernd,
tighten the hold down screw so that it is difficult to adjust the gap but "not" impossible to adjust by using a small flat screw driver to move it out. If you move it out too far then loosen the screw and start again.
When I write move it out I mean to pry it out.....gently.
Sometimes it takes me five times before I get it close.
If you find the dizzy cannot be adjusted then off it goes to repair. I don't know of anyone over your way. Here in the states the guy many of us Mustang owners use is Tim O'Connor. He has a website.
Auf,
Mike