Edison GEM on/off speed control adjustment

[coecky]

After re-assembling my friend's GEM, I have a problem with the speed control: when I get the right speed with the knob fully clockwise, the rotation will not stop when I turn it off. If I modify the speed control (small screw inside the knob) so that it brakes when turned completely counterclockwise then it will not reach the normal speed when fully opened. What am I doing wrong?
Rgds,
Alex.