Found out why my carb was moving.
It seems that the metal colar on the base of the carb is held into place by three holes which are filled with the plastic (mold) of the carbs body.
Because of the different matterial and heat the collar gets loose from the plasic carb body and allows some movement.
It doesn't matter how hard you set the grap screw... since it doesn't hold the plastic carb body underneeth that metal colar.
I also don't know whether this affects the tuning in any way. This slight movement does create a small gap between the metal colar and the plastic body which results in a possible air leak from the upper o-ring area of the carb.