Malvin,
As Raimey said, the screw is an adjustment screw for the set trigger.It does not affect the normal(unset)trigger pull.Often,people trying to adjust the normal pull with this screw, make it not "hold" in the set mode(ie won't "set").They are sometimes unscrewed enough they are lost. To "set" the trigger, push it forward until it "clicks", then it has a very light "pull".
Mike