The spring is probably stretched. You should probably replace the spring with a new one. If you really want to shorten it just clip some of it off and bend the cut end over in a J shape to hook it back on. If you trim too much, it will operate at too high of speed and damage the motor.
Remove the spring, clip a small amount off with wire cutters, bend the end to hook in place. Also consider buying a replacement spring from the manufacturer.
