I have 2 different types on my engines one has a spring and the other has a double nut holding it in place and one even has a slight taper to the barrel grove that can be used to close the barrel slightly more if needed. but on my engines this screw HOLDS THE BARREL IN if you would like to remove the barrel from the carb just take the screw out and barrel will fall into Hand. However this is not what I would like to happen in the air, if the screw was missing, hense the name it stops the throttle from comming out or throttle stop from falling on my head ect.......

Just a thought