Well, what to do if the screw is damaged beyond use and still stuck?
Drill it out. Select a small bit to drill a starter hole in the center of the screw head. The cross-point head design makes it easy. Just to the depth of the cross point.
Switch to a larger bit -one just slightly smaller in diameter than the outside diameter of the screw head. Drill off the screw head while avoiding too much contact with the aluminum carb top.
Once the head is removed, the threaded stud can be turned out by hand,
Had to do this on two of four evil carb top screws. Sounds scary. It's not.
Monte