When you say throttle casing are you referring to the switch housing? Is the head of the screw stripped or is it turning in the hole? If the head is stripped, get a set of easy outs and give it a go. If it is spinning in the hole mabe you can get a putty knife and slide it in from the other side and apply upward pressure as you turn the screw iin question.
John
So you talking control housing ...you can use a drill and bit the size of the head and by drilling you'll remove the bead of the bolt.Then romove the contol housing and grab the little bit left with a vice grips and unscrew.H
You could also take a Dremel and cut a slot in the head of the screw and use a flat head screwdriver to get it out. I had to do this when I replaced my rocker cover gaskets and the bolt broke off in the jug.