BE SURE TO DO THIS WITH THE CARB OFF OF THE CAR.
The small screws used on most choke plates are almost impossible to remove with any kind of socket, or wrench. Here is the best way to remove these:
Generally there will be a small piece of screw sticking out the bottom of the choke plate.
The easy way:
Using something like a dremmel tool, grind the head of the screw off. Get as much as you can without grinding any from the shaft. Using a small vice grip remove the screw.
The hard way:
As above, grind the head of the screw as close to the shaft as possible. Start with a small drill bit and drill a hole through the screw. Step up to a bit almost the size of the screw and drill it again. If you used the right size bit, you should now be able to get the bolt residue out of the hole. Run a tap through the hole.
When reinstalling the screws be sure to peen the screws a bit so that they don’t work loose and fall into the engine.
You can find throttle screw sets for your specific carburetor at my carburetor parts site.