To add to the good advice given so far:
(1) The Rochester screws are 6x32, the Carter YF screws are either 3x48 or 4x40 thread, probably 3x48. DO NOT THINK ABOUT USED SCREWS!
(2) Before you start drilling and tapping, at least read the following procedure, it will save you MUCH grief!
First, heat the throttle shaft in the area of the screw(2), and allow to cool.
Next, use a centering device (a diamond burr works very well) to start a small hole in the center of the broken end of the screw.
Use a number 50 drill (0.070) and drill a hole maybe half way through the screw.
Now, use a DULL number 48 drill with a variable speed drill, and attempt to gouge the drill into the hole in the screw.
Once stuck, continued drilling will cause the screw to screw out the bottom side; AND THE ORIGINAL THREADS IN THE THROTTLE SHAFT WILL BE INTACT!
Once you are ready to reassemble, a HALF DROP of blue Loctite on each screw will prevent them from backing out, DO NOT ATTEMPT TO PEEN THE SCREWS! Make sure you center the throttle plate in the bore.
EDIT: I posted before reading the entire thread. This issue is quite common when Dr. Goodpliers works on the carburetor, and does not remove the peening from the reverse side. I have had 4-barrels come in with 8 twisted off screws (practice makes perfect!) I suppose it is possible that one could break without help, but that would be quite rare. Not nearly so difficult to repair as one that comes in with a broken not-so-easy-out stuck in the screw!
Jon
