I'll try to explain my method. Jerry's pipe tap ain't bad either.
I slit the offending part with a tool that fits the occasion. Tiny drill bit, dremel. cold chisel, or thin the ID with a drill. Obviously this is delicate because you don't want to touch the carb threads very much. Normally I can stand a slit in the threads because a new bolt does not care about a vertical slit. BUT in your case it may leak. Anyway, since you have some stick-up to grab with a tight adjusted vice grip, twist it inward like trying to coil up a spring. If it is split or thinned, it will peel away from the carb. Maybe a piece at a time but eventually it cleans up. Grab the fitting wall, not the whole filling OD, when twisting and winding it inward. (after slitting/separating)
None of this is easy. We all use all kinds of ways to get threads fixed. The brass fitting will be harder than the aluminum/zinc carb. Use that to your advantage.
Jon's toaster oven with controlled heat is good except your next food item will smell like gas. That's just extra flavor to a Stovebolter.
