I have no idea how I read in your post that the car was on 403s. What an idiot, sorry.

Have you checked the carbon brush in the dist cap?
If your main HT voltage is down then lowering the mixture can result in a misfire.

That said, so can turning the mixture screw too far when you have a sensor failure which, if this is a sudden problem is also quite likely.

This is one of those problems where it's probably cheaper and faster to just get her on some good diagnostics that can read weber properly and see where the problem lies.