I replaced the entire distributor with a good used one but the problem was just the vacuum advance itself. It finally got seized enough to fight with the mechanical advance and wiggle out the little set screw that holds it in. So it basically was sticking out and saying "Here I am dummy! Replace me!" The truck ran crappy like that for a long time before I figured it out.
This may not be your problem but you have similar symtoms as I did and this is what my problem was so it's worth a shot unless you just really want the weber. A new vacuum advance is like $120 so you will want to find a used one to troubleshoot. I got my entire distributor for $20. It was off like an '82 or '83 22r. I had to swap the pickup coil plug but other than that it was identical to my old one.
