At this point, try lowering the jetting. If too rich, maybe some fuel is leaking into the venturi.
As for piston spring coils, that's beyond my expertise.
Of interest, this gentleman has a lot of interesting Rochester 2 videos:
https://www.youtube.com/user/MikesCarburetor/videos
If I was in your shoes, I'd send the center carb to a pro, and for fun tinker with another to learn while you wait on getting that one back in your hands.
Carb issues are really tough on a forum.