Woah awesome!! Lots to look into here. Many thanks!!
I did a pretty thorough cleaning on the carbs but didn't replace any o-rings because the float bowl ring seemed fine to me (pliable, proud of the surface), as well as the idle mixture screw ring.
Since the carbs are apart waiting for the new jets I'll double check everything and see if I can't get them spotless, and make sure I've hit every internal passage.
Meanwhile I'm working on a total rebuild of a '74 CB350f which had seized up and been seriously neglected.