Acting on Walrus' advice, I pulled the carbs again and checked the plates. This time I pulled the original vented plates out of my stash, sealed them u,p and reinstalled them in the carb leaving the new jets and new throttle shafts in place.

The idle behavior is much better and the carbs now pass the "spray-test." However, no matter what I try the plate on #1 will not fully seal. I always get a crescent of light around the edges. It is almost like the plate is too tall or too narrow...

Because of this persistent leak, I cannot get the #1 carb idle down any lower and the minimum balanced idle is about 1200 RPMs. The idle is much smoother and the vacuum is constant.

This has been a relatively consistent problem with all my carb. I run out of idle screw before I get the idle down under 1000 rpms. From what I've read, this usually means air getting in, but if the shaft and intake gaskets don't leak, how's it getting in? Only the butterfly plates?

It is a little odd that about 10-12 flats is the stable idle mixture. Seems a bit rich for the #1 carb but would be consistent with a leak around the plate.

The question now is why won't the plate seal up? I've never thought of the plate as a wear-item and I don't see any grooving or damage to the intake passageway (venturi?) surface, so I'm not clear how the plate could cease to seal up correctly...

My thought here is that I should order some replacement butterflies. Would that "fix" the sealing problem?