I spent the afternoon tinkering with my various carburator sets and since I had them off for R&R, I switched the throttle plates from my spare parts HS6 set into the HIF6s. They are indeed slightly larger, but the inner and outer bevel allows them to seal against the bore. The trouble is mainly that there's some sort of small vent hole in the HIF6 bore. This hole is perfectly bisected by the HS6 plates, but was entirely separated with the stock HIF plates. Unsure what that does, but chances are, it was yet anotehr contributor to the overall problem of #1 being unadjustable.

Seems like #1 is far more twitchy than #2. That's troublesome because it was fully rebuilt using kit SU-23 which included a new needle and all the gaskets.