I wonder if the 500/550 floats would fit Mark, never had the 2 sets dismantled side by side before but the do look very similar, with the 500/550 floats having the tang on to stop them dropping too much it would fix the problem if they fitted.
You're exactly right about the tang: I've often wondered why Honda didn't have them in the 350F carbs? All their other carbs, unless the float bowls are very shallow, have had that little stopper to prevent this issue. I don't know if the float will interchange, though: there are 2 different float types in the 550, so that would add some confusion to the puzzle...
I've also noticed on other carbs, when using the modern aftermarket float valves, that I've had to adjust that tang to prevent this very trouble. It always stems from the same issue: the sprung needle tip is sitting further down in the valve body. In some cases this has been due to shorter float valves, and in others it has been due to deeper holes in the float valve body. I've also seen them taller in the past, but that was in like the 2005-2015 ear, Keyster parts mostly. In some of the taller ones I've had to reshape the float's stop tang with a bit of an offset downward and then back to horizontal to get the right bowl depth. This step is spooky, though: it hearkens back to the days of the Old Factory's 657a carbs in the early 750K1 - those often came with an almost-too-short float tang to begin with, which taught lots of us how to 'shrink' the middle diameter of that brass bracket to 'lengthen' the float tang because of an out-of-spec [shorter] float valve assembly. I still see these once in a while, tricky to fix without just outright replacing the old brass floats with plastic ones, which all have a longer tang.
The brass floats make a more-even 750 throttle response at highway speeds (which are back at K1 speed levels again now), because they bounce a lot more and tend to stay more 'sunk' than the plastic ones, which deepens the float levels at higher speeds. For the 'purist' this is sometimes desired.
