OK - Ive checked the following resources:
IJNAviator1937-45
RAAF Directorate of Technical Services Detail No. 119/A/30 for the MK. II (HAP) [A6M3 for our purposes]
JICPOA 5981
Curtiss P40C vs. Model 21
Spit Vs. Zero - A.L. Walters Group Captain, No. 1 Fighter Wing, RAAF - Darwin
J-Aircraft.com
Warthunder
WWII Aircraft Performance
Wiki
And several other sites using various queries like Sakae 12, Sakae Development, Mitsubishi Engines and other terms.
Conclusion:
There are no easily found documented anecdotes stating the SAKAE 12 engine suffered from cut-out in negative G maneuvers. Compare this to the fact that the Hawker engine cutout is widely documented and cited. The absence of even one resource citing a negative g cutout makes me think that it was not a characteristic of the down-flow carb.
I do recall, within the last four or five years, seeing a diagram / animation of a schematic carb with the neg. g. issue, versus a downdraft version. I will continue on with this...
EDIT - This YT Video also does NOT mention negative G cutout - where if it was a characteristic of the Zero, they would have mentioned it...Certainly.
https://www.youtube.com/watch?v=teXzEti5PVYStill reading this:
http://www.wwiiaircraftperformance.org/ ... -dec42.pdf