I agree with Klem. I guess the effect starts now with anything less than 1G. In my around 300 hours of flying, most of which I've spent flying atmospheric piston engine airplanes with carburetors pretty much similar to WWII design (no neg G capability), I haven't seen this happening, not in heavy turbulence, nor in powered stalls (which is a zero G maneuver on full RPM).
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