Seeing the construction of this tank, and then assuming you are running with the overflow capped and no muffler pressure to the uniflow line, my thought is that when you take this inverted, you will get a slug of fuel trying to backflow from the “hopper volume” into the uniflow vent. The fuller the tank is when you go inverted, the further this slug will go.
This can happen because the viscosity and density of air is so much less than the fuel. You could get a chug-chug-chug of air going in thru the flooded out tube, and the resulting pressure pulse and flow restriction to the fuel can’t be doing good things to the output flow to the engine.
If you like to kick your tanks out at the rear in order to get a clean cutoff, the baffle holes shown would not be ideal. I would also try a notch in the baffle at the aft center point. From the picture, it looks like the baffle butts right up against the aft cap, so no joy there, either.
The plumbing arrangement is also a bit confusing. For a conventional gear plane, especially an “old school” model that sits with a pretty good deck angle, having the overflow vent at the rear gives up a fair bit of usable tank volume. It almost looks like someone reworked this tank and converted the overflow tank exit position to the uniflow.
Other than that, this tank looks like it could win any ProStunt contest in America up through Expert. I would upgrade it and start practicing.
Hope you have a nice tanksgiving,
Dave