Setup a Cox .049 for Stunt
By Dave Hull
Based on your description, it is almost certainly caused by one of two things: tank venting, or fuel pickup location. The stunt tank uses over and under vent tubes that should be on the inboard side of the tank. If you use a tank body with no integral vents, but instead use a vented backplate in the "upright" position (NVA up), then the fuel will block both vent holes and block fuel flow inverted. Think Sparklets bottle--glurk, glurk. Next, the fuel pickup must be outboard, and should be at the very back of the tank. If you cut the end of the tube at an angle, you can push it against the backplate without blocking the flow. I always use the vinyl tubing and anti-collapse spring. I hope the crude sketch helps.
Let us know what you find, or how we can help.
Dave "McSlow" Hull
PS--I would also check to see if the upper vent tube is blocked. Maybe a dirt
clod or chunk of grass got in there during one of the inverted landings.