Pilot,


The fact that your engine sags when you pull positive G hints of a fuel tank location issue...

It might be mounted too low, relative to the engine...

The correct level is: fuel tank center-line must be level with the center of the carburettor's barrel (the fuel nozzle).
There is very little latitude for variation from this level (±3/8").

Also, the fuel tank must be isolated from any part of the fuselage, using latex foam and be as close as possible to the engine.

The J'Tec Pitts muffler provides insufficient fuel pressure as well...