Check your fuel tank to make sure it is correctly vented. If not properly vented it will restrict the flow of fuel. Judging from positioning of the 25 gallon fuel tank in the 3-view drawings of the J4A, there is plenty of gravity to provide fuel to a C85-12. You might do a fuel flow test on the ground at various attitudes by jacking the main wheels to simulate a nose-high attitude. You might also check the carb if you haven't already, there is a small screen where the fuel hose threads into the carb. Make sure its not clogged. I would recommend getting to the bottom of this before further flight - good luck,
-Joel Marketello, restorer of a J5A (N397PA).