If the carburetor is on top and the engine runs that way, it's pumping fuel "uphill" fine. Right? If the carburetor is on the bottom and the engine won't start, and fuel leaks from the carburetor, then it's pumping "downhill" and also getting a siphon effect too? Sounds to me like its flooding out due to fuel siphoning down to the carburetor and having the choke on too. When it won't start and the fuel is dripping, pull a spark plug and see if it's soaking wet. If it is, might want to clear the cylinders out and get the raw fuel out of the crankcase. Start the engine upright, run it up and clear it out good and set the plane right side up and see if the engine continues to run properly. If it sputters and coughs and quits, it's getting too much fuel. I'd be looking at fuel tank height, and the regulator/pump assembly to ensure those parts are working properly. But that's what I would do. YMMV.