Once the engine is running, the fuel tank position shouldn't be so critical.

Not a lot of info to go on, but...

From the info you've given, it sounds rich, as it will idle for a few minutes then quit. I would suggest leaning the idle mixture out in small increments (1/16th turn) until the engine will idle reliably indefinitly. It sounds like you're close.

This is assuming that it is quitting while idling. If it is quitting when accelerating, then it might be too lean on the idle. If it stumbles while accelerating, but eventually cleans up and runs, it's too rich. Be sure to get the high speed mixture set close to optimum first, before adjusting the idle mixture.

An inverted engine is not the demon many people say it is, and can be made to run as reliably as any other engine. Been there, done that.

Dave