ORIGINAL: soarrich
First check to see if you fuel pick-up line came off inside the tank. Don't change the engine, they run fine upside down, that's not your problem.
The worst thing on a maiden flight is reversed ailerons, that's the big killer.
Agreed.
I run a couple of those same engines upside down on planes where I have done nothing to the tank height as well... they work flawlessly even with tank height offsets of greater than 1 - 1.5".
Tank height affects siphoning, which is primarily a problem on the ground, not in the air... it's overstated as a cause of in flight problems all too often here.
If this were as problematic as stated, we would all be experiencing deadsticks whenever our planes go nose up ( ooops greater than 1" difference when this happens! ).
Usually things like this are fuel line/tank/clunk related...
On my maidens, once I have the plane up, I keep them flying upright with only light banking on the turns, gaining altitude as much as possible.
Only after a few minutes of otherwise good flight do I roll the plane while fairly high, to check for potential problems with the engine/fuel delivery.
I tend to keep the planes high but close in, to facilitate getting them down in the event of a deadstick.