ORIGINAL: dredhea
That certainly sounds like siphoning to me. If siphoning is happening when the engine isn't running, it must also be happening when it is running.
Once the engine is running airflow through the carb controls fuel flow via the Bernoulli effect and the needle valves.
Siphoning by gravity has almost nothing to do with it.
The next time you break in an engine on a stand, try hooking up a 2 foot plus length of tubing to a tank.
Start the engine up, check the RPM's... now move the tank up 2 inches and watch the RPM's...
Also move the tank down 2 inches... no much difference eh?
When you go nose up or down, the tank is a good 6+ inches below or above the carb and yet the plane still flies fine.
If the engine is unduely siphoning when it is off, it may still be out of tune, as a tuned engine set to idle will at worst siphon or drip somewhat slowly.
Once on, it doesn't care...
Most of my engines run inverted just fine BTW including my ASP's and Magnums.... 24 inverted engines on planes.
I've never modified the tank height in a single ARF that I own after inverting the engine...
On a couple of planes with "T"'s I merely clamp off the fuel line while fueling or while the plane sits on the ground. Once I go to start it I remove the clamp until I refuel. No big deal.