I have found that keeping the tank outlet and carb inlet at, or close to, the same level works best.

A similar event like the gentleman above described: After being run well on a bench, an inverted 91, would not idle correctly AT ALL when in the plane.
Reason : Because the tank's output was at least one inch higher than the carb's inlet ( viewed/measured as the plane sits normally on it's gear) , I failed to accept the fact that this "might" cause problems, even though I took notice as I was assembling the ARF!
Fix: Relocated tank (major alteration), and she purr's like a kitten!

Another thought: Unless you are committed to your current project, for your first nitro model, I recommend a model with an upright engine installation.

Just my $.02 Good Luck!