This is all good information but one particular item has not yet been addressed. You never mentioned what aircraft this issue involves. If it's an ARF then I would seriously think that it is NOT tank position. Most ARF's are designed to have the tank centerline in line with the carb, or darn near it. The best situation is the shortest possible fuel lines, a properly installed fuel tank with foam protection from vibration, and a properly installed fuel feed line (inside the tank), clunk, and both vent and pressure lines. If you don't use a vent (two lines, one to the carb and one to the muffler pressure tap) and if they are as short as possible, and if the clunk is free to move as it needs to without getting stuck, then it could be an engine that simply needs to be run.

CGr