ORIGINAL: CGRetired
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.
So true... even if you invert the engine the difference that results has VERY little effect on siphoning and NO effect on how the engine runs, contrary to what is often posted here.
ORIGINAL: CGRetired
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.
Note however that with fuel levels less than 1/2 tank and especially by 1/3 tank or less, the tank WILL become uncovered during downlines, leading to air bubbles in the line, as he has directly seen with his nose down test...
Short of replumbing with a header tank, or a fuel bladder, or finding a way to PERMIT the clunk to travel forward, there is no real cure for this with a standard setup... often this somewhat "normal" condition is seen as a problem, especially with bigger engines.
With a smaller engine, we normally don't run the tank down to the point that this is a problem.