I've never had this happen using a quality Pitts muffler on a .46 or .55 AX. In fact I have only seen it one time on a .91. Of course anything is possible and it would be easy to check and verify or rule out.

Also things like the tank position will not be an issue here. There are tons of these Revolvers out there (mine included) with side mounted .55-AX's and tanks in the stock position and they perform excellent. Tank plumbing could be a factor though if it is incorrect or has a leak.

The point is that you will almost never fly at the recommended needle settings. Those are given as general guidelines so that the engine will start in most places and conditions. Yes, run it rich for the first couple flights but the manual settings will often be too rich.

Now all of this is just an educated guess based on experience with a lot of my own and other peoples engines. There is no way to be 100% certain without seeing it.