The Magnum XLS .52A two stroke is an outstanding engine. I also removed my remote needle valve setup during initial break-in when I saw it was prone to leaking air. 10% nitro is plenty for the Magnum XLS .52A, although they seem to prefer a hot plug (O.S. A3, Enya #3) when running lower nitro.

Trying to insulate the tank from vibration by surrounding it with foam would be a good basic step toward reducing inconsistent runs. The XLS .52A is a very powerful engine and it certainly could foam up your fuel tank if run with an out-of-balance prop or spinner. Also, I've found that 11x7 two blade props weren't too much of a load on my XLS .52A. I've flown it extensively with 11x7 two blade and 10x7 three blade props.

Magnum engines use a true chrome sleeve and will require a longer break-in period than some other two strokes before the idle settles in. After four months of running, however, I'd expect the XLS .52A to be good to go. Mine seemed to settle in nicely after about eight or so tanks of fuel.

Everyone seems to be ignoring ceecrb1's comments about fuel leaking out of the front of the engine and messing up the inside of the cowl. This could be indicative of:

A) Fuel spitting out of the carburetor due to an overly rich low range, mid range, or high needle setting
B) Fuel spitting out of the front bearing
C) Fuel leaking out of a crack in the crank case
D) Fuel leaking out of an imperfect cylinder head gasket seal
E) Fuel leaking out of a worn, cracked, or split carburetor o-ring
F) Fuel leaking out of a loose or broken carburetor-to-crank case mounting

I would recommend pulling the engine out of the plane in question and running it on a test stand to see if you can determine the source of the leaking fuel at the front of the engine. This would go a long way toward isolating the source of the problem so the engine can be properly repaired.

Put it on a test stand with a fresh, hot long plug and some fresh fuel. Use a balanced prop, perhaps one with a slightly lower load like an 11x5, for bench testing as the engine won't get as much air on the ground for cooling as it does in flight.

If there is no obvious fuel leak from the front of the engine on the ground, and the engine tunes well and idles and transitions reliably on the test stand, then your engine problem likely isn't directly attributable to your engine. If the tank outlet and carburetor intake aren't the same height, the plane will more easily richen or lean out at various angles while in flight.

The forums are littered with requests for help from folks having problems with O.S. Max engines, the difference is folks usually don't recommend shelving the O.S. engine in favor of another two stroke. Work through this problem and you'll not only be rewarded with a great running and reliable Magnum XLS .52A, but you'll gain a lot of general knowledge about engine set up and tuning that will help you in the future regardless of whatever brand of engine you're running.