Your Magnum XL .52 RFS is a ringed four-stroke engine and will require considerable break-in before it will run reliably. The owner's manual should advise you to run the engine extremely rich, keeping it under 5000 rpms or so for the first ten minutes of run time. After that, you will want to continue running it very rich, leaning the needle slightly with every subsequent tank of fuel.

Your 15% nitro fuel is fine so long as it contains some castor oil as lubrication. Magnum doesn't recommend 100% synthetic lubrication for their engines. I'm more curious about what kind of glow plug you're running and what you've done so far to correctly break in the engine.

If you're not running an O.S. F-type plug, that would be the very first thing I'd recommend you try. More information about your plug and break-in proceedures will help us help you.