15% vs. 20% nitro will not cause the engine to die for no apparent reason. The engine will needle in easier with the 20%, but the engine will run fairly well on 15%. I broke my .63 in on 10% as it's cheaper and all you are really concerned with during break in is keeping the temps low, which running rich does, and allowing the ring and valves to seat in properly.

I agree that you were probably running it too rich during your first few runs. Lean it out a bit and it should start to behave more normally. If it doesn't there is more than likely an air leak either in the tank/lines, air box, or the valves need to be adjusted. There is nothing mysterious about the YS engines. They have a pump and the intake tract is different from other 4-strokes, but otherwise are just another engine. Break it in the same as you would any other 4-stroke.

John