These engines take a little bit of running to finish breaking in. The liners are chrome and the ones I've seen have a tight pinch at the top, so they need a little running time. My advice would be to burn a couple of tanks of fuel on the ground, or on a test stand before final tuning and flying. I've only seen them run on 15% nitro.

Stumbling or dying could be a sign that your low speed needle valve is a little rich. Also, it is a good idea to replace the glow plug after you break an engine in. These engines don't require a whole lot of time doing that, but you may find that either a Saito SS plug or Towers plug will fix this symptom. The OS 8 plug is too cold for 3 other sizes of Evolution engines that I've seen. I have a friend who is using the SS plug in his .61 without any problems.