As mentioned above, I have also found that they seem to like a warmer plug. I have 3 Evo's and have either a Hangar 9 Super plug like the one it came with, or an O.S. type F or similar. I am thinking that perhaps all the extra fin area on these engines make them run cooler, and thus they need a warmer plug? Oh, and I too use either 10 or 15% nitro fuel...usually Powermaster...and they run great