With a 10%-15% nitro fuel, and O.S. Max #8 plug should be fine. Most likely, the engine was never broken-in properly. It would be best to mount it on a stand with a prop and spend the time to get it well broken in. Unless you can fly a heli at full throttle and anticipate autorotation, it's not easy to get a good breakin when the engine's mounted in the heli. A lot of good engines have been ruined by attempting to break in an engine while it's mounted in the heli.