These engines do have a good piston seal after they bed in nicely - I backflip start mine most of the time. If the engine was pretty new and tight, you're still breaking in and loosening up. You should see around 11,000rpm or slightly better on a 13x6 APC. Depending on your elevation, you might make a little more power on 10% nitro fuel than I do with my engines using 5% nitro fuel. If it will hold a rich needle setting and throttle well, go fly it to finish breaking it in. They only get better with time.