I have a Castle MMM 2200kv and a SC 2400kv. In my opinon of what I know it's just how castle motors are. Both mine spin hard even without the pinion attached our the motor hooked up. If thats what it is your talking about. Nothing to worry about if thats the case. Gratz on the sc10 as well. I actually just ordered a FT SC10 today.... with a Castle MMP 5700kv. Add another to the list.