Arching is normal, just depends on how much. Make sure there isn't oil on the comm and the power supply is clean. Most of these motors are RTR, meaning the brushes are curved to fit the commutator. I would be more worried about breaking in the gearbox first because these motors are stock and do not run at a high RPM all the time like say a slot car motor would.

I know because I used to re-wind motors that I raced, twist the comm to change timing and even balanced them too.

Happy CN12?