You need a run in cord, The reason to run at half speed is to stop carbon chipping,It takes about 5 or 8 minuts,
If your carbons were worn out ,I am willing to bet that the armature has over 800 hours on it.
The new carbons have new springs and the the ground strap,which can be tough on a motor that has that many hours on it,
Its always better to bench run the motor to make sure nothing funkys going on with it. The bearings usually are in need of service also like changing the upper and lower,also the fans should be balanced also.
This service can only be done on motors that are repairable,Like older lamb/ametek.
One of my pet peeves with manufactures, is why should a sealed motor that needs a lower bearing for 5 bucks be replaced.
STUCK IN THE OLD DAYS.........................
regards
MOLE