Yes the carbon brushes, if they are new and have been replaced, have to hone themselves into the motor so you might notice an ozone like smell or blue sparks. This should clear up the longer it is used. Burning rubber smell would be coming from the belt, make sure it is installed correctly and is not slipping. If the belt is OK, it can also possibly be an old smell trapped in the motor from the last owner that is finally being blown out.