The brushes on the commutator sparkled strongly. The cause was in a damaged commutator that did not have a centric run. Part of the commutator was lower than the rest because one wire was broken and the owner did not stop when this happened. The brushes bounced, so it sparkled.
I reconnected the wire and straightened the rotor and now it works as it should.
I repaired (turned) the rotor in the machine on the original bearings. This is the easiest way to ensure centricity.
