a) The bushing rod is excessively worn due to friction between the rotor and the magnetic pole. b) Poor contact of graphite brushes with the commutator. c) The surface of the switch is scorched or etched with oil. d) The weld between the conductor and the commutator is damaged. e) Poor connection between cable and clamp. f) Solenoid switch contacts are not working. g) The battery is not charged enough
a) Replace the bushing rod with a new one, clean the surface of the commutator. b) Scrape the contact surface of the brushes and adjust the spring force. c) Sand the surface of the commutator, remove oil stains from the surface of the commutator. d) Re-cook. e) Tighten the nuts to obtain a good connection. f) Restore the solenoid switch contacts. g) Charge the battery.