Starter failure or slow cranking often occurs when you energize an electrical starter. In such cases, you should perform the following checks:   

1. Whether the storage battery is dead or of little electrical power; 

2. Whether the wires get loose or fall off, leading to the switch failure;  

3. Whether undue mechanical wear of carbon brushes contributes to the loose contact with the rectifier;  

4. Whether the field coils or the armature windings short-circuit; 

5. Whether the rectifier gets stained, resulting in poor contact of carbon brushes with the rectifier.