1990 Johnson Evinrude "ES" 9.9 thru 30 Service Manual, P/N 507871, Page 287Get this manual
Starter System Voltage Drop Test, 2030
By making systematic check from the positive battery terminal, through the starting circuit and back to the negative battery terminal, any component or electrical connection having excessive resistance can be located below
Avoid accidental starting of engine while servicing, twist and remove all spark plug leads
Clean and retighten, or replace, any connection, cable, or component having greater than specified voltage drop