JR: another possiblity is a faulty solenoid. If you have access to a volt-ohm meter or digital multi meter, and you understand how to use it, measure the resistance of the solenoid contacts. The resistance should be very low, say less than 0.010 ohms. Another check can be made using the dc volt scale and measure the voltage drop across all of the elements of the starting circuit, including cables to and from the solenoid, from solenoid to starter. There should also be very little voltage drop across the solenoid contacts during starting. Also be sure that your earthing point, that is, the connection between the battery and the bulkhead is sound.