On the older starters a lot could be wrong.
1. Bad or old battery cables or corrorded connections. Inspect, upgrade and clean connections
2. You absolutely must have a fully charged battery in good shape or you are chasing one problem with another problem.
3. The starter relay and the wiring from the button down to it and the wiring from the circuit breaker to the relay and to the solenoid must be in good condition.
4. The solenoid does wear and can be rebuilt cheaply. Often you can flip the copper washer over and rotate the contact posts and get more life out it without a rebuild.
5. Starter motor, gear reduction gears all have to be in good shape. The brushes and commutator and field and armature all must be in good shape.
6. The fork must not bind in it's travel and the Bendix must not bind and slide freely.
Get a service manual and use it. IT is the best tool you will buy.
