What things could make a starter motor in a 1972 Chevy pickup fire sometimes not others I replaced the starter over-rode the parking gear and replaced the ignition switch Still no go?

Answer 1

The battery ground wire to the frame/engine could be making a poor intermittent connection especially important when high starter currents are needed.

The battery cable connection to the battery could be just corroded enough so it's intermittent.

The starter solenoid wire could be loose/corroded.

The starter battery cable connection could be loose at the starter..

Starter solenoid could be sticking.

Motor winding insulation could be shorting out & failing reducing the motor strength.

Battery cell(s) has intermittent short (unlikely, get a battery test)

Suggestion: clean cables, check ground connections, get battery checked (free at most parts stores), remove starter solenoid start wire. connect jumper wire and engage manually by jumping to 12v to see if something in the harness is intermittent. Alternately, connect a wire to the solenoid start signal (ie the small wire) and route it inside the car to a voltmeter or simple trouble light(ie 12 bulb to ground). When the starter doesn't fire, verify that the meter read 12v when engaged. Or, just replace the starter and see if it repeats after doing the above.