If the starter has an attached solenoid it may be that the solenoid
isn't engaging the gear with the flywheel, unusual, but worth
checking. The action is, you energise the solenoid, a bridging
contact flies forward to both increase the solenoid supply, thereby
providing enough power to push the gear forward and, at the same
time, energise the motor. If it fails, sliding part dried sticky/oxidised
grease, or rust, it whirrs but doesn't engage.
If got into how it works because I found three solenoid wires, one
a dry joint, so unusual, but not unknown. Try a breakers yard for
a replacement starter, it's a pain in the arse fixing them.
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