We need to understand the symptoms more clearly. If the starter motor turns, you probably wouldn't hear the click of its solenoid.
Theory: Ignition switch powers a solenoid (integral to starter motor). Solenoid is a heavy duty relay that engages motor with starter ring on flywheel then puts 12V onto the motor. Motor runs and cranks the engine. Any reference to connections includes earth return to battery (thick black wire from gearbox).
Scenario (1) - nothing happens when turning ign to start. Fault may be with battery, supply fuses, ignition switch, connections to starter motor or solenoid (in starter motor).
Scenario (2) - click or repetitive clicking noise - probably battery low or poor connections at starter motor (solenoid is engaging and disengaging).
Scenario (3) - clicking noise, no cranking. The click is the solenoid. Lights dimming if on at the time (indicating that the solenoid is connecting the motor to 12V, drawing much current, reducing battery voltage). Fault probably in starter motor (pre-engage jamming) or issue with starter ring or engine too tight (both less likely faults).
Scenario (4) - clicking noise but lights do not dim. Fault in wiring to starter motor or fault in starter motor (not connecting motor to 12V).
Scenario (5) - starter motor cranks engine but too slow to start it. Faulty battery or poor connections to starter motor or faulty starter motor.
Scenario (6) - starter motor cranks engine at normal speed but it doesn't fire. Not an issue with starter! Something such as fuel, air,compressionor ignition is missing or ill-timed.
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