First bypass all the circuitry and jump directly from the battery to the starter motor with a known good battery
No problems then the starter is OK
try again jumping from your own battery.
No problems then your battey is OK
Jump from the battery to the switched terminal ( one the starter is attached to ).
Starts OK the starter wire is OK
Jump between the hot ( battery side ) terminal on the starter to the small control terminal
Starts OK then the solenoid is OK
If starter rotates lumpy then chck the valve lash as the starters are only jut big enough to turn over a decompressed engine and excessive valve lash negates the decompressor.
