[Neil E.]

There is something else to look at. The starter relay is signaled by your handlebar switch and closes to send power to the starter motor. If the relay contact is badly worn from arcing, it may not be passing enough current to turn the motor under load (compression). It could be good enough without the spark plug installed.

The ground path is also important. The battery, frame, and starting motor must have good grounds. Make sure every point of contact is clean (no paint or rust).

Take an old nut and tighten it on the starter motor stud. Connect jumper cables to a car battery. Connect the negative cable to the stater motor body. Strike the positive cable to the stud nut and see if the bike cranks (sparkplug installed). The old nut is a sacrifice piece, it will be damaged by arcing.

If the engine cranks fine you have a bad ground or a bad starter relay.