What’s all this talk of “solenoids” and the starter “jumping out”??
It’s not a car…. It’s a motorcycle and it has a starter RELAY and a starter CLUTCH.
Now… back to reality…. A fully charged jump pack connected to the battery , all connections clean and secure, relay has a healthy “click” and if that starter clutch just spins with a whirring noise, the starter clutch is the obvious faulty link in the “chain” of events needed for cranking the engine over.
The starter clutch is bolted to the end of the crankshaft engaged by means of a spline, so that eliminates a sheared woodruff key causing the problem.
Check the bolt is tight and secure though.
One other thing, I’ve seen many motorcycles jump started using cables connected to a running car, never have I seen that cause a problem to the electrics or the electronics.
I’m not saying it can’t happen, but I’ve never seen it or even heard of it happening, and I’ve been working on motorcycles for a long time.