YL82
Perpetual Restorationist
What I mean by an isolated engine is cases where the engine is poorly grounded to the frame. This is usually encountered after a newly painted frame. The engine simply doesn't touch the frame because of the heavy paint. The starter motor being attached to the engine cases means it has to find a different route for the cranking amps required. They will run through the ground wire in the harness (black) and burn the wire. I've also seen paint so heavy in the bolt hole for the ground cable, just behind battery box on the frame, that it would ground good enough to run lights and kick start but smoke the harness if you hit the starter. And last is the battery box ground. Never run your ground cable to the battery box. It has rubber dampers on all 4 mounts and will send all starter amps through those tiny wires in the harness and they just won't take the load. I still think you have a ground issue but can't imagine how with what you say you have done service wise. Hope this helps.
Looks like my ground cable is connected right to the damn battery box. Yes, even with my infantile understanding of motorcycle electrical issues, I can see the rubber dampers that you mentioned and why not to try to ground there. Damn PO!
I wish I had recognized this much earlier on my own.