The engine is grounded to the frame at the right-side rear frame member. The main wiring ground connection is a gusset attached to the frame backbone beneath the tank. Photos are attached.
The starter itself is a ground connection; if its internals are dirty that will cause numerous problems. If the ignition switch contacts inside the switch housing are burnt,
that can intermittently interfere with the starter's function. If you rap on the starter relay with a screwdriver handle and the starter will work, the starter relay contacts might be oxidized or burnt and cleaning them could restore function. If the battery positive or its ground connections aren't tight that will also affect starting.
If you're hearing a
whirring noise instead of a buzzing, the starter sprag clutch could be clogged and will need to be cleaned by one means or another to restore reliable function
