When I took the engine on my 79 CB750 apart, we noticed the starter clutch/drive assembly was not securely bolted together. This may have caused the spinning problem that would occasionally happen when cranking the starter. I never got the chance to re-test it, as I sold the pieces parts to someone else.
I think there were 3 or 6 bolts that held the clutch to the starter ring. I can't recall the exact parts count, but I'm sure there's a picture of it in the FSM.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.