73 850 gearbox

I have a 73 850 with a gearbox that shifts fine without the motor running but locks in 1st gear when started?

Your clutch if freeing off without issue?

New clutch installed I figured that was the issue from the start

Primary chain too tight? Loosen it to test...

While checking the primary chain also check the clutch is secure on mainshaft an that the small circlip that locates it has not failed.
If it has failed when you pull in the clutch it can let the assembly move along shaft an bind on the end face of the sleeve gear which makes hole gearbox spin as if clutch is still engaged.
A friend of mine as had this happen to his bike after a full rebuild recently.