Can I ask what kind of oil you used?
its important to use a "jaso" rated oil anything rated for motorcycle use would be the stuff.
I did this once with a sport bike i rebuilt .. i put regular car oil in it because its what i had.
I know you tried new oil but are you sure it was motorcycle oil?
heres a link to a blown up view of a 1980 bikebandit didn't have a 78 listed
http://www.bikebandit.com/houseofmotorcycles/1980-yamaha-motorcycle-xs650g/o/m8271
its important to use a "jaso" rated oil anything rated for motorcycle use would be the stuff.
I did this once with a sport bike i rebuilt .. i put regular car oil in it because its what i had.
I know you tried new oil but are you sure it was motorcycle oil?
heres a link to a blown up view of a 1980 bikebandit didn't have a 78 listed
http://www.bikebandit.com/houseofmotorcycles/1980-yamaha-motorcycle-xs650g/o/m8271