Great thread, thanks to the OP for starting this. I traced a leak back to the tach cable port on my 78 CB750 and found that the PO had installed a new o-ring/gasket on top of the old one. I was able to pry both of them out, the 'new'/outer one came out with no issues and looked to be in great shape. The inner/older one took some persuasion, but came out without doing any damage to the port walls, just messed up the gasket a bit. I put some bond on the newer gasket, re-installed it making sure it was fully seated in the port. Let the bike sit overnight then drove it to work this morning with no signs of the oil weep that had been plaguing me. What an easy fix, thanks to everyone who contributed, as all of the posts helped me get this problem resolved.