When I first took the bike out for a 5 mile ride at about 45mph, I noticed a new feature: hot oil gently spritzing my right pant leg with the fury of a tiny sun. When I stopped to take a look at it, I found that oil was seeping slowly from the port where the tachometer drive cable entered its housing on the right side of the engine head. The cable was secure in the housing, but there was a couple millimeters of play in its seating against the oil seal. This is probably why there was a leak. I think that over the 40 years since the bike was assembled, the seal had hardened and shrunk.
I ordered new Honda part 91211-286-003 to replace the oil seal that was apparently failing. That’s part 19 in the schematic above.
At first, I had a lot of trouble getting that oil seal out of there. It was apparently fused to the aluminum of the housing. I tried using combinations of penetrating oil (Seafoam Deep Creep), dental picks, wood screws, and tweezers, but nothing worked. I looked to www.hondatwins.net for guidance and found a recommendation to just pull out the whole tachometer drive element (#5 in schematic), and the oil seal would come with it.
After some wiggling and gentle but firm yanking, the whole assembly dislodged. I had pretty much destroyed the old seal while trying to remove it earlier, as you can see.
I replaced the seal and re-assembled the connection. The play in the seating of the cable is now almost gone, and the leaks appear to have almost completely gone away.