Mysterious leak from rear of engine between engine casings.

Recently I had a clutch throw bearing replaced on my 1975 R90/6 as well as the neutral light switch replaced. Now the engine casing directly in front of the rear tire is leaking either oil, gas, or transmission fluid. Any suggestions?

My hunch would be the output seal from the transmission that links to the drive shaft. Take a whiff of whatever is leaking there, I doubt it would be gas that far back. 80W90 oil has a distinctly different odor than engine oil, it would be readily apparent by smell. My hunch is the output shaft seal on the back of the transmission, it is not difficult to pull the old seal and put in a new one altho it is time consuming. On a bike this old it is to be expected seals will leak, post what you find.

I like the "follow your nose" theory. Gasoline and gear lube do have very distinct odors. Also, check the underside of the rubber boot that encloses the space between the transmission and drive shaft. If it is leaking, there will be wet goo there. These are notoriously difficult to get properly fitted and pinched off. Check the tiny ribbon clamps to make sure they are tight, particularly the one on the swingarm side. The drive shaft tube should hold a small quantity of gear oil. 150cc's if memory serves correctly.

Could also be the seal where the clutch arm is mounted on the back of the transmission.

You may see the oil on the back of the transmission, but oil leaking forward of here can blow most anywhere else on the bike.

The way to find out is to clean everything well. Then sprinkle baby powder and run until the oil shows up.

You should be able to smell the difference between gas, gear oil and motor oil. Gas will also evaporate quickly.