This subject has no doubt been visited as much as any subject about airheads. But, I hope to add some helpful info. My rear main seal finally started leaking after more than 20 years. I had put in a Chicago Rawhide seal those two decades ago, and it performed admirably all this time. So, I decided to go the same route with another "generic" seal. It is actually the same rear main seal as an Acura. After getting the bike all back together, I started it. It sounded fine until it heated up. Then the "chirping" started.
It is almost common knowledge that is a rear main seal symptom. But, since I had just put in a new one, I sought other answers that might explain it, and that would not necessitate me tearing the whole bike apart again.....I hate doing stuff twice.
I rode to the Falling Leaf Rally last October. The bike performed admirably, getting 51 and 52 mpg, loaded with camping gear, running two-lanes. What's up with that? But, idling at stop lights, or starting after fuel stops was totally embarrassing....Chirp, Chirp, Chirp.
I finally decided it was time to do the repair over again with a new BMW seal, this time. Might as well rule out the obvious, that the generic seal was not up to snuff. When I got it apart, I found the tensioning spring from the back of the seal hanging on the crankshaft behind the seal. So, it looks like the spring popped off the seal when I was driving the seal into the case. That would explain it. Not enough tension on the seal to actually seal properly. (I don't think that accounts for the extra mileage). And, no, the bike doesn't seem to be running lean.
I talked to a Mercedes mechanic friend of mine, who said that he was paranoid about those springs, and packs them in grease to keep them from coming off during installation.
So, the good news is no chirp, and the first tank on non-alcohol gas, running pretty easy on some back roads, gave me 53.6 mpg. Coming back running in a head wind dropped to 47. Headwinds are terrible things.
