Rear tire has been going down about 10 PSI per week of late. Cannot see a nail in it. Soapy suds didn't seem to bubble anywhere. Just kept airing it up and riding it.
Today it was flat. Completely.
Put air in it and I noticed this little thing didn't appear to point straight out to the side anymore? Maybe 5-10 degrees to the front?
Seemed it was pointing forward a bit more than normal. Yep. I could turn it with my hand. I guess with all the recent air nozzle capers I've somehow managed to push it around a bit and loosened it up. Maybe nicked up the O-ring a bit too. I could turn it towards the rear and tighten it up to slow the leak, but I decided I may as well remove the thing once and for all to prevent another loosening occurrence with the air nozzle. Nice idea, but by pushing on it (it had a fierce Schrader valve in it for some reason) that it just didn't like getting pushed about.
Looking at the BMW rear wheel online fiche for the stem, it showed a D=15mm so I thought it was close to a rubber 0.625" regular rubber valve stem. Turns out the rim is some 0.425" (or close) hole and nowhere near D=15mm. Love how BMW standardizes on stuff (Not!). Back for a smaller one to the auto parts house....
Broke the tire down and off one side and went back to the generic rubber stem. No leaks. Needed a valve-stem puller too and some Yamaha Tire Mounting Lube (good stuff!). Tire still has maybe 400 miles on it and I got a new one, but am stingy right now and will reuse the old tire and cuss some more in a few weeks changing it again along with the rear drive annual flush.