Past chain/sprocket troubles:
http://www.ninjette.org/forums/showthread.php?t=63170
http://www.ninjette.org/forums/showthread.php?t=78956
The chain and sprockets are only a few months old. I'm running 15:43 JT sprockets at the moment with a cheap "Volar Motorsports" O-ring chain from D2Moto.com. At first the installer accidentally messed up one of the pins when removing a link and it had to be pressed back in after I found that it had pinched and torn an O-ring on the other side of the master link. That was resolved and, just to be sure, I bought a new master link from ForwardPowersports.com which looks identical ("Factory Spec" brand).
Anyway, after replacing the rear rotor and perfectly aligning the rear tire using the string method in early August, I rode 130 miles to work on a way-too-tight chain, which I corrected as soon as I noticed, which undid all of our meticulous tire alignment! It was caused by the "chain seems to tighten when you tighten the axel even though you set the adjuster nuts" phenomenon. *shrug* It seems that I may have done some kind of damage because I started hearing a pop or grind every now and then shortly after that, usually when starting from a stop or leaning/turning.
I adjusted the chain twice since then and it got notably worse and louder when I did it today (the second time). It was difficult without my rear stand, which I already got rid of, but I had been keeping an eye on things and hadn't seen anything out of the ordinary until today.
Each tooth appears to be developing a bump right where the tooth normally starts tapering to the squarish tip, but only on one side. This little bump/ridge seems to catch on the chain link and make a grinding noise as it is forced down anyway.
Now, I need to cross the country in a few days so I need to make this servicable again quickly. I don't know if the chain is causing this or the sprocket. The sprocket looks even, so I can flip the sprocket over and see if it starts wearing the other side of the tooth, right? If so, new chain. If not, I will have hopefully bought myself time for a new sprocket, at least. Maybe someone at tomorrow's SoCal tech day will have one after swapping their sprockets.