The XR still won't start/run. With starter fluid and almost all of the energy I could muster I got it started but it would die almost immediately. The longest it ran was about 10 or 12 seconds.
I was a little nervous about doing this kind of work myself, but I pulled the carb based on the service manual guidelines. Clean as a whistle but I went at it with some more carb cleaner anyway. Needle and jets all looked fine. Float had no irregularities and was free-moving.
Since the tank was off the bike, I tested the petcock in both the ON and RESERVE positions and the gas flowed freely. Tomorrow I'll dump it and put fresh gas in, but I doubt that's the problem.
Spark plug is good. Did the "touch it to the side of the motor" trick and got blue spark. Fuel filter is in/behind the petcock so opening the petcock while the tank was off showed no slowing of fuel there.
Air filter is a new-looking K&N that was cleaned and oiled before the last ride and shows no signs of dirt or crud.