Drove around today and am getting good voltage numbers.
To summarize, i was experiencing poor charging volts on my 78 xs500 when the engine was at operating temperature. I did some tests (see Guylr's very helpful posts on this) to try to find what was causing the problem but found nothing definitive. I felt I ruled out problems with the voltage regulator and rectifier, the field coil, and wire connections. I had no shorts to ground. I decided to replace the stator because it has been the reason for poor or no charging in several other cases.
I replaced my stator with a NOS one purchased from eBay. It was described as being for 73-77. The new stator was different from mine in that it was missing a fourth wire, for connecting to the harness to a relay. It turned out that it didn't make a difference and I left the single plug on the harness disconnected.
At idle (1200-1300rpm) with headlight on I get a voltage of 12.15 volts, with headlight off about 12.6v. At 2000 rpm headlight on 13.3v and off 14.3v. When riding, at 5000 rpm with the headlight on 14.88v, headlight off it drops to 14.6v. I haven't experienced the heat related decrease in voltage I was seeing before.
I would appreciate it if someone can confirm the idle voltage both with and without lights on.
As an aside, I also experimented with the ElectroSport ESR352 combination regulator/rectifier. They claim it is plug and play, but it is not. It will blow your main fuse in its original wiring configuration. To make it work I had to swap the red and black wires on the rectifier side, and the brown and green wire on the regulator side (this matches the wire colours on the harness connectors for the rectifier and regulator). I also had to pull out the brown wire female terminal from the plug coming from the field coil, and connect it to the negative terminal on the battery (I used a short wire with alligator clips at both ends). It seemed to give lower voltage at rev. I went back to using my original voltage regulator and rectifier since it works and they are hidden.
If any one else here has a running xs500 I would appreciate it if you could measure your idle voltage with lights on and off, to confirm what I am seeing. Thanks.
Dan