[digginfool]

Okay. Went and purchased an infrared thermometer. The rear cylinder, driver's side, is 150 - 170 degrees cooler than the other three (220 compared to 368 - 388 for the other three). Thankfully, that's not the cylinder with the shattered spark plug, although it was only the external portion of the insulator that broke. I don't see how linkage could affect only half the carb and the popping suggests that fuel is still flowing through that half of the carb and into the cylinder, where it's not burning, and dumping raw into the exhaust. The plug that came out of that cylinder was one of the ones that I thought was a bit dark but still brown, so I don't see how it could be a burnt valve from leaning out, which if anything would cause that cylinder to be hotter than the rest as well as a different color. I'm officially stumped. I'm going to look at the cap again to see if I missed something.