Checked intake and exhaust for leaks via smoke testing. Found a leak in the muffler; so that's another future project, but intake and exhaust looked fine around the actual engine.
Update: Still idling rich, rockers looked fine, nothing wrong with the valves. Going to soak cylinders 1-3 in seafoam, give it an oil change, and see what happens.
Well, I finally found out what the problem was... Here's a photo of the intake valve on cylinder 1 on the near side of the intake port. Looks like some blowby on that valve. Probably only happens sometimes and at low RPM. Looks like I'll be taking the heads off this weekend....