I recently bought a 78 xs500. The only thing wrong with it is that it is idling between 2 and 3k. Which screw on the carbs is the idle speed? and what is the normal idle speed when it is warmed up?

You'll be shooting for 1200 rpm. I don't know if the xs500 uses the same carbs as the xs400, but the idle speed screw on my carbs is underneath them at the front. It's a big knobby screw.

But if you didn't tweak the idle speed to begin with, I'd think an air leak is the culprit for your high idle. It's pretty common on these bikes to have the carb boots crack and let in air between the carbs and cylinders, leaning it out. Examine them for cracks and replace them if they're shot.