Is this rotor usable?

[NoDOHC]

I think that RETed is correct, that leaner mixes will protect from carbon buildup to a large extent, as will careful selection of engine oil or running premix. I get carbon buildup in my '91 driving around town (where it runs open loop and filthy rich) but it goes away after a long highway cruise (due to closed-loop, stoic fuel mix - or maybe just extended operation). My '86 (which run 16.2:1 AFR (lambda of 1.1) everywhere but WOT) never experiences the carbon-buildup-characteristic knocking that the '91 does (although this may have something to do with the 9ish 7,500 rpm shifts that it experiences everyday on the way to work). My 13,000 mile teardown (which is still a sore subject) indicated very little carbon buildup on the rotors (although they were polished).