Well, it could be that in a plane I just never get enough raw fuel on the firewall to find out. It's just exhaust residue which should have no Nitromethane left in it.

Have you ever noticed cured epoxy turning orange a couple of years after a plane was built? I wonder what causes that. Could it be a bad resin/mix ratio or just the faster curing epoxy?

I've always used Acetone for cleaning CA. Where do you find pure Nitro?