What I found worked well on my '85 was to soak the carbon areas with WD-40 then let it sit for a day or two in an unheated area (mine was in the truck outside). After a little damp air hits the carbon, it just bubbles up like paint with paint stripper. A wire brush then takes it right off. Found that by accident, as I had soaked the piston tops with WD-40 to keep down any rust as I worked on the engine. Once morning, saw the carbon all bubbled up, pistons cleaned up pretty well:



Otherwise, something like gun cleaning solvent should do the trick (designed to remove carbon deposits).