When the carbon has been loose and flaky, I've used the rounded end of one of those double-wide hacksaw blades to gently scrape off most of it. Then finish with a wire cup brush or a 3M abrasive wheel (the black sponge looking thingy). Trick is not to scratch anything.
Hardened deposts I've just gone straight to the cup brush or wire wheel and patiently polished it out. Be very careful to stay away from gasket or o-ring surfaces.
I've never decarboned a piston in the cylinder so as to avoid pushing carbon down between piston and cylinder wall. That's just me, though.
Merritt Island, FL
When a person asks you for advice, they don't want advice. They want corroboration.