The 1.9 turbodiesel engine in our dirt buggy leaked by the injector, after we pulled it out to stick a piece of metal wire into it to check for TDP after re assembling a timing belt. It blew some air, but after getting the engine hot, apparently the carbon was burnt off and it sealed.
The best way to clean carbon deposits (also to free stuck piston rings) is to soak in DOT 4 brake oil. It will eat through it very good. But once it has soaked past your piston rings, change the oil as it will eat engine oil too: Its crazy thin after getting some brake fluid into it.