This is the sort of topic that makes me scratch my head.
First question: Is the carbon really hurting anything? What evidence do you have that the carbon is hurting anything? yeah excessive amounts cause predetonation - that ain't good.
Second question: If the water/Seafoam, or whatever fluid dislodges the carbon, what happens to the dislodged carbon? it blows it out the engine's ass mostly and burns some along the way.
Does anyone think the carbon magically dissolves and is evacuated through the exhaust valve? Engage your brain. (Given the firing time of an engine the carbon chunks would have to completely dissolve into a gaseous state and leave the engine in a few milliseconds during each firing cycle).
they aren't dissolving so much as breaking free and out the ass.
Do an experiment, take a chunk of engine carbon and place it in a cup of water, or Seafoam. Does it dissolve instantly. Nope.
well yeah this "experiment" hasn't even begun to simulate the internals of an engine burning fuel at 2500+ degrees, try it that way and let us know what ya find.
Or (more likely) the carbon releases in small chunks and gets lodged in a piston ring...or between the piston and cylinder wall.