[Christ]

Seafoam is reported to work well when carbon and soot buildup are concerns.

However, a mist of plain old water will also clean your intake and engine internals, just be careful to use a light mist with the throttle partially opened.

Unfortunately, short of a before/after teardown, there is no way to really prove that the smoke and soot you saw coming out of the tailpipe wasn't just the stuff they put into the engine not burning properly.