I would worry about the "black clouds" clogging your catalytic converter up. Once upon a time, I gave a similar treatment to a Toyota Corolla and killed it's catalytic converter.....the label said it would be safe, but I think too much junk came down the pipe at one time for that "old cat" to handle. I opened up the old catalytic converter once I got it out, and it's beads and baffles suffered a melt down in places.

I think you would be better off running something though the fuel system that gradually cleans up the deposits...like Chevron Techtron Fuel System "Clean-Up"...or the like, rather than going for the one big blast approach. If you think you are really carboned up, add the fuel system cleaner to several tank fulls...alternating a tank full with cleaner with a tank full without, until you think you have got it. You will want to change the oil at 500 to 1000 mile intervals over this teatment too, as the oil will really get dirty fast if your engine is dirty inside.

Good Luck!