I once tried to "tune up" - or at least refresh my ancient and long gone Morris minor by using Redex poured into the cylinders - which I think was one suggested way of burning off the carbon. Fortunately !!! I was at the time in a relatively remote area. When I eventually set off, the car worked OK, but the smoke billowing out of the rear was of the kind mentioned by Roald Dahl in one of his factual books. I think in that "story" there was an idea to protect oil wells in a country - was it Iran or Iraq? - or another one? - by creating smoke screens so that enemy (presumably German at the time) planes wouldn't be able to see the wells as targets. As he mentioned, it was an imaginatively stupid idea, as the smoke could be seen for miles away, giving pilots a pretty good idea which way to fly in order to find an oil well to hit.
By the time I got to the main roads, most of the smoke had gone, though I felt that it was a bit anti-social for a mile or two.

