MIG-29 Question?

[Fly26]

Today at RIAT during the Typhoon/Spitfire Battle of Britain display (which was outstanding in itself) the Polish Mig 29 was waiting at the holding point prior to its display slot, when roughly every 5mins or so it did a brief engine run up emitting a cracking amount of black exhaust smoke. I'm just curious, to what causes so much black smoke, and is this a normal pre-takeoff procedure when holding? Incidentally the answer was explained over the speaker during the display but it was perfectly timed with a climb on re-heat, so I don't think much was heard by anyone north of Swindon! Funnily, it was the only aircraft I could smell as well, which I thought was a brilliant two fingers to the emissions driven world we are constantly reminded about in modern life! Anyhow, up there with the best displays of the day.....thanks to all the crews, both air and ground, excellent day.

Thanks in advance for the answer...F26