ok, so me and my little bro just finished riding today, he stopped his bike for about 3 seconds then went to start it up it backfired and sounded like a large caliber rifle shooting off. it ran for about 2 secs with black sooty smoke shooting out, i imediately yelled SHUT IF OFF!! and he did. as i was walking away he started it up again about 3 seconds later the black smoke billowed, i once again yelled shut it off but he didnt hear me and he drove away. What bogged my mind is that there was no longer any black sotty smoke bellowing out anymore. I think that a valve had gotton stuck momentarily, but i barely know anything about 4t's only 2t's. Any advice what might have happened. Hes not gunna ride it untill we check the valve clearances and check it over, that will probably be on the weekend. Thanks in advance.