neighbours bike not revving

[mikee112]

Hi guys
this is a question on behalf of my neighbour
He has a '98 diversion that he rides every day to work, about a week ago it suddenly coughed and splutterd and then refused to rev past around 4 or 5000 rpm, I heard him on his driveway today revving and swearing, so i went over, and this is what he told me: first he bought a used set of carbs and fitted them ( over reaction I know )
still the same, then he bought a used set of coils and fitted them, still the same !!
anyways like I said I went across to his house to check out the bike for myself, and this is whats happening: The bike starts perfectly and idles. when you rev it, it revs fairly cleanly to around 4 to 5000 rpm then starts to splutter ( revving it slowly ), if you blip the throttle hard it will rev all the way to the redline, but if you rev more slowly or hold the revs above 5000 rpm, it splutters, and backfires (alot) shoots black smoke from the exhaust ( and sometimes flames ) and just won't rev any higher, although the carbs and the coils were used items ( and therfore who knows what condition they are in ) it still seems strange that replacing them made no difference :what:
Anyone got any ideas
Thanks