a friend of mine just bought a 2001 wr426, and it seems to backfire an awful lot. it runs really nice, other than the backfiring - with the choke on. after letting it warm up for a while, turn the choke off, and the bike dies. there is an fmf powercore iv2 on the bike - could this be the reason the bike isn't running well? there was also gas in the bike that was 2 years old. we drained as much as we could, put in new gas, and it still dies once the choke is off, unless we give it some throttle. also, there is a small amount of oil coming out of the black overflow tube. does this bike need some jetting work b/c of the new fmf pipe? should we try putting the original pipe on the bike? thanks!