My 2005 WR450 runs well, but over the last year it keeps mildly-backfiring on downhills or decelerating. Have had valves retimed, air filter cleaned and recleaned, plug changed, etc. Bike runs really well but keeps having this same problem. I asked mechanic if FMF Q2 might need to be repacked, and they said that wouldn't cause the problem. Any other ideas from the Yamaha Dream Team?