My friend just recently bought a 2000 YZ426F today. When he got home and took the bike for a ride he noticed the bike backfiring. The bike doesn't really backfire when being revved in neutral once it's warmed up. The only time it backfires is if you cut the throttle while moving in gear, usually after a moderate throttle opening. Me nor him don't know a whole lot about 4-stroke bikes, is this normal or has anyone had past experience with this, any tips or help would be appreciated. Thanks.