Just bought a new WR400 (Canadian model). Got it out on the trails and it's running great, except for that damn backfire when you let off the throttle after opening it up in 3rd or 4th! Does anyone else experience this? Talked to my dealer and they said this is normal for a big 4 stroke with lots of compression. How soon should I drop my oil and change the filter on a new bike? Any opinions? Love the bike, it's blowing my socks off. I upgraded from a 1980 TT250 to the WR!