Thanks to all the great information on this site I just completed the BK mod on my 2000 WR400. Just got the WR a fiew months ago, got a great deal and decided to try the 4 stroke. I ride in northern Michigan with my 14 yr old son, exclusively single track. My previous bike was a CR250 and I have allways felt 2 stroke was the way to go. 4 strokes rule, the more I ride it the better I like it, smooth and totally maintenance free. My only complaint with the bike is that is sluggish in the middle, to wheelie over the whoops I have to open the throttle all the way. Hopefully the BK mod will correct this. The jetting is 170 main,OBDRS jet needle on 3rd clip and 42 pilot, pilot screw 1 1/2. I ride at approx. 800 feet. Thanks for all the great info on this site. I feel pretty confident working on 2 strokes, looked at the WR carb and was very intimidated, stuff all over the place. TT site had all the information to get me up to speed.