I just put the JD jet kit on my 01 YZ426 and I have a problem - I CAN'T KEEP THE FRONT WHEEL ON THE GROUND!!! I complained here once before about how the top end response seemed a bit flat, but after installing JD's kit it's a different animal! Low end power doesn't seem to be much different, but the low end was very good to begin with. I have a Thunder Alley pipe which supposedly gives good low end gains over a stock pipe. The top end is where I really noticed a gain. Stock main jet was 162S (what's the S for?) and the stock needle was OBEJP with the clip on the fourth groove. Per the JD recommendations, I installed the 170 main with the blue needle, clip in the fourth groove from the top. My idle is a little strange now. The bike stalled a few times on me while riding, but it's 10 degrees warmer than what I normally have been riding in, and I didn't touch the fuel screw yet. My bike is also backfiring on deceleration now, more than usual. Prior to the JD kit and when I was riding in 30 degree weather, it would pop once or twice, but now it backfires more. Could this be cured by the fuel screw? I didn't even bother with the O-ring yet because I don't have bog that I can feel.