02 Canadian WR 426 grumbles and pops when revved..

OK heres what I have...02 WR 426 Canadian, YZ timed, lid removed, Big Gun Race, BK @.5, 168 main, 45 pilot, OBELN 3rd, fuel screw around 1 1/8. Bike pops when revved and stumbles a little under acceleration. I popped the screen in the muffler to ride this weekend and it got even worse. Any help would be much appreciated. Im in Southern Mich so around 500ft sea level

My '02 WR426 has 162 main, 42 pilot, YZ timing, BK/GB mod, no lid, Aussie/Canadian YZ muffler, stock needle 3rd clip, fuel screw 2 turns out give or take an eighth of a turn or so depending on temp' etc. I normally ride at sea level, but when I play up in the hills I never have any trouble either. Someone closer to your area may be able to give you better info' based on altitude, temperature etc, but I think you may be a little rich, but I am only speaking for my conditions over here. Also I recently had a bit of a play with my BK/GB mod' screw, and actually got the bike revving even crisper off idle than when I originally did the mod', which I didn't think possible. Just remember to mark where the screw is before you move it, as even an eighth of a turn makes a big difference. Good luck with it, Chris.

Try adjusting the fuel screw first. I got rid of similar problems on my bike by going out another 1/4 turn.

I have the best results by adjusting the screw while the bike is at at fast idle. Then turn the fuel screw all the way in and back out until the rpms change/increase. I run at that spot where the increase occurs and the bike seems to "free up", which on my 426 at 4000 feet is 1 3/4 to 2 turns out. It runs best at 2 turns out.

I have played with my jetting but am back at stock with the bike uncorked. If I put in the GYT-R insert I am too rich with stock jetting.

If you don't have an extended fuel screw with a "T" on the end, get one. Any brand will work. My bike requires adjustment depending on temperature and humidity which varies greatly this time of year in Wyoming.

You may need more than a fuel screw change, but just a thought on somewhere to start.

What color is the spark plug? I agree with Chris that you sound a little rich.

When I pulled the plug it was black, but that wiped right off and the plug looked like new...Its very puzzling. I'll try fiddling with the fuel screw. I tried that before but never noticed any change in sound from the exhaust. How fast of a idle did you adjust yours at ??? :)

I'm guessing around 2500 rpm, but it's hard to tell. I just turn up the idle adjustment until it sounds like a fast idle (not where I would ever leave it to ride) but not to where it is loud and sounds high revving (sp?). When I recently changed from 1 3/4 to 2 turns, I got ridding of the popping on deceleration and it became more crisp and quicker off low-end.

The front wheel is almost like the kick stand, it only touches the ground when I'm parked! :)

Removing your baffle's screen would make a lean condition worse. The light, easily removed black residue on spark plug may be caused by a slightly rich idling (pilot) mixture and is quite normal. Have you sealed your air cut valve?

I would try position #4 on your needle and/or a larger MFJ...Check out my setup.

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