Problem w/ YZ Timing

I have an 02 WR 426 and recently switched to YZ timing. On the first couple of rides I noticed that at constant throttle, anywhere from 1/8 to 1/2 throttle I get a real distinct miss. Like a constant sputtering. I tried jetting all over the map and nothing changed it. I didn't have the problem with WR timing. After a week of testing everything I could possibly think of, I decided to go back to the WR timing. I did and the sputtering is gone now. Has anyone else experienced this? I'm wondering if all YZ's do this or if maybe there is a difference in the CDI box. I really like the YZ timing and I'd like to switch it back but that sputtering is really irritating. Here's my setup currently:

Free mods done, including grey wire, 42pj, 165mj, DRR #3 needle, Yoshi pipe. As I said I tried all kinds of different jetting and it change anything. Let me know what you guys think. Thanks.

Instead of saying "all kinds of jetting" I wish you would've been a bit more specific. IMO as soon as you lose the goofy airbox lid and tiny exhaust insert you need to go to YZ jetting, that makes a MUCH bigger difference on how the engine breathes than the cam timing, obviously.

So either the timing change uncovered a jetting glitch, which is possible I suppose, they do seem to idle a bit rougher with YZ timing, or it could be that your ignition timing is on the verge of being too retarded and the new cam timing made it worse.

I guess if it were me I would leave the WR timing, put a YZ needle in there, rejet for main and main air jet, then try the YZ timing again. In my experience the only jetting change the cam timing change prompts is a slight one on the fuel screw (provided the bike is jetted properly to begin with) due to the rougher idle.

Hope this helps.

Thanks Hick for the advice. To be more specific about the jetting. I tried the the DRR needle clip positions 2-5, DRS needle, clip position 3, ELP needle clip position 2-5. I also tried a 40pj and 45pj and ended up back at a 42pj. I tried 155 main, 160 main and 170 main and ended up back at 165. I didn't have any other jets to try or else I would have. I just found it odd that even though the jetting changes created other problems it seemed that nothing changed the sputtering problem.

What is the recommended jetting for an 02 WR426 with YZ timing at sea level? I know it's posted all over the place but everyone seems to have something a little different.

Thanks for the help guys.

I've never ridden below 4k ft. of elevation, so I could only guess at sea level jetting, but I would think that an ELP 5 (but probably not # 4) and 45 (maybe 42) pilot would be in the ballpark.

As for your problem, it does sound like a clip position or maybe pilot circuit lean condition, but as you stated changing these things didn’t help it. If the problem disappeared after changing the cam timing only then I guess it can’t be a jetting problem.

I’m stumped. I have noticed different idle characteristics after changing a WR to YZ timing but no issues at higher throttle and higher rpms.

My next best guess would be that your ignition timing is off, I think I said too retarded but it could also be off in the other direction, and the different cam timing will affect how it runs at a certain ignition advance at certain rpms, this is why the YZ (I’m told) has a different ignition curve. Add in the fact that the ignition timing changes w/ throttle position via the TPS sensor and I’d think that adjusting / testing that sensor may not be a bad idea.

Hope this helps.

Hi MTbiker,

Just wanted to be sure you rotated the exhaust cam the right direction(1 tooth clockwise). Others owners and shops have rotated the cam the wrong direction. If not, I'll let the jetting gurus help you.

Good Luck!


Bryce, I am absolutely positive that I turned the cam one tooth clockwise. Thanks for the thought though.

Hick, Thanks for the idea of testing the TPS. I had thought of that, but wasn't sure if I wanted to mess with it. I guess it can't hurt as long as I mark where it was stock. The only other thing I can think of is to try a YZ or Vortex ignition, but that's a lot of money to shell out without knowing if it will help.

Does anyone know if Stock YZ's have the same sputtering problem at constant 0-1/2 throttle?

Thanks again for your help.

Check out my set-up, I run from sea level to 6000ft. Does not run rough at any throttle position. Needle clip position 2nd from top. I'm thinking you would have to go to at least 172-175 mj as Yoshi pipes realy breath..SoCal

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