02 WR Jetting question to the jetting experts


I have a 02 WR426 with totally stock jetting. It has all the typical mods: gray wire, airbox lid removed and throttle stop trimmed. In the SoCal desert at Ocotillo Wells occasionally I get some popping when I close the throttle abruptly but I've been told by numerous people not to worry about it. This past weekend I was in Baja -Tecate, Mexico at about 4000' and was frequently getting some LOUD popping-backfiring when I closed the throttle abruptly. What should I adjust on the carb to compensate for this. Is it safe to assume at the higher altitude the jetting was too rich?? Any advice would be greatly appreciated. Thanks.

All other things being equal higher altitudes require leaner jetting, but different weather and/or climate can have an equally significant (and sometimes counteracting) effect as a change in altitude.

Have you removed the exhaust insert? I’m guessing that you have.

Your WR was jetted from the factory to run with the airbox lid and restricted exhaust. Now that you have opened both the intake and exhaust your engine has more in common with a stock YZ than a stock WR. I’ve always thought it was obvious that YZ jetting would work better once you take that crap off (doesn’t everybody?), and in my experience it has. But I digress.

The popping on a closed throttle is caused by an overly lean condition at lower throttle settings, so your idle mix, pilot circuit and perhaps needle are the possible culprits. A richer idle mix, smaller pilot, and smaller pilot air jets would be the typical prescription, but your WR already has a pretty small pilot air (75 as far as I know, vs 100 for a YZ). IMO you should start fresh with YZ jetting. On a few different ’01 WRs I have tried stock ‘01/’02 YZ jetting and it is very close here in the So NM desert, 4k to 5k elev, but a bit too rich in the summer.

The main difference between the WR and YZ jetting is the taper on the needle, the pilot and main I believe are the same, and the pilot air size is different. That’s only two part numbers, probably around $15 for both.

I don’t want to ignore your main reason for posting, which was the popping. The easiest way to mitigate that would be to try a richer idle mix (turn the idle screw out to richen, stock setting should be 1.75 turns out, don’t go over 2.5 or 3 or it may fall out). If this doesn’t cure it then you obviously need a richer pilot. But if you change the needle taper then all bets are off and you need to start over.

Hope this helps.

I'm baffled. The bike comes pretty rich. Going up in elevation and up in temperature should have made it richer. Popping is a sign of lean. Your friends are correct, ignore light to moderate popping on trailing throttle. My bike, and I think yours, comes with a air cut valve, aka a decelleration enrichening circuit, that reduces popping dramatically on WRs compared to other bikes.

My guess is that the new backfiring was not jetting. I would replace the plug just to eliminate it, if it were the problem, you would hear it on leading, not trailing, throttle. A better guess would be an air induction leak. A great guess would be a loose header pipe (the allen bolt and nut on a WR250F) at the exhaust manifold or a badly dinged header pipe.

Good luck,


The more I think about it, the more I like the air induction leak. It is the only guess I had other than jetting that would be worse on deceleration.

Good luck,


I should have added that an exhaust leak can also cause popping on closed throttle deceleration, so you should probably check that first.

Originally posted by Mark Cantrell:

I'm baffled. The bike comes pretty rich.


Relative to what?


You're one of the guys I look up to for jetting issues. What I meant is the bikes come rich from the factory for somewhat normal conditions, say within a 1000 feet of sea level, 70's. By rich, they like lean but rich is 'safer', i.e., too lean combined with hard running can cause catastrophic engine damage. As the altitude goes up or temp goes up or humidity goes up, they are even more too rich. But if it gets cold, they start to lean out. The factory protects itself from this by intentionally shipping the rich.

My experience (WR250F) is that it runs better leaner until it starts to vibrate or stutter or pop. It came a long way from that and the YZ's appear to be even further. So coming too rich means there is more performance to be had by jetting leaner.

If you don't agree with any of this, I'll defer. I'm interested in what you have to say.


Hey Mark, just as a counterpoint, I found that my stock 01WR250F was a bit rich the day I brought it home. I had to use the hotstart to get her lit the first few times. Of course this was with it all plugged up and in pretty warm temps (90's at about 2000-3000 ft). The jetting was actually pretty close once I unplugged it (considering the stock needle).

It seems to me that the recent trend here (and what has worked for me) is to richen up the middle (needle) and then lean out the main and pilot. And that with also restricting (richening) the main and pilot air circuits. Not really strictly a rich vs. lean change. Really more matching the fuel delivery curve to the engine needs at different rpms and throttle openings.

BTW, seems like you had mentioned somewhere about some difficulty jetting with the Vortip. I'm curious about how that is going for you. I had heard one other person make a similar comment long ago and more recently I had finally come to the conclusion that the Vortip just was not working for me. After removing it, I have found jetting to be easier and more consistent with what others have reported here. I don't know if this is strictly an issue with using a restrictive baffle or if it may be somewhat particular to the design of the Vortip (like maybe some weird back pressure pulse or something). Anyway, I could not get mine to start and run well without going way up on the pilot (48). Once I pulled the tip out, I was able to drop the pilot to a 42 without any other jetting changes (and that seems backwards to me from an exhaust restriction point of view). Anyway, wanted to share that and see if you had reached any success or conclusions about jetting with the Vortip.

Originally posted by Mark Cantrell:


You're one of the guys I look up to for jetting issues. What I meant is the bikes come rich from the factory for somewhat normal conditions, say within a 1000 feet of sea level, 70's.


I can only speak from my own experience, but it seems that most TTalk denizens below 1k or 2k (the majority) who have bothered to try different jetting have all ended up richer, particularly on the WR/YZ 400, be it clip pos., main or pilot, it seems bigger is better.

Since I live in the high desert, EVERY bike I’ve ever owned was too rich as delivered, some ridiculously so. A few bikes I have owned I changed the jetting before I ever rode them (my ’01 YZ 426 among them). I also think that rich jetting on a two stroke MX bike, any brand, is harder to ignore (easier to notice???) than on a four stroke.

The reason I don’t like getting into jetting discussions on the WR side is that the jetting on that bike is pretty confused as soon as you uncork and uncover. What I meant to provoke by my “relative to what?” comment was that an uncorked and uncoverd WR with the stock D taper needle at sea level is probably BOTH too rich and too lean, depending on throttle position.

Three of my riding buddies have WR 426s, all three have a YZ-taper needle (BK mod, adj. PAJ, yadda, yadda, yadda), all are uncovered, one is uncorked, one has a PC T4, and one a WB pipe.

And all ride around a mile of elevation. So I should probably keep out of the jetting discussions of the “rest” of the guys who aren’t out west and up in the clouds like me.

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