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02 WR426F two jetting with stock exhaust?

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Hello friends, I'm new here and this my first post:

Since July of this year, I'm a happy owner of 02 WR426F bike, but... (I'm sorry) I have jetting problems. I have read many advice here in TT, so in august I went some days to the mountains, between 4200 and 6300 feet, I changed the oil, airbox lid cover removed, and I dared to modify the needle clip from #4 to #3 position, PJ from 1,5 to 1,75 turns out. Improving with it enough to have good holidays. (Easy starting and acceptable idle, but backfiring, very sensitive to the weather, not too much power, only wheelies sometimes up to 3rd gear at full throttle).

When I came back to Barcelona (sea level) I went to off-road Yamaha service. So I gave them it in order to make the first revision, jetting and YZ timing.

According to them, the WR should have enough power, the only problem were stock muffler. It must be more open like a "Thunder" for example. This change would be more effective than YZ timing?? which they didn't advice me.

They returned me the bike jetted to 1,8%CO by gas analyser.

The spark plug was changed only with 800Km; the previous got out dirty, black, like a rich jetting, (but backfiring?).

Now the bike is almost a Europe stock 02 WR426F as follow:

Stock exhaust (it can't be opened to take out spark arrestor)

MAJ 200 MJ 165 PJ 42 PAJ 175

Needle OBDQR #4 PS 1,8%CO

airbox lid cover off

Good starting, good idle, similar power as above described, and go on with backfiring.

- Does anybody know if is possible to get better jetting and power with stock exhaust?

- Is possible to modify 2002 stock exhaust with a plug in the city and open in the circuit?

- Is needful some other exhaust with "plug / not plug" to improve it? (not loud / loud)

- Can I get a correct sea level jetting, and easy change to 5Kfeet jetting? (two jetting?)

Thanks to all

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First of all let me assure you that the yz timing will give you the biggest performance gain out of any mod you can do to these things,i'm not familiar with the muffler you have over there all i know is that the aus muffler we have just dosent cut it, if you choose to go with the yz timing i recommend you change your needle to an "e" series needle, i use the OBELN clip #3 with a 160 mj but you realy need to work out your own jetting because most people here are riding in conditions so much different to ours, sure they can help you get close but not perfect unless you ride together, my brother and my self had the advantage of having the same bikes so we could change somthing and then swap bikes, hope this helps.

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The YZ timing mod is worth it, don't listen to what the shop tells you... You may very well need a new muffler, as you have the same sealed one that we have in Australia, you can cut them open, then re-weld them, but it is fiddly and rather hit & miss... Look for a pipe available locally... ask at another shop if you have to...

As for jetting, well, once you get the timing done and the new muffler, then you will have to work on your jetting... the important thing is to write down your results and the conditions like temperature... then you can easily reproduce the same results if you go up in the mountains again... you will know what to set everything at...

Good Luck,


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Well, I have not still modified neither changed the stock muffler. I'm thinking about.

Still I have some doubts:

:confused:Is possible to improve the Yamaha service jetting with the same muffler?

After reading enough jetting posts, I think to understand that jetting means

to find the exactly proportion air fuel, for each rpm's by several jets.

Then, to jetting correct, :confused:is needful to modify all them?

To try jetting (close/open exhaust, sea level/altitude) I have to check many parameters:

Idle, engine response at many throttle positions, flooding, etc.etc.

But one of main jetting indicators is the spark plug, and I'm a bit confused with it:

Rich jetting, spark plug must get out dark, black.

Lean jetting, spark plug must get out white brown.

Buckfiring is also symptom of lean jetting

Then, :confused:how is possible my spark plug got out black with buckfiring?

I hope not bore too much to all with my uninformed questins

Really is almost incredible :) to find answers about WR's problems

on the other side of the world

Thanks to all


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I have never really understood or subscribed to the theory that backfiring=lean jetting. I was always taught that backfiring is caused by unburned fuel igniting in the header which is a sign of rich jetting. That has been my experience in both motorcycles and cars.

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I agree with you, if the plug is black and you are backfiring you are running rich.

I use this method based on assumptions and performance.

1. Pilot jet and fuel screw control front half of throttle (close enough)

2. Needle and Main jet control last half of throttle.

3. Most activity is in the front 3/4 of the throttle.

First I set the main jet w/needle in a mid position. (ideally I am in the conditions where I will do most of my riding) I ride it around while whacking open the throttle and backing off quickly. If it backfires (as Wicked described) I move the clip up to lean it out. If I cant get it to backfire, I move the clip down until it does, then go back up one position. SOFT POPPING when you back off underload is OK. I now have the back half of the throttle right.

Then I will turn fuel screw down to one turn out and start with a 38-40 pilot jet. I will rev the bike, on the stand and check the idle, if it hangs, indicative of a lean condition,(takes a while to return to normal) I go out on the fuel screw, 1/4 turn at a time until it drops nice and then 1/4 more.

Then comes the seat of the pants test, do you like what you have created? Does it start on the first kick after warm with or with out the hot start? Does it start in two to three kicks when cold? Mine does all of these and pulls very strong for what I do (trail riding)

However my plug comes out grey, usually a lean sign, but everyone I have talked to on this board assures me that I should not worry about the plug.

If any one see's any issues with how I do this let me know, but, I don't think I am missing anything.

Hope this helps :)


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