Need help w/ mods and stuttering blue stump puller

I am brand new to 4strokes and just getting back into riding after 8 yr hiatus. I have an 02 WR426 and absolutely love it most of the time. It is completely stock with the the air snorkle removed, baffle pulled and the throttle stop chopped (street legal to boot, I've never seen so much attention to a bike rolling through town!). I had no idea how many other trick mods there are for these bike until I found this site! Carby on my 90 CR125 was pretty easy to take apart even for a novice such as myself 8 yrs ago. These puppies look a little bit more involved (TPS, vacuum hoses everywhere) and I am a bit nervous about doing the BK mod that has been discussed in some threads. Now that you know I am no master mechanic I have the following nagging problem.

Every so often the bike just spits and sputters like something is totally retarted in timing. Then magically it goes away after many bad words. My buddy has the EXACT same bike and set up as mine and his does it too. It tends to happen more often it seems we have noticed, on pavement when cruising around 50 to 70 (guessing) but it also will occur on dirt. Muggy weather also seems to make it occur more often. I would say it happens between 1/4 and 3/4 throttle but if held WOT it clears up and runs OK until you come back into that same range.

Any suggestions of what we can do to clear this up? We live in the NW lower peninsula of Michigan. Altitude is around a 1000ft and doesn't vary more than 800ft +/-.

If we want to modify our bikes any further, which order is the most logical of the remaining mods to do for the 426 (gray wire, YZ timing, BK mod, YZ needle and jettings?) We plan on riding probably right until the snow flies and the Skidoo's come out. What will temps in the 30 to 50F range do to our jetting?

For the most part they run great and the power is unbelievable. It just seems like there is more potential that I would like to tap into and would like to clear up the stuttering problem.

Thanks for listening and any help you can provide.

Welcome to 'THE" place to get the good oil on the WR...

I can't help you with specific jetting recommendations, but you need to get that grey wire cut ASAP... and if you do want a kick in the pants mod, then go to YZ exhaust timing... then you will start to realise that you have been living in a 'performance fog' since you got your bike... :)

Others with specific experience of your area, altitude and temps will surely chime in with some jetting pointers...

Remember, the search feature will turn up so much good info that your head will spin... :D:D



Thanks for the reply. I have done quite a few searches already and the knowledge is mind boggling. Seems like I haven't found a whole lot of people with the exact same set up as mine YET w/ the 02. Have seen alot on the 01 but I am not sure if the same should apply with the "remapping" Yamaha claims to have done to the 02. Sounds real close though. I will try the gray wire as it sounds the easiest.

Taffy claims that the needle should drop down or one clip up. That would be a move from #4 to #3 for me.

Any pointers on working on the carb out there? Do I have to take it off the bike or just disconnect some cables and hoses, loosen the boots and twist around. I am totally in the dark on this yet and I don't have all night to work on it generally.

Do you need a new gasket when you do the YZ timing change?

Thanks again

You may consider disabling the Air Cut Valve on your carb…

You can do most jetting changes with the carb in the boot and the throttle cables still attached, but IMO it is often easier to just take the thing off. A lot of guys will remove the throttle tube cover on the bars, disconnect the two cables there, loosen the air boot and head clamps and pull the carb out the left side and leave it dangling by the cables.

You can reuse the head cover gasket if you change the cam timing.

Now that you have removed all the WR silliness (exhaust plug, throttle stop and air box lid) you should try YZ jetting, the only difference is the needle part number. Post your altitude and temps for more jetting help if you want, but IMO when you lose all that extra WR stuff you basically are left w/ a YZ motor, so why not use YZ jetting (more or less).

To my knowledge there is no material difference between ’01 and ’02 models carb wise, though I suspect the accelerator pump diaphragm is different, and I thought the ignition was unchanged as well but I could be mistaken about that.

Disconnect gray wire - no chance to ride it yet.

Hick, thanks for the carb removal info.

Next I want to try the ACV mod. What are people plugging the hole with on the engine side of the ACV? Many different methods to this mod I've read. Some turn diaphram around and take spring complete out, some leave spring in and cut pin from diaphram... yada yada. Any method better than others? I would like to be able to put back to stock if I ever had to.

Another quick ? for you guys about the air box lid removal. Do you remove the whole lid or just the little snorkle like portion? I know it would get a lot more air with that entire lid off but will that compromise sucking dirt and dust into engine? Right now I just have the snorkle portion off.

I do have a UNI FILTER.


Lose the whole lid. As far as I'm concerned its only true function is to make the bike quieter. I've owned many dirt bikes, all with "normal" air boxes and never had a problem. The avg. air filter is properly engineered to deal with any dirt/debris.

If you encounter water crossings on a regular basis, or any basis really, you need to remove the silly cap that Yamaha puts on the WR air box drain. If you want you can replace it with the YZ part, a 90° turn-down piece that drains the box.

For really deep water crossings I guess you would benefit from the snorkel and the drain cap, but IMO the performance trade off is not worth it unless all you are doing is fording deep streams...

Now that we've answered THAT question:

If you unplug the exhaust and remove the air box lid you MUST try a YZ needle or I will flame you into oblivion. :)

Rode w/ out gray wire last night. Started real bad at first, but could have been cause I had the fuel tank off and there wasn't enough fuel in the carb yet. Seems weird but it's happened before like that.

I also removed the whole lid from the air box which maybe I shouldn't have done right off the bat. One mod at a time right? Anyway, I thought power seemed a little less than before! (it was louder too BTW) Then I raced my friend on gravel and pavement (remember his was the same setup as mine) and he was pulling me slightly :D (unlike before we were even.) To me the gray wire hasn't done a thing and it feels like I am maybe getting too much air with the whole lid off at the stock jetting I have right now. TRUE?

Hick - I know you insist on the YZ needle - but won't I really have to change a lot more jetting if I start down this road? If so do you have any suggestions where to start? What is the stock YZ needle #, EMM? It is all so new that I am having trouble keeping up with all the info. :)

Again altitude is 1000ft & temps this time of year can range from 45F to 90F during fall riding time.

Bear with me, I am learning day to day from all you guys.

Sorry to sound so ignorant but what does IMO stand for?


Originally posted by Blue...ByeU:

Hick - I know you insist on the YZ needle - but won't I really have to change a lot more jetting if I start down this road? If so do you have any suggestions where to start? What is the stock YZ needle #, EMM?

The whole “more air” thing is why you need the YZ needle. You can change that single part only and be better off than you are now in my opinion (IMO). To exactly duplicate YZ jetting you would only need one other part, called the Pilot Air Jet. Stock WR is a 75, YZ uses a size 100 (PN 7A9-14943-27-00). I don’t have the PN for the stock YZ needle, just ask for an ’01 or ’02 standard YZ needle, should have OBEJP stamped on it.

So stock YZ needle is an EJP, stock WR is a DRR I believe. Stock clip position is always # 4.

I would think stock YZ jetting will be a great starting point for you, and may even be the best ending point.

The best twelve bucks you can spend is on a YZ needle. Once you install that go drag race your buddy again and report back to us.

Hick - thanks for the YZ jetting info, saved me from searching through all the posts on this subject.

Here's one my local dealership came up with - He said if you change to YZ timing, you will have to lose something somewhere (probably top he said) cause you aren't changing duration of cam opening. Is this logic correct? Seems like nobody has ever complained about it before on this site at least.

Comments anybody?

the exhaust cam timing is out by anybodies standards. to bring it back in the ball park is all you're doing with the timing. there is a range of cam timing about 5 degrees that "rocks" the power profile. at this stage though with YZ timing it's on the extreme edge and at it's peakiest.

so to say changing from WR timing you will lose something is wrong. the timing, as said, was way out.

most people will tell you that by the time you've got the jetting spot on you've not lost anything and if they ever did it has been said you lose a tad at the bottom.

i didn't find it that way. i found the performance boost so good higher up the revs that it made the bottom end feel weak. but a feeling is all it was.


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