wr426f misses at constant speed and 1/4 throttle, help

[wrkaholic]

Guys, I bought a '02 wr426f a few weeks ago. It was jetted rich for where I ride (Hungry Vally - 3000 ft). I twisted the carb in the frame and changed the pilot and main jets. I went from a 48 pilot to a 42 and from a 172 main to a 170. It fixed all my problems except for one.

When at constant speed (i.e. dirt road) and about 1/4 to 1/3 throttle, the bike has a miss. If I roll the throttle, it's ok. It seems to be rich just at the spot. Anyone know how I could fix this. Is it the needle setting? It seems I have to take the carb. completely out to change the needle setting, right? Any advice and help would be appreciated. PS. I think the needle is set to the stock position.

[Hick]

The stock YZ needle works much better once you remove the exhaust restriction and air box lid, so if you are going to bother to remove the needle to change the clip I'd encourage you to spend the $12 or so.

But it does sound like you are rich on clip position. Stock should be fourth from the top, given the huge main and pilot in there I don't think we can assume the needle is in the stock position or even is the stock needle. If you rotate the carb around the other way you should be able to get the needle. You need to remove two allen head bolts to remove the top cover, this exposes the slide. In the top of the slide is another allen head keeper screw, the needle is underneath that.

Hope this helps.

[wrkaholic]

Thanks for the reply and tip. If I change the needle to the YZ needle, won't that mean that I have to rejet the pilot and main also? Also, if stock position of the WR needle is 4th from the top, that means I have to go fifth from the top to make it leaner, correct?

[Indy_WR450]

Just changing to a YZ needle will make your situation worse. Check what type of needle you have in the bike and then go to a larger diameter needle. I mic mine to verify which needle to use. I run from 0.01070" NDJN needle (Cold Weather) to a .01077" NCYQ needle (Hot Weather). The stock WR needle is 0.1090" and rather lean when you change pipes or baffles. Needle diameter and taper are variables you will want to look at but my experience tells me you should start with the needle diameter. Needles cost $13.00 from Yamaha so you dont want to guess a lot. I like the NCYQ with a smaller main jet combination in hot weather. The 426 is different but I would research some more before you just buy a too rich standard YZ needle. The stock needle works better with uncorked pipes. ?

[Hick]

If I change the needle to the YZ needle, won't that mean that I have to rejet the pilot and main also? Also, if stock position of the WR needle is 4th from the top, that means I have to go fifth from the top to make it leaner, correct?

The only difference WR to YZ 426 jetting-wise is the needle and the Pilot Air Jet. Pilot, Main, and Main Air Jet are the same. The smaller PAJ on the WR (75 vs. 100 for the YZ) actually works okay with the stock 42 pilot fuel jet, but if you run a 45 you will definitely want the 100 PAJ.

Also, you have the clip position moving in the wrong direction. To go leaner you want the needle to stay down in the main, letting relatively less fuel out for any given slide postion. So you move the clip up which drops the needle down.

Hope this helps

[Hick]

Just changing to a YZ needle will make your situation worse.

Not necessarily. Why would you say that? The stock WR jetting works okay with the bike corked up, but once you lose the restrictions, you are for all (jetting) intents and purposes operating a YZ motor.

Check what type of needle you have in the bike and then go to a larger diameter needle. I mic mine to verify which needle to use.

A good idea, but the PN/Needle Code will tell you the diameter. Also, he states the problem occurs 1/4 to 1/3 throttle, so that would be on the taper and beyond the straight diameter. Maybe I don't have enough zeroes in there, but I'm getting a diameter of .10847 for the stock WR 426. It is 2.755 mm, somebody check my math.

The 426 is different but I would research some more before you just buy a too rich standard YZ needle. The stock needle works better with uncorked pipes.

In my experience the stock WR 426 needle most certainly does not work better with uncorked pipes. You are the first person I've heard say that.

That don't mean you ain't right and I'm wrong but I wouldn't bet on it. ?

[Pooley]

This may be the CDI causing the miss. I also have this problem and I searched for "throttle miss" and like terms and it was identified to NOT be a jetting issue. It was a while ago (last fall) on this and the 250F board. You may be able to clean the response a little with jetting however it sounds like the CDI issue to me. BTW list the mods you have and that will help you get info on your jetting if you need to adjust. I would try searching for the throttle miss issue first before trying to adjust anything else, especially if it runs fine and does not foul plugs.

This is a known problem with some 03's and you will find a ton of info regarding the new bike. Our info will be back on about page 50 for the 01's but its essentially the same issue that Yamaha never fixed. I guess that they figured that no one who notices this miss is not actually racing so it is not worth the effort to fix. Also search for Vortex ignition which will eliminate this but for $$$.

Just my 2 cents.

[wrkaholic]

Guys, thanks for the info. My bike has the airbox opened up, high flow air cleaner, e-series pipe with 12 disks and silencer insert (metal). Grey wire cut, YZ throttle stop, Full throttle acceleration and rollon seems good now that I changed the jets (48 to 42 pilot, 172 to 170 main). The miss happens when I accelerate hard, get to a constant speed and let off, then just give a little throttle to maintain the constant speed, voila - miss. A similar example is "cruise control in a car". I am trying to give it just enough throttle to keep going 40 mph and it misses. If I try to accelerate from 40 to ??, it does not miss. I am thinking of adjusting the needle to leaner. Do you think that is a good idea? Also, do you think the 170 main is too rich? Should I go to a 168 main? What are the stock pilot and main jets? Thanks in advance for your help.

[Indy_WR450]

Hick,

Just to correct my opinion. I am refering to the stock YZ needle works better with an uncorked pipe (not stock WR needle). Sorry for the confusion. The WR motor with WR cam timing will need a different needle than the WR motor with YZ cam timing. At a quarter throttle my needle diameter has a bigger effect than the taper. Taper has a bigger effect from 1/3 to 3/4 throttle openings. Just my experience with the 5 needles, WR and YZ cam timing and pipe with insert, uncorked, and aftermarket pipe as well as twin air filters. That is why I am saying to pick the right YZ needle diameter for his mods. I agree that the stock YZ needle will work for the high flow modded bikes with YZ timing. ?

[Hick]

The WR motor with WR cam timing will need a different needle than the WR motor with YZ cam timing.

I also thought the different cam timing might affect the jetting, but in my experience I just couldn't tell. Whatever the affects are, it is too minor for me to detect riding one bike after the other (uncorked WR vs uncorked and YZ timed WR, both ran equally well with the E taper needle).

I changed the cam timing on my '00 YZ with some adjustable wheels and didn't notice any difference. Of course, the changes I made weren't as drastic. If you radically changed the valve lift all bets are off, but I'm not sure if I can see how an exhaust timing change would substantially alter the way the motor breathes. Just my own thoughts on this issue. I guess it is good to see some different opinions and experiences.

Anyway, I agree with you that it is likely a jetting problem. I have heard of CDI problems leading to fouled plugs, and I know the 250Fs had CDI issues, but I haven't really heard that on this board.

Wrkaholic, your last post makes me want to agree with Indy on the cause. I'm leaning towards diameter and/or pilot circuit and not clip position (when you accelerate I'm thinking you are now on the taper, the "cruise control" comment tells me straight diameter and pilot/idle).

Mabye the first thing you should try is a leaner idle mix. Go in 1/4 turn increments, unlike a two stroke the idle mix screw governs fuel, not air, so going in (clockwise) is leaner. The idle mix is recessed in the bottom of the carb bowl.

BTW, I'm fairly certain stock main is 165, YZ I know is a 162. This makes sense when you consider the D taper needle in the WR, the E taper needle will open things up more quickly and will prefer a smaller main all else remaining equal. Both bikes come with a 42 pilot.

You said you were at 3,000 ft. I would think you are borderline 40/42 pilot, stock main should be okay, but a 162 may be better, particularly with the YZ needle. If you do get that YZ needle try fourth clip first (this is stock setting on most bikes actually), then go to the third which might be better for your altitude.

Hope this helps.

[wrkaholic]

Hick and others thanks for your reply. I ride in Hungry Valley (3K' to 5K'). I talked to the previous owner and he told me the needle is a EKP in the 4th position. Based on your posts, I think my next step is to move the needle to the 3rd position (down, leaner) and change the 170 main jet to a 168 main jet or even a 165. What do you think? I was able to change the main and pilot by just twisting the carb. Can I do the same when changing the needle or do I have to remove the carb. Thanks for all your help guys.

[Hick]

Hick and others thanks for your reply. I ride in Hungry Valley (3K' to 5K'). I talked to the previous owner and he told me the needle is a EKP in the 4th position.

That is nearly identical to a stock YZ needle, to be exact it is one half clip leaner.

I'm at 4,000 ft. in the desert, here is the jetting I ran on my '00 and '01 YZF:

160 main

200 main air jet (stock)

40 pilot

1.5 turns out on the idle mix

~ 85 pilot air jet (via and adjustable screw I bought from Sudco)

EKQ # 3 (this is just slightly leaner off the pilot than your EKP)

So your jetting is pretty rich relative to mine. A 170 main jet is at least a few sizes too large for you, so this may be sort of polluting other circuits. To that end I think you should put a 162 or at least a 165 in there before you do anything else. Then ride the bike, if the problem persists I would suspect the pilot jet before clip postion but who knows, relating the condition over the internet is an inexact science at best.

Hope this helps. Please let us know what happens so we can get this problem sorted out.

[wrkaholic]

OK, changed the main jet from a 170 to a 160. I left the needle how it was, EKP in the 4th position. I took a ride around my coldasack (sp) in first gear and the problem seem fixed. Hard to tell with such little room. But, I am definitely moving in the right direction. It seems stock yz jetting with the EKP works well on a wb e-series with 12 disks. thanks so much for your help guys, you have made this pretty painless. It was a nightmare for the previous owner and he seemed to be moving in the wrong direction, ie. he kept putting in richer jets (48 pilot, 172 main with the EKP needle). No wonder it was missing.