Jetting Qs

200th (i had to take it even if it was to waffle on)

i've asked brian to archive this in a sensible format because obviously it needs tidying up and needs to stick to it's educational value.

no reply yet.


:) Boinggg!


Keepin' it alive Taffy...

I'm trying to get the most out of stock pipe with vortip to keep noise down.

The bike: 98 WR400, de-octopussed, airbox lid off, YZ timing, stock everything else.

75'F, 1600' ASL

ran some tests with DTM needle.

MAJ 162 throughout testing.

1st test:


PJ 38

PAS 3/8out

clip #4

better than previous (way rich) jetting, but slow top speed


needle to clip 7

same top speed, now 8-stroking from 1/4 throttle up.


MJ ->150

barely faster on top, 8-stroking from 1/2 throttle to full


PJ ->35

clip 6

showed promise, 8-stroking changed to weak spot, everything else still a little soft.


PAS ->1/2 out

0- 1/8 throttle is a tractor. Idles smooth, super slow. PS wants 2 full turns for best idle.

1/2 - 3/4 throttle better but not good enough.


clip ->5

throttle sensitive to fast opening, probably lean. barely rideable. 1/2 to full roll on pretty good???


PJ ->38

idle poor, stalls easily. Rich 1/8-3/4+

7 1/2th: :)

PAS ->5/8

idle slightly better, but it really wants something around 3/4 throttle.

-I was quite surprised by the PAS change to 1/2 out. night & day difference below 1/4 throttle, yet made an improvement to near 3/4.

I want that tractor pull with low idle from test #5, but from 1/2 throttle up it's no good.


Is running way up at c6 killing me?

Can it possibly need to go leaner on the main?

Is this the effect of the fat-tipped D taper?

I only have 162 & 200MAJ, do I need smaller?

I need some guidance. It's discouraging to have that great low but have to give it up to use the whole motor!

I'd like to try to make it work with a D needle if possible, since Taffy's already covered the E's.

Where do I go now!!!!!

[ June 09, 2002: Message edited by: tbronc ]

The "D" needle should take a bigger main jet than Taffy was using. The tip/point is bigger and more restrictive to fuel flow, even at DTM#6. The exhaust tip may be giving different results though. Try both bigger(158?) and smaller(145) mains to be sure. Is your accel pump disconnected? It needs to be for this type of testing, to be sure. Taffy's was disconnected for most of the tests.

that's a hard one to call as JD is implying.

i would have thought that a 158MJ with a vortip and D series or even the 160MJ may be needed. i would then try to drop the needle a clip if it's "eight-stroking" at mid throttle. if that all works go try your PC set up again.


Ok, thanks. I'll try test #5 again with 155 & 160MJ. What effect would this have on pilot(I already had the 35PJ in)?

-it would be so easy if there weren't so much overlap

accelerator pump is happily unhooked.

It's been a while, but I finally got some tests in.

MJ 155, 160, 168


PJ 35

PAS 1/2+ a hair out

DTM clip 6

fine on top, still no good 1/2 - 3/4. 168 main just made it soft everywhere.

really getting frustrated here, thinking about giving my needle a hot date with sandpaper, then come to my senses.

tried going to WR timing.

slower building power (no surprise), with a surge of power as rpm's hit ~7000. Occasional stall at idle. Louder.

MJ 168 =>160

lean off idle, lean 1/2-3/4. Maybe better with AP hooked up.

So I think "it could use more everywhere", and remember I initally measured the floats as 25/64 in., .052" lower than 9mm.

Raised float level to 23/64"

noticeably better. good under acceleration, as rpms even out w/throttle (1/2-3/4) I still have a spit & sputter. Seems I can change the character at 3/4 but always have a prob there.

I intend to play with the clip here, do you think that would do it? The voice in the back of my head keeps telling me to "get an E needle and just go ride", but I'm too stubborn.

what's left to try? MAJ?

let me add that 3rd gear roll-on wheelies went away with the timing change. It'll come up with a little help though.

Originally posted by tbronc:

...The voice in the back of my head keeps telling me to "get an E needle and just go ride", but I'm too stubborn.

what's left to try? MAJ?

Get an E needle and just go ride. :)

Turn in your PAS an eighth of a turn.

Your jetting is almost exactly the same as mine, except for the needle. Mine runs perfectly. Why fight it?

mainly because there's potential for stronger bottom, and smooth power.

O.K., made real progress yesterday.

Went as I said to try changing clip positions.

First tried clip 7, since I thought it was lean near 3/4. It got worse. Tried clip 4. Bingo! What a surprise that was. Sot that cough & stutter 1/2-3/4 was 8-stroking, just when I was starting to think it was lean.

At clip 4 there was still a small stutter about 1/2 throttle only noticeable while cruising down the road.

clip 3 best smoothest this machine has been since I decided to run the vortip. no surge.

Pilot circuit is now a little lean. Some stalling when doing the "John Deere" impression.

I learned not to think too much about everyones comparisons today. I was sure that since so many did well with an E taper, I would be lean near the tip. Not so in my case.

I guess the PC is leaned off so far that I could raise the needle til it was richer than an E in the right clip. And now that I'm close to the right clip the pilot needs tweaked.

Will try 155 main also.

After that, AP gets hooked up and WOO HOOOO! It's done.

[ July 18, 2002: Message edited by: tbronc ]


you pretend to be a convert but your not are you! :) go on admit it rich is best!

get your needle at mid throttle right first. it still sounds like it could go down one clip. but instead of doing that lower the MJ in your tests first.

if you do need a lower MJ you'll find that the needle will have come right naturally. THEN it may want to go leaner/may not :D

only then should you check your PC. as has already been pointed out to you the PAS is a little too far out which causes a 20mph vibration. so tweak it in after you've done the rest.

and that APJ? run without it for at least a week whilst you feel the bike. FEEL the bike.

the APJ is a shroud/aveil which stops you feeling the jetting.

take your time be patient, don't worry, be happy! :D


It's no lie to say I've been all over the place w/jetting. But rich bites it. I would have told you that 3 yrs. ago. That's why I never jumped on the 175MJ, 48 pilot bandwagon. I mean, why go richer when your pipe's already black from soot??

I'm right there with you an the tests, taff, except that yesterday I twisted that PAS down to 45 (1/4 turn). No popping at all afterward. I must be learning now because those were my thoughts also with the needle/main. Thanks.

Come on Guy's...don't let Funktree and Taffy be the second longest post in TT History.....


Bonzai :)

:) Got mt DTM today and fitted it......had to redial the PC in slightly, but only a bit, well happy, I've final running a needle that is not stuck down the throat of the ET.

My (nearly) final settings are

MJ #162

MAJ #170

needle DTM

clip #3

SJ #62

PJ #35

PAJ #55

I,m gonna try lowering the PAJ tomorrow to get rid of the popping.... :D

I hope that anyone who reads the tbronc series of tests recognizes that he is running the DTM needle and small main jet in his bike with the quiet and RESTRICTIVE vortip exhaust baffle. It may not be a surprise that DTM#3 was the stock setting for a corked up '98/99WR (US Model) exhaust.

The only 4-stroke Yamaha continues to use the Dxx needle in is the WR426 with it's standard restrictions. The WR250, YZ250F, and YZ426 all have the Exx needles.


I just love doing that

Bonzai :)

Well almost a year since this thread started, I did'nt even have my WR426 then. I have read alot on jetting but have done no changes. My 02 426 starts great runs what I call great. If you wick the throttle real quick it will stumble slightly, it might back fire once in a while when hot on deceal. 600 miles and never fouled a plug! The bike seems to rip but am I missing something? I ride from 1500ft to 5500ft average temp is 60-75 degrees, air box lid and snorkel gone, throttle stop cut and uncorked. I live in the north west corner of Washington state.

What would be recommened jetting specs.

Nooffseason, you could try my signature (Taffy inspired) jetting, I’m close to your elevation.

I have to admit I was reluctant to run lean jetting on a $5500 toy, but since I’ve re-jetted the bike STILL starts easy, and responds instantly to quick throttle blips. As far backfires go, there are NONE, no matter how hot it gets.

You’ll have to do the BK mod on the accelerator pump also if you want to truly get rid of any kind of hesitation IMO.

Thanks TK421. What about performance gains?

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