Jetting Qs

I just did a couple mods to my 01 WR. I went yz timing and pulled the airbox lid off. It runs pretty good overall but Im having some trouble down low.

It seems to not want to come back down to idle. Im not sure if this makes sense. After being full throttle it wont come back down to idle. It sounds like my idle is way to high, but if I sit for a minute it comes down. I am also having some backfiring on deceleration.

It running like a bat out of hell on the top.

Its only a deceleration problem. Right now Im runnin;

170 main

45 pilot

EJP needle #4

100 PAJ

fuel screw any where from 1/4 turn out to 2 turns out, neither way seemed to fix my prob.

I have a 42 pilot and an EKP needle around, should I try either of these?

I was also thinkin maybe my float height is off, how do I check this?


A 48 Pilot should probably solve the problem, or a 75 PAJ, might also solve the problem as well.

The backfiring is a sign of a lean mixture. Also an indication of a lean mixture is the engine not wanting to return to idle and racing.

As a temporary solution, go to 3 turns out on the pilot screw and revs will drop quicker as well as the backfiring should be reduced.

This will also confirm that a richer mixture is the way to go.


thanks I will try the 75 PAJ. Thats what came in the bike.

a 75 PAJ will make the bike even leaner


since its called a pilot air jet I assumed that it would control air flow on the pilot circuit is this wrong? its located on the air side of the carb too.


Ok what am I missing.

Larger fuel jets, more fuel, richer.

Larger air jets, more air, leaner.



It does control air to the pilot jet, so less air will result in less fuel coming through the jet.

I think.

However it works, the smaller PAJ will lean things up, I have mine set at around 80 or 90 at 4k feet (and a 40 pilot). I was told a good rule of thumb is larger pilot – larger PAJ. So I would think if you are going as large as a 48 you may want something BIGGER than a 100. That is what prompted me to buy the adjustable PAJ, a bunch of guys on this board riding at lower elevations were running 48 pilots and PAJ settings higher than 100.

Hope this helps.

Still having same problem even with airbox lid back on and 45 pilot fuel screw out 3 turns. Im thinkin im lookin in the wrong spot. Could an exhaust or air leak cause any of my probs? I really want to race on the 15th I need to get this damn bike dialed before then.


If your at 3 turns out, your bike needs to have a larger pilot jet. Try a 48.

I changed my bike to YZ timing in June and at the same time, switched to a baseline jetting configuration that was posted here at TT (Thanks James Dean). The bike starts first or second kick, any and all the time and if it pops a tweak of the fuel screw and it's reduced.

It's all in my signature below.



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, EKP #4, 50PJ, 175MJ at 500-1000' Thanks James Dean!

Shouldnt a 45 be big enough with the airbox lid back on? Are we pretty sure this is a pilot problem?


Look for an air leak. A #45 pilot jet at 3 turns is plenty rich. Using a #48 pilot will be about a turn further in at 1 - 1 1/2 turns. Also check the carb clamp, float level, hot start circuit.

The #75 pilot air jet will be richer than a #100 pilot air jet (for a given pilot jet/screw setting)as some of you thought. More air making it leaner.

I had best results with a #48/#100 for easy starting, great torque, and consistent idle. The #45/#75 had a clean feel and would make for a smooth/slower idle, but was not my preference for harder riding. I had a fast idle situation at one point with the #45 pilot and I'm fairly certain it was an air leak. It never recurred in subsequent tests.


[This message has been edited by James Dean (edited September 03, 2001).]


the low speed circuit is known as the enrichening circuit. more air will suck more fuel. less air (75 PAJ) will suck less fuel making the bike run leaner.

a few months back i offered that a relationship existed between the PJ and the PAJ which was something like

42PJ 75PAJ

45PJ 75PAJ

45PJ 100PAJ

48PJ 100PAJ

48PJ 125PAJ

etc etc.

i'm not armed with official kiehin info so maybe you can enlighten me?


How would I find an air leak? A guy recently did a leak down test and it came out good, but thats only for the motor right? doesnt cover carb and airboot? If I had an airleak would it be only a low speed problem what would happen at full throttle with an air leak?


After getting the pilot air screw (thanks to you),I made a study of using different air screw settings. Opening the air screw would lean the mixture unless the pilot screw(fuel) was increased also. The same will hold true for increasing the pilot air jet size.

48 pilot jet- equivalent idle mixtures:

pilot air screw, pilot screw

5/8, 3/4

1 1/2, 1 1/4

2 1/4, 1 2/3

The name is probably associated with this circuit because it commonly does not have an air screw. This is an enrichening circuit only if extra fuel is allowed to flow with any additional air.

Your relationships look good for keeping a steady mixture and extending the range of effect from the pilot circuit. At which point higher up in throttle some richening will occur.



i think we're singing from the same hymn book but on different pages!

while you're here i'll just update you on my machine.


EFP on 5th clip


1 1/4 turns of PAJ (125)

PS out one then for one lap two turns

no APJ

had to do two hours on sunday with a bike that;

snapped off a closed throttle beautifully

misfired from (i suppose?) 1/8 throttle to under 1/2 throttle.

then of course goes like snot!

i seemed to improve the bike recently by coming down from 50PJ to 45PJ as well as a corresponding drop in the PAJ screw.

yet going up a 50m rock face it was misfiring, but so calmly that it never stopped the bike reaching the summit?

it just shouldn't have made it.

i don't think we were ever able to delay the start of the APJ on these 98/99 models were we? i know you/we were able to shorten the pump stroke.

i realise that my problems are right in the middle of that (APJ) area.

i find the problem so inconsistant that i believe the ignition system is a real player in this and as such i'm getting a price for the vortex from MT racing.



Interesting that you have been using the EFP#5. I fouled a plug after running close to that jetting (EMN#7~~EFN#4). This was with the accelerator pump and a short stroke=.7mm. I would hope you would try EFM#4 to reduce the misfire between 1/8-1/2 and add fuel below 1/4. This would be the same diameter needle as your stock WR had (2.715mm). Without the APJ fuel this should be another solution with good results.

The '98 and '99 FCR has the bendable fork to adjust pump timing(start point). I didn't have one to see how it was adjusted. There is a specified gap for it, something like .6mm. The '00 has a adjustment screw for this setting.

Anyone have any idea how I would check for an air leak.

When it is doing the fast idle try spraying the carb boot connection with WD40 and around the flange. See that the hot start is in all the way, push on it. Choke off also.

With the #45 pilot and #75 pilot air, expect the pilot screw at 1 3/4 - 2 1/2 turns.

With a #48 and #100 expect about 1 - 1 1/4 turns.

sorry to talk across you funktree!...


i'll drop the needle and raise the PJ/PAJ as necessary this weekend and then let you know.

99 model

the fork shortened the amount of stroke but the moment you turned the throttle you were moving the arm which was also a stop away from the pump which is sprung loaded. so whatever happens the moment you open the throttle the pump starts to squirt.

if i don't get closer this weekend i will start to look at the accelerator pump again.

yesterday i collated all my jetting numbers ever since i got "excited" and i will try to get some meaningful picture.

if you remember my original write up on removing the APJ i did a lot of WOT. i then said what a great trials bike it had been (off a closed throttle) and finally i said how good it was "popping the front wheel through the woods". maybe i just struck lucky with all that oxygen?

anyway, i did my gearbox in and then came the winter and the foot 'n mouth in the spring so this (8 months later) is my first chance to look at it.

let you know.


Hey James Dean, what would the wd40 show?

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