To the honorable, DR.J, Professor Taff and Aussie Andy

Firstly I would like to thank you all for the time and effort that you guys have put into the jetting and to share that info, I think that is what makes this web-site so great! An international co-operation all with there own and differant ideas but trying to get to the same place, WR Nirvana.

Perhaps you could summerise exactly what you've done and how its changed the bike, for people like myself who are a bit mentaly challenged in this area.

I'm particually interested in Taffies results as he has a '99 like myself, by the way Taffy have you got your "open exhaust" yet as I think this will have a large effect on the bike aswell.

Aussie Andrew, do you have any problems running Optimax? I kept fouling plugs when I used it, and I heard a lot of other people had the same problem aswell.

Thanks again for the time and effort,



cheers mate, nice one!

two-dads are in the process of making an insurance claim as my parcel has been mislaid on the way over.

i put the engine in my bike yesterday. i shot blasted the frame saturday & primered it. sunday AM top coat & engine in before tea.

i've got grease nipples at the headstock, S/A pivot (x2) & down the middle of the linkage bolts (x2. also the rear brake!


now mine sits exactly as it did after the last run when the gearing went to 15t & the needle was my DVP.

i'm not allowed off road-mummy says! (nothing to do with foot & mouth!)what i can say is that for the cost of a 50PJ & a PAJ Screw, it is cheap & easy to try.

the bike was brilliant at trials that day i posted the 6 test runs (snap & 3/4...)with the 'D' needle it was brilliant all the way without the viscousness of the 'E' needle.

the 'E' needle i probably did all in one day & has to go down as my pinnacle & culmination of twenty years of testing. the gearbox ruining the final run it would seem. again the 50 PJ & the PAJ Screw are the only expense.

the pipe will only make my MJ go up 5/10 in size & possibly needle up one but i strongly doubt it.

JD has got his APJ down to a minimum & runs a 48PJ. i have no APJ & run a 50 PJ. you take your choice.

you know where my money is.

you've got too much going on in your head splint. don't piss with fuels.

go try my settings for $20.

get a 2.3mm drill bit & have some fun.



The jetting can get confusing quickly with all the codes and things, I'm sure others will be interested too.

To summarize: I gathered together a bunch of optional pilot jets, air jets, main jets, and a dozen needles with the idea that Yamaha had not done it's homework on jetting. The needle straight diameter, taper, and clip position dominates how the bikes run with the Keihin FCR carburetor so lots of time was spent looking at these features. Clark gave us a big head start by picking the DVP out of the needles closest to stock and it still is a good choice.


Here are 3 options to pick from:

1)Needle DVP - More responsive than stock, clean running, controlled revving, lean in mid-range relative to "standard" Keihin needle.

#48 pilot jet (**)

#100 pilot air jet

3/4-1.5 turns out

DVP needle clip #4 or #5

#172-#180 main jet

all other jets stock

2)Needle EKP or EKN - More mid-range power, wheelies quicker, "The engine and the power that it produces now feel more wired to my brain"(John in L.B.- see "EKN needle results"), not always as tractable, '01 YZ426/250 use this, "standard" Keihin needle taper.

#48 pilot jet (**)

3/4-1 1/4 turns out

#100 pilot air jet

EKP#3 or #4 clip (or EKN#3/#4)

#165-#172 main jet

main air jet #200 (or modified as below)

3)Needle FHP - Very aggressive response. Borderline rich in mid-range. With careful tuning this adds more mid-range yet, wheelies or breaks loose, accelerator pump is optional, aggressive riders may like it better but expect more adjusting to dial this in. This is richer than "standard" Keihin. Andrew and James are still adjusting to this.

The "works" -

#48 pilot

1 1/4 turns out pilot screw

1 1/4-1 1/2 turns out pilot air screw (~#125)

FHP#4 or #5


main air jet passage drilled to 2.8mm (7/64 drill)

main air jet drilled to 2.30mm (#43 drill)

Reduce accelerator pump to 1-1.5mm stroke or disconnect the pump.


(**)A #45 pilot jet may be preferred at higher altitudes and warmer temps, expect 1 1/2 - 2 1/4 turns out on pilot screw.

An adjustable pilot air screw is a good add-on to any of these, as Taffy said.

Remember, you have to jet your bike for your conditions. These are starting points. Use some care and watch for signs of overheating. The settings above should get things close for most bikes.

Then have fun roosting! :)

James Dean

[This message has been edited by James Dean (edited 03-26-2001).]


what else can I say,James & Taffy have have said it all,With the optimax fuel I've noticed my bike is running slightly richer than James' with the same jetting, I use a SPLITFIRE tripple platinum spark plug it has never fouled even under extremely rich jetting, I've also noticed with this plug the bike is less likely to stall at low revs in too high a gear, It chugs it's way out of gullys and over logs & rocks even in 3rd gear at walking pace.

hope this is of some help,


Taffy are we going to see a picture when it's finished? Your bike sounds cool!

Dennis :)

Andrew, where did you purchase your adjustable pilot airscrew from? And do you think the following setup would work??

'99wr4 yz-timed

white bros exhaust (8 disc)

Ekp #3



#168 main

maj drilled to 2.3mm

disconnected pump

RE. drilling: if I drill the maj to 2.3mm do I also then drill the main air jet passage to 2.8mm? Or is that only for '00 models?

Thanks again, o mighty oracles of T.T.


the bike might look a little tidier but, like me, it isn't a thing of beauty!

the subframe change you can't spot, well hardly.

the yokes you won't spot either & because my sticker kit is for a YZ tank & not a clark, this won't be stuck on either.

anyway i'm trying to get my clever mate up at easter & get a load of foggy's done.


why the 50PJ & not the correct pilot air jet? no offence to andy but he's whacked into the MAJ & got it done. it's me that's using the PAJ Screw.

my settings were


PAJ out 2 1/4 turns (=No140)

APJ disconnected

PS out 1-1 1/2 turns

EKP clip 7

MJ 165-170

std exhaust

airbox lid off

white bros filter

taff cam timing

of the above splint only the main jet & your open pipe would need fixing. my MJ is now 165/168 & I think yours would be 170/172.



Without the accelerator pump you will need a richer clip on the needle. It will bog and/or backfire. Use a clip position between EKP#5 - #7 as Taffy said. With the pump working EKP#3-#4.

Andrew and I are on the '00WR's with the newer version carb. Taffy's pilot adjustments are a little richer than my '00WR.



i could have put this on either of the two posts near the top of the menu at present.

i was on '1-turn' of the pilot screw & in my next test took it to 1 1/4 before going back to '1-turn'.

during the 72 posts for "at last ..." i went up & down. every time i get asked this i get so fed up with it.

"did i say 1 turn, oh maybe i meant 3!".

never mind, close to running the blue beast. i've got sunday completely clear!


i'll be like a new spring lamb!





i could have put this on either of the two posts near the top of the menu at present.

i was on '1-turn' of the pilot screw & in my next test took it to 1 1/4 before going back to '1-turn'.

during the 72 posts for "at last ..." i went up & down. every time i get asked this i get so fed up with it.

"did i say 1 turn, oh maybe i meant 3!".

never mind, close to running the blue beast. i've got sunday completely clear!


i'll be like a new spring lamb!





Thanks for the response. Now go make some noise and have a good time!! :):D:D


Splint, they are a Keihin distributer in Adelaide, it looks like James & Taffy have answered the pilot, AP & clip pos issues, I'm not exactly sure of the air passage size on the "99" before drilling check approximate size of the air passage and go for a ride with no MAJ if it feels lean from wot-rev limit dont drill the passage just resize a MAJ to #230

Have fun :)


Taffy, you said in your post, "why the 50pj and not the correct pj?" Standard mine is a 48pj, I put 50pj down as its what you specified to do when diconnecting the pump isn't it??

Thanks, guys.

read it again splint. you've got to be so careful.

50 PJ with the correct Pilot AIR Jet (PAJ).

in other words you need the 50PJ & a PAJ Screw. as you now know, this makes your 1/4 throttle mixture variable at the turn of a screwdriver.

the PAJ sits in the face of your belmouth. you remove this & put in the PAJS, & screw it all the way to the bottom & then back out the correct amount. in this case i hit it right first time at 2 1/4 turns to make it the equivelant of the No140 PAJ.

PAJ No140 to go with 50PJ ok?

what you're doing is allowing tons of air to go down this enrichening circuit & it picks up lots of fuel making it richer. but it also gets a better reaction when you open the throttle as there is plenty of air able to get through the passages.

if you ever did 'blow' painting at school you'll know that until you blew hard the ink came out in big shitty lumps & didn't react to different low speeds of huffing. when you blew flat out & then slightly altered to even more flat out!!!! you'll remember that the ink reacted to even the slightest change in your huffing!

try blow painting by blowing through a 75PAJ & then a No 100PAJ. (wr 98/99 &???). same as your school days per chance?

i hope that that is as clear as mud!

having got this well mixed "mixture" going through this passage you now want to control how much of it you get.

this is why we adjust the pilot screw under the carb. this doesn't control the mixture. it controls how much of the mixture you let out.

so you say "how come when you turn it out it richens the engine?"

ok, imagine the nile flowing, 1 mile wide, ******* huge river. & it flows pink water. next to it is a canal, 50 ft across & it's flowing far more die so the canal is rich red. by controlling a gate i can only control how much of this richer die leaves the canal & joins the nile. so i've made the nile redder (richer) but only by opening the gate & it's pre-mix of (rich) red die & water.

so now you know!

the PAJ circuit as it is known works either side & around 1/4 throttle. this is where your Accelerator Pump Jet is working at it's best & needed most. yes! i know it squirts for 3 seconds & by then you would be at the red line in first AND already in second gear(not to mention in the chip shop!). ancient or what!!

this is why people try to make them pump less time. so now you'll note that JD has come down from 3mm of pump to, like 1.2mm or something! howeever you have no control over the size of the APJ's nozzle which is a DOUBLE DISASTER!!!(only the nozzle you can see 'squirting', is available -so tough!)

the fact is the APJ is only needed at around 1/4 throttle same as my 50PJ & PAJS.

if a 45 PJ = a No 100PAJ. therefore surely

a 48 PJ = a No 125 PAJ?

it was myself who pointed out that the PJ's were going up from 42 to 45, 45 to 48 & in clark's case to 50PJ & yet the air to lift it out the bowl hadn't!

so go back to the blow paint idea. when we had a No75 or a No100 PAJ we were getting enough air through that tiny hole to draw enough fuel for tickover. but at 1/4 throttle the thing couldn't let enough air through. it had a glass ceiling!!

a bit like me asking you, splint, to walk breathing through a straw. "fine" you say, "i can just about breath enough through that". "ok", i say "now jog you bastard!".

so, 1/4 throttle is lean due to enrichening circuit unable to lift sufficient fuel.

in my 2nd ride in "at last snap & 3/4 throttle response" i went from 100PAJ to the PAJS & turned it out to 1 1/2 turns (No 125) to go with the 48 pj.


the 1/4 throttle vibration on a CONSTANT throttle was gone. yippee! why a CONSTANT throttle? because there is no Acc pump Jet here. the APJ should only help on an opening throttle not a constant throttle.

the bike had been running lean at a constant 1/4 throttle on the 48 PJ & No 100 PAJ. ok?

if you look in your manual it says that the pilot screw should be left at 1 - 1 1/2 turns out. so when i see people at 2 turns & 2 1/2 turns i chuckle. THEY NEED A LARGER PAJ!!!

so called carb specialists are still recommending that you just keep on winding that PS out to 2 or even 3 turns.

back to your APJ.

air reacts quicker than fuel. this means that, when we whack the throttle that we get a lean or "flat" spot. this is because the air reacted to your wrist action but the fuel didn't. it's heavier so it's slower.

this is why APJ's are fitted. they are crude, neolithic, stupid & should be shot at dawn.

as you can tell i don't like them. never have-never will.

let's move up the revs.

the above "enrichening" (50PJ & PAJS @140)now helps 3/8 throttle & we now enter the needle's area.

as you know my needle went up 2 or 3 clips. this is no problem as the bike could do with richening here.

finally, full throttle. WOT. this is the point at which you open the throttle to the stop but the bike is of course still revving hard to get to flat out at the RED LINE.

so straight away there is a difference between WOT & redline OK?

maximum torque is the point at which you feel the hardest pull on your arms. about 7-8,000 revs i guess. THIS IS WHERE WE NEED THE MAIN JET. this is where your engine was made to work, this is it's nirvana.

from now on in the engine becomes less efficient. the fuel has essentially caught up with your wrist-remember the blow paint- it is now reacting VERY VERY (read too efficiently!) to the air flying through.

now we're starting to have too much fuel as the air slows down & the pulses of the inlet valves are so close that it's just a constant stream of air across the top of the ET.

time for less fuel. seriously!

no more stop & start, this stuff is FLAT OUT over the ET. not a lot of it but it's flat out. the fuel in your float bowl doesn't get the suck, stop, suck, stop. it just is told to get the **** out. like this, we have too much fuel.

red line. 11,000rpm. less air or at least only roughly the same. more pulses from the inlet valves but the engine is less efficient & there is no more air going in than at max torque.

ever seen a straw bailer work, straw or no straw it keeps going. we don't need all this fuel. we're talking only 1, 2, or 3% less. yet the throttle is wide open & the needle is right out. the main jet could do with being smaller. so what do we do?

we have another air bleed circuit. this one is the Main Air Jet (MAJ). this time the air doesn't draw up fuel, it joins it as it is coming up the Emulsion Tube (ET) & can only weaken the mixture by joining in.

instead of only the fuel being sucked up from the base of the carb due to the vacuum of you opening the throttle & the engine drawing in mixture hard, it gets a dose of air! so this time it's leaning it off. this (MA) Jet doesn't work much until past say, 1/2 throttle +, & it works to the redline weakening the fuel to match LESS/EQUAL air. one could argue that a MJ is for 7,000 revs & NOT the redline.

so there you have it.

one final thing on my jetting. you may have thought 'how the **** in the space of four hours did i make those jetting changes & essentially get it right?

it was very easy, when i tell you, you'll say whatthe&%$#@!!

i sat riding at tickover & kept trying to open the throttle hard. it wouldn't rev until the throttle had been opened 3mm (or 1/8"). so i decided that it was the straight portion of the needle dangling 3mm too low in the carb. so i lifted the needle 3mm, which is 3 clips & the next thing my bike is called "trigger". da-dah!

right from that moment even though the bike vibrated due to the APJ starting it's job, i knew i had found the holy grail & no matter how many pops & bangs; i had to lift that needle 3mm & stick with it.


[This message has been edited by Taffy (edited 03-28-2001).]

To summarize:

-Jetting is throttle position dependent, engine speed dependent, and load dependent. In addition to temp and altitude, ALL these need to be addressed to achieve the best possible performance.

-Yamaha and Patrick Burns have good reasons to recommend 1-2 turns on the pilot screw. The '98 YZ manual recommends to only change the pilot jet and pilot air jet TOGETHER. "...while making sure not to adjust one without the other. Do not adjust the pilot screw." (pg 7-4)

(See- )

-A richer and well atomized low speed mixture allows opening the throttle harder.


Thanks Taffy, I actually thought I had written down paj screw in my post but I obviously hadn't. The way you have explained it though it makes absolute sense not to run the fuel pump or drasticly limit its effect, especially if it operates for 3sec or so. Cant wait to try this setup just have to get the screw! Hope everything goes well for you on Sunday.

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