Jetting Qs

thanks JD

i'm glad you're still around.

it's only been a short day of testing but as you can tell the rich needle has allowed the pilot system to come down with favourable results.

i died with embarrasment when i saw how i pleaded with you (and all) to go to a 50PJ and that i couldn't wait for a 52PJ! alla long all i needed to do was winde the PAJ back in!

that modified jetting write up has really helped. i feel that the needle works lower than the pilot system (ie first). for folks joining in, look for a list of links from 14 days ago "keihin tuning" by patrick burns.

from now on i will treat the PAJ and PJ as twins but not the same and not always working together. those graph lines he demonstrates on the PAJ are quite interesting, although he fails to say where the peak of the PAJ's powers are in the rev range and just describes it as 'higher up!'

the ET.

i've thought about this and i would have to lean the jetting as you say as soon as the taper start has gone out the way with say an EKT needle (or very similar).

the problem is that the straight of the needle is hanging in the narrow part of the ET and they overlap by about 6mm (+-1mm). the narrow part is going to be drilled from above to match the wider part of the tube that already exists. i'll take off 2mm and i may even put a 1mm spacer under the ET to push the ET lower.

with the overlap reduced to 3-4mm i will then go one further if all ok. i believe slightly more fuel must get through the gap at tickover and the needle straight will come into it again!

so just to be clear, i won't be drilling the narrow 2.90mm part at all but the wider tube above it.

after this weekend i liked the _ _ M needle again and i'm sure that it's right or slightly rich. the _ _ P was too lean and i don't really want it back. i guess that after the drilling the _ _ N which i presume to be correct/lean would become correct/slightly rich in the drilled ET.

i've never mentioned this but i have loads of .5mm washers that fit the needles perfectly. they are 20 years old and come in one of those boxes that garages have full of assorted clips etc. autozone (maybe you don't haver them in the states).


The nozzle diameter and needle straight diameter are very sensitive to the dimensions you are changing. I tried to modify a nozzle on a Mikuni once and was unsuccessful. The brass cuts like butter and it went too far oversize.

Since then I just rework the needle profile instead. By putting the needle in a drill press and marking the taper start, the corner can be moved 2-3mm higher with a fine grit wet/dry sandpaper or emery cloth. The total width of alteration ends up about 5-6mm. It makes the change focused on a narrow throttle range and not so much overall. It affects nothing past 1/4 throttle, when the change has cleared the crest, so previous testing results up higher remain unchanged. The depth of narrow passage enters into it just as you have said and the results are very good, very linear, hitch free response.

It will be interesting to see wich needle and clip works with your changes. The .5mm washers will be great for fine tuning. Hopefully they won't bind the needle by holding the clip too tight.

[This message has been edited by James Dean (edited September 17, 2001).]

more test results JD.

i practiced a little on saturday before racing on the sunday. i raced it without glitches, coughs or rough running. that's the best it's ever gone. didn't stall once. whatever my brain wanted the throttle delivered.

was;165 MJ, needle EMM on clip 4, 42 PJ, 1 turn PAS (100), 1.5 turns PS.

test 1

MJ 165 to 162, PS 1.5 > .5

no top end, ran cleaner at the bottom. turning the PS right in makes no difference. bike is throttled in upper revs.

test 2

PJ 42>40, PS .5 > 1.5, MJ 162 > 165

far better at the bottom, gradually getting rid of that bwoooor noise as the engine simply lights up instead. top end back to normal.

test 3

PS 1.5 > .75 turns.

poorer as the bike starts to rev up (just before the needle?) also when i come in and snap the throttle and let go the revs won't die! mr burns knows his stuff!

test 4

PS .75 > 1.25, PAJ 1 turn out(#100) > .75 (#85).

didn't feel any difference at all.

over these four tests a kind of closed throttle response has started to improve but you couldn't make the bike respond from tickover. 4 mph yes. the bike rolls on the revs but you have to wait.

test 5

PS 1.25 > 1.75, needle lowered from EMM #4 > EMM #3.

bike is great everywhere and when we get on a long straight and into 4th/5th gear it responds to the throttle instead of hardly moving. again that same old problem though-the snap just literally went AWOL!

future tests

F needle for the top end or is it the pipe?

38PJ and PAJ to suit?

EMM down one more clip? (or even a half clip)

MJ up to 168/170?


i hope you read this one and give me your opinions. you are the only other bloke running this lean a Pilot Jet.


i'm miles from your pilot system now. look at brett's jetting! can only speak as you find right!

i've had my engineer friend drill into the top of the ET and will try it soon. 3mm down and out to 3.6mm from 2.9mm.


[This message has been edited by Taffy (edited September 23, 2001).]

The pipe must be impacting results here. The smaller pilot and the lean clip position are beyond my test results on the '00WR. A #45/#75/1.75 turns was clean and steady down low with the EMP. This was with uncorked US/WR and YZ exhausts. I preferred the 48/100/1 turn for it's smooth torque and easy starting.

Looking at your results, after shifting to leaner clip positions the 3.6mm oversize and 3mm deep looks like it will have you going further yet to compensate.

I feel the FHN needle suits a YZ exhaust or more free flowing setup. Unsure about the higher gears due to test conditions, the lower ones revved quick under load and sometimes broke loose. Good for short shifting rather than high revving.

Given any thought to add back the pump? A leaner clip #2 is the same as a few KTM520 riders use. Main jets run all over the map, have to test to see. The small pilot is also often used on the DRZ.

Keep going and see where it leads. :)


agree that the pipe may be having something to do with it. i'm making a proper sleeve for the stroker tailpipe which i've only used in those pictures i posted.

the bike is just running so well and yet i couldn't touch top runners on their YZ426's. never mind.

i'm not going to try this modified ET until i've done the more obvious stuff first that i've listed above.

now that my rear tyre is getting shagged i shall dirt ride adjusting the lower end on saturday and then road ride to check the MAJ/MJ needle on the sunday-i hope!

i don't need the APJ as the bike is running so well, as i've said the only time the bike doesn't respond is at tickover in first when you open the throttle it just laughs and thinks your taking the piss!

i can't believe that nobody else has this problem by the way. if i put the EKP back in right now on clip 6 or 7 and ride out then snap the throttle the petrol tank will be infront of my face.


I couldn't believe it either when the stock '01YZ426 came out with EJP#4(EKP#4.5) and the accel pump still flowing. I thought surely the plugs would be fouling all over the place. There hasn't been that much of a problem from what I've seen. Just the occasional high altitude rider needing to move the clip leaner. 2-stoke riders almost always move the needle clip a step leaner and maybe they do it still without mentioning.


I wonder if you could really run a 38 in a 400 at lower elevations, but I’ve never owned a 400. Perhaps you should try a _ _ P needle again and start with a 42 pilot and 100 PAJ and go leaner from there. Or have you already tried that??

As it was explained to me, long ago, by someone at Sudco, which I later posted here, adjusting the clearance in the fork does not shorten the stroke but alters the timing of the pump. I have never had a slant FCR apart however and cannot confirm this.

I always thought you ran incredibly rich jetting (EFP #5!!! 50 Pilot!!), even knowing you had deep-sixed the AP (or did you turn it back on?) altogether and ride a 400 which seems to want richer jetting than my 426. Also, recall that I never, ever, ride below 4,000 ft. Don’t know what that is in meters, but it is high and hot here, and this impacts jetting greatly.

I do know that running a pilot smaller than 42, on a 426, closed/trailing throttle response is definitely better with a smaller PAJ (80 to 90 PAJ with a 40 pilot). After reading the Burn’s tuning article you refer to I arrived at the best PAJ (after settling on 40 pilot) by trying low AND high rev, neutral/trailing/very low throttle settings. The difference was more obvious the higher the revs were.

As for needle and pilot as a pair, I ran a 40 pilot w/ EKP in my ’00, and also run it in my ’01 w/ EJP. BUT the 40 did not work as well with the stock ’00 needle, EKQ and for a time I rode EKQ & 42, then EKP & 42 before realizing I was too rich on the pilot again with the new needle.

Finally, I have a bit of a restrictive exhaust on my 426 as well, a new FMF Q silencer, almost as quiet as a KTM. This did seem to richen things up, especially at lower settings. At one point I got to ride, back to back, at even higher elevations (8,000 ft.) an ’01 w/ E-Series, 40 pilot and EJP-3, then mine with same. It was richer off the pilot (and maybe straight portion?) but seemed equal once on the taper. So it seems that the more restrictive exhaust has the greatest effect on jetting at the pilot circuit.

To summarize, I believe I am way out of my league here. You and JD can continue to drill out the MAJ, modify the emulsion tube, and custom grind needle tapers. But I know that your experiences have helped me, as well as several friends with Keihin FCR bikes (KTMs, DRZs, several WRs and one other YZ). Thanks to you (and others here) I’m the local jetting guru of sorts, and at this altitude stock jetting for most bikes is rarely even close.

BTW JD, EMP #3, 45, 162, 1.5 turns really woke up my friends ’01 400 EXC. He says thank you very much.

thanks for joining in hick

4,000' is 1,200 metres

concur over the idle area being the most affected by an open pipe. but with less restrictions at entry and exit the air tends to arrive too easily like six sons being called in for tea! followed by a lag and the same again. this means the jetting gets richer to make up for the lack of air speed between pulses (note; not less air altogether).

i agree about the needle straight. having an ekP i had a 45 pilot and the PAJ was #125, i missed out an _ _ N and went to this nice rich _ _ M (EMM ) needle and of course this has dropped the pilot as a result to 40 and counting.

if you look at your original keihin needle charts you'll see that they show a graph of where the needle straight, pilot and taper all work.

there are some trapzoid type (god i hope i know what i've said!) shapes and the needle straight shape is glued hard against the left bar indicating that it is the straight that dictates your engine starter jetting.

yes i am still running without the APJ. and it's going really well. i hope to bump into some Brits that would like to check it out sometime.

yes it was rich i agree. it was one great days testing and a broken gearbox (on the last run can you believe!), followed by foot n' mouth followed by a long hard summer of work.

if you had taken your EKP to clip 7 and turned off the APJ then snapped the throttle you would have to have agreed.

all my jetting efforts in feb were to keep the snap and do the jetting for the rest.

this last fortnight it has been to get the rest of the jetting right first and watch the snap disappear.

i've ordered the EKT needle and i stand ready to drill it out to 2.95 (from 2.9). JD says i'll be slightly rich from 1/8th throttle but we'll see how it goes. i'd better make sure the needle straight is lean in that case for the overlap to the pilot circuit which he expects to make up for it and juice up the engine.

seeings that you got down to an EKP and 40 PJ i don't feel too bad now. your teaching me hick! you had the nerve to follow your hunches and go down and down on the PJ. off you go my son and make your own way in this world!!!!

reading your thread hick makes me think that the '_ _N' is the right needle and that our PJ's have been flying everywhere because of those on 'M's and 'P's etc.

- -P =45/48 PJ (48 too rich IMHO)

_ _N =42/45 PJ (40 even?)

_ _M =40/42 PJ (38 even?)

we'll see in the next fortnight. keep you posted.

i can hardly believe the numbers you lot have got your APJ's down too!!!

like, i saw on the other side "0.3 of one second". can't be far off running them without now hick now can yer!



Good to hear the 400EXC is working well and the jetting reference is helpful.


The .3 second measurement has me wondering if thats enough to do anything too. If the delay is short and the throttle isn't completely closed, the pump can't refill. This makes for no squirt at all on the next throttle application. I had that condition testing and was always careful to set the delay back to reset the pump. Then measured the pump stroke length rather than time the flow (~1.3mm). This seemed like a reliable way to measure and set things.



I just received my EKQ. Your riding conditions are the same as mine. I'm under the belief that this needle will work when I use my less restrictive US pipe, but probably not so well with my nice-n-quiet-n-restrictive Euro pipe. Do you think I bought the wrong needle? It sounds like you prefer the EJP. What clip position to you recommend I start with? Keep in mind, I'm still WR timed.

Actually I think my current jetting is pretty good with the Euro pipe, but I want to try to make it work with the more free flowing exhaust because I the bike hits harder with it.

Regarding the BK mod, I found out some interesting info on it. Supposedly, the accellerator pump squirts enough fuel for 4 carbs, because this carb was originally used on superbikes with the quad carb system. In that system, one pump fed all four squirting tubes. If this is true, I bet this is where BK originally got the idea for the mod.

more tests today JD.

start set up is 165MJ, EMM needle and clip 3, PJ 40, PAJ #85, PS 1.75 turns.

test one

tried PS right in and it didn't stall?? tried running at 1.25-1.75 and it didn't make much difference.

test 2

MJ 165 to 170MJ

bike is very slow to rev out to limit.

test 3

170 to 160MJ

jet propelled at the top and pulls to max quicker than 170 or 165.

test 4

drop needle from 3rd to 2nd clip.

can't feel any difference really. both EMM clip 3 and 2 are sharp.

test 5

160MJ to 155MJ

just as good as 160 and no problems except when on steady 4th/5th gear roll and snap when it struggles.

the above could be cured with MJ up to 158, needle up or MAJ from 235 up!

rain stopped play.

ordering a 38 PJ this week and an F_ N needle this week.


That would likely be an FHN, wouldn't it? EMM and FHN have their tapers start close to the same location, within ~.5mm.

The pilot jet appears unresponsive to pilot screw changes. This sounds like it's on the edge of getting too lean, the 38 will tell.

Main jet results were interesting. Wish you had a YZ exhaust to compare under the same conditions.


cheers JD

this thing is running really well. i go trials riding on it and you just can't make it stall or anything going up 15' mounds etc on tickover.

with the exception of the large straight needle-in-a-bucket mod i now need just to fine tune.

i need someone else here in the Uk to ride it as it needs quantifying.

the EMM is now on clip 2 so i presume the FHN would be the same, the next available needle is the FBN which is er... half way either side.

for the needs of these tests an open pipe is pretty much an open pipe and i think when i get the Stroker tailpipe done i will try it and about 75% of the lads on this entire web site will have a more relative graph for their own steeds.

i felt last week that the bike wouldn't rev out and i must have misunderstood the MJ drop in test one (165-162).

the bike not stalling when i turn the PS in? last week i turned it in after one test and the bloody thing did stall and i thought that at last i had reached the "lean" point. this week i screw it in and it didn't happen. if only i'd made a note last week. i thought i had-but never mind.

i bet a pound to a pinch of salt that the 38PJ improves it yet again. that "bwoooaar" noise is getting shorter and shorter as the engine just lights up. according to Patrick Burns "i may have too large a PJ if the bike doesn't stall when the PS is wound right in". all this on a 40PJ! it's either that or i need to winde the PAJ out a bit to even the mixture up. but it's just getting better every time.

remember i asked you for your help three weeks ago with the midrange misfire? the reason it stops WELL SHORT of max rpm is because these bikes need more fuel at midrange (max torque) than they do at the top. this i correctly deduced in Feb and aussie andy and you got to 235 MAJ.

that's what i've run today and it's my hunch that that isn't enough. 2.5mm (#250), even 300 (3mm) could be on the cards. i'm probably going to try 250 then 280 then 300 MAJ next week. i hope it will be easier with the bike on the edge of leanness, to read it.

according to Mr Burns i will have to go back up slightly on the MJ which is just as well as i have now got jets from 155 to 180 and i'm sick of the expense. PJ's 40 to 52 etc etc.

without the APJ it is soooo easy to work on these.


A very careful inspection of the MAJ passage will show that there are limits to how much air will pass around the base of the nozzle. The space where the air turns to go up is going to be the next restriction. This may well be why the FCR main air circuit "sucks" according to Patrick Burns.

I aggree with the assessment of mid-range vs top end mixture needs. It's not just a Yamaha problem, same thing is going on with the KTM's and makes for differing preferences between riders. Some want more torque while others want more revs.

Do the RPMs drop straight to idle, or do they hang a little high(lean) and then drop?

The opposite indication (of richness) at idle is if the RPM's drop low and come back up.



it's only done that the once as indicated above on 23rd when i dropped the pilotscrew to 3/4 turn and the revs wouldn't drop when i snapped the throttle. when i put the PS back to 1 1/4 it was ok.

this would indicate lean and so i put the PS back to 1 1/4 and dropped the PAJ screw to #85 and all was well.

i haven't had the opposite effect of dipping and coming back up at any time.

i've thought and thought about at which point on the 23rd the bike stalled by turning the PS right in and i think now that it was test 3.

as i said last week the two circuits are kissing cousins working side by side.




i noted the _ _M needle when high on it's clip was slightly rich and i wish it was an _ _N by the time we get to where we are which is EMM clip 2 the needle is about OK as is. some of it could be that the pilot system has come down as well of course.

i guess that if enough needle straight sits in a hole it will act as a resistance over distance.

i've just purchased the EKT needle and i want to get the CSA of the space right to suit the needle.

i can bore the ET to the right size i just need the gap to be right. in february you calculated that a T or S needle would be about the same as the M given a 2.95mm ET.

i can buy a ream for £10($14.40)any size i want.




2.95 etc.

can you also tell me what it would be for an EKX?


[This message has been edited by Taffy (edited October 04, 2001).]


I aggree with the observation on added length increases the resistance to flow.

I can't emphasize enough how important Patrick's observation is on selecting the right needle straight diameter. With that and the right needle taper, jetting becomes a textbook operation. Manuals are written with the common misconception that factory tuners will find and deliver this magic combination. (i.e. OBDQR & OBDRS) Without those, it's almost like riding on square wheels. In Patrick's words "if the needle root diameter isn't right... you're pretty much screwed... and if you didn't set the float level to 9mm, just go jump off a cliff and get it over with." I like his honesty. :)

I rechecked and confirmed the numbers from before. A 2.95mm ET with the EKT will give the same CSA as a needle between EMM and EMN with the stock (2.90) ET.

An EKX equivalent needs 2.99mm.


[This message has been edited by James Dean (edited October 05, 2001).]

well i knew after last week that finding anything else major is going to be pretty hard but today was like work. i didn't enjoy it.

i started off with the same as i finished last week. 155MJ, EMM clip 2, 235MAJ, 40PJ, 85PAJ, 1.5PS.

as my 38PJ isn't here and i didn't go to get my stroker pipe from my engineer i knew that the tests would all be about the MAJ/MJ set up. i also had time to try my modified ET.

by the way test run "nil" is always to run it "as was" ok?

test one

155-160MJ, 235-250MAJ

wouldn't rev out, very slow.

test two


good midrange but slow to rev out.

test three


reved out

test four


better still, simply quicker to rev out.

test five

160-165MJ, 180-235MAJ.

pulls hard at mid/higher revs and very quick to flat out.

test six


even better still.

test seven


not 100% certain but felt better, run had to be changed due to land owner chasing me in his 4 x 4!

test eight

EKP needle clip 1 (same as above EMM clip 2), modified ET with drilled out top.

result; no different to the normal ET. bike just a little harder to start.

test nine

EMM clip 3. gone up one from above test(eight).

bike rich on the needle and popping etc.

ET no different.

this test was all work and hardly any improvement. my MJ is up 10, the MAJ is down 55 so it's well richer in top.

will try the reamed out ET with the EKT needle but i know that it won't work; even now! the taper start is miles from the top of the ET and the whole pilot circuit has changed since March. i'm not happy that i'm going to get this right.

i think i owe somebody from the DR site an apology as i nearly bit his head off when he dared to suggest that a bigger MAJ was no good.


That's a long list of tests! Thanks!

Back up on the main jet and down on the main air jet. Ever considered some dyno runs? It would be interesting to see.

Someone emailed me dyno run results on a DRZ using the EMP needle and #158/#200, it had a beautiful flat A/F mixture ratio across the revs. If it ain't broke... don't fix it. He also had a KTM that showed a shift to richen at higher revs. A slightly bigger MAJ looked like it would have helped there.

Good to hear the modified ET is working with EKP. Is this at 2.95mm and how deep is it altered?

I would surely like to know how a more open exhaust compares.

Test seven was an interesting one. rolleyes.gif


[This message has been edited by James Dean (edited October 07, 2001).]


with lean needles at 'D' and 'E' the patrick burns tests aren't valid to us and there is a case for me going down to a 160MAJ and THEN i can come back down on the MJ.

mr burns does say that the ET is shyte and that your CO reading's will go out the window at high revs. so i think your friend will find that a 165 should replace that 158 and he'll produce more power even though the CO readings would high.

his mistake not mine, sorry. am i saying the dyno is wrong? no, but why do think that after dyno work everything should be backed up on the track?

the MAJ can't break down the fuel it can only make the mixture leaner and that ISN'T the same thing.

the ET was the one i had drilled from the top remember? 3mm down and 3.6mm wide and it DIDN'T work JD. there was no difference.

the EMM needle misfired on clip 3 so that tells you that clip 2 is right/rich and i might even try another clip down. well you never know. and as benny hill used to say in his german scetches "learning all ze time, learning all ze time!".

i know the 'reamed' ET won't work because since two weeks ago the needle has dropped two clips and the pilot system has leaned right out. in march it was 50PJ/140PAJ and now it's 40PJ/85PAJ. just a teansy weansy difference wouldn't you say!

the 38 PJ will be here this week but i have three races in three sundays coming up so mucking about will be difficult.


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