Jetting Qs

Larry, in addition to the above needles, ob-dtm stock in '98WR.

ob-dvp = 5BE-14916-DP, as labeled on the bag I still have.

Taffy, have you tested any "F" taper needles yet?, not to rush you or anything :)

Also, I think I'll get another thread going to try to get a hold of all jetting related changes between years. It seems like there's alot of comparisons that may not be apples to apples.


I've tried all the needles you are about to with the identical bike setup and none of it worked when using my uncorked US pipe. The difference is that I just kept the PJ/PAJ settings the same. Taffy later pointed out that this ratio is what's important, and after my testing I agree. I found the most awesome power at EKN-4, but very rough running. If this works for you, and I think it will, this will be great news. Can't wait to hear your results.


i haven't tried the 'F' needle yet, sorry. i can't walk away from this and not clear it up completely. i will try and go out this thursday, the trouble being that i've a new MX on the rear and i've got 20 miles of roads. oh well!

by the way i came off six times on sunday and three of those were front end wash outs. i came 11th from 200 riders and yet i've only done one season and more or less since september. not bad for a 40 something beginner!

i guess if i want to move forward, i need a spare pair of wheels. especially when i'm prepared to put my body on the line.

i think this 'F' needle is going to make only the smallest difference when rolling on the throttle in, say fourth or fifth. we'll see.


as soon as you've started your bike from cold a few times on this new needle; let me know.

you'll notice i hope a clear improvement from **Q to **N at starting and the first two or three yards.

but we're coming close to the stage where, because you're ALL running APJ, you'll have differences and will have to make a decision. just remember that my bike runs spot on without it and all you have to do is disconnect it to find out!

i don't know why you don't need the choke. that's a strange one?


your settings haven't changed! are you telling me that nothing was better? how much testing and how much talking? you've had the same settings for over a month. tell us what went wrong.

the book says that the float height for the '02 YZ should be 8mm and i take it that the WR is the same. all the older models are set to 9mm and this is what patrick burns recommends VERY highly. some of you had better check it out.

i've heard nothing from slashman, woody has said nothing of his dyno woork (or anything) where are they? how's it going?

as for me, i've bought a 152, 150 and 148MJ as well as a 155MAJ. i'm ready to come down on the MJ from 155 to say 150 and have to put the needle up at least a clip. as you know that's great news to me-the higher the needle the better. that leaves just the PC as the variable.


Hi there, I thought I’d put in my tests to future aid fellow readers and gather your thought’s on my experience.

I own a 2000 model WR400, all stock bar the snorkel taken off the air box lid. I haven’t removed the lid completely due to tackling creek crossing regularly.

Problem I experienced, like most all WR/YZ’s around here(Aus), is backfiring on deceleration and/or engine compression braking.

I live in Australia, NSW, riding in a elevation typically of 500 to 1000m. Right now the weather is ranging around 25 degrees celcius and will soon rise into the mid 30’s.

For a fact I know the WR’s we buy here come configured stock with DRQ needle, oppose to in the USA with DRS needle. I’m not sure on PAJ & MAJ sizing though, and could be the fix of my problem.

As for stock config –

DRQ position 4





1 & 1/2 ps

Tests –

1. In the carby’s stock form, nil to wot feels good. Cruising at half throttle I get constant power but feel the bike surging in power slightly, then backing off, then coming on again. Other than this minor complaint, the major issue is backfiring. Engine will stall if the PS is wound all the way in, and is not happy till after a 1/4 turn out. This is a good sign. From here to about 2&1/2 there is no power change or fix to the mention problem. Once at 2&1/2 the backfiring is cut down by a good 70%, the higher you go though, backfiring is cut out but from 0 to 1/2 throttle is rich causing slow and uneven pickup.

2. Moved the clip position to #3. Basically I’m on a new bike, that’s how good it feels. Reason – throttle response has improved by 10% causing near lifts of the front wheel on changes of gears. Again though backfiring still exists. Turning the PS between 1/4 to 2&1/2 turns now affects power. Backfiring is cut down a lot much like last time after 2&1/2 turns. But yet again there is a compromise and that is throttle response and power is degraded. Yet the manuals etc state the PS should only affect 0 to 1/8 of the throttle opening. Doesn’t seem to be the case with me.

3. Replaced the 165MJ with a 170MJ. Why? Cause I had one on hand at the time. At 1 turn of the PS from about 1/2 to wot the bike is just fantastic. Throwing roll on throttle wheelies and real good strong punch up top. Backfiring has been cut out by 90%, but only at 2 turns of the PS(note – not the extra half turn). At this point, the mid to top end power seems to suffer by 5% but I can live with this, and the occasional backfire.

4. New day. Replaced the 42PJ with a 45PJ that I went and bought. Power is boggy, backfiring has gone, but the bike is really only happy at wot. Bike is happy to idle with the PS turned all the way in.

5. Replaced the 170MJ with the 165MJ. At 2turns out of the PS. Backfiring has gone completely, rode for 4hrs. Good power all through. Compared to test 3 though, 8% power lose at wot(revs climb slower), throttle response has gone back to nearly the same as stock(pos#4). Still happy with this, and considered leaving it at that.

6. A new day. Thinking… Ultimately the bike performs better from 1/2 to wot with the PS at 1turn. But this shouldn’t be the case cause the PS has nothing to do with this part. Thought and did – moved the clip position from #3 back to #4. Thinking that 1turn of the PS would be all that is needed now with the 45PJ, and thus satisfy the mid to top end better too. My mistake. Bottom to 3/4 throttle the mixture was rich and the bike performed badly. So badly that I could whine the PS all the way in and the bike would continue to idle fine!

So from my tests I have concluded that the 45PJ is too rich, and as much as it cures backfiring, it robs bottom end power and throttle response compared to the 42PJ.

I have proved that a 170MJ, a needle taper size Q, with a clip position of #3 is highly desirable. As for needle diameter I believe I need to get it increased from size D to?? either C or E which ever is richer. Going on the WR manual needle diameter graph, this should only affect 1/8 to 1/4 throttle mainly which is were I need a richer mixture. Now I’m hoping with this richer needle I could get away with a 42PJ, and that backfiring would be cured, not only that, the PS would only need 1 to 1&1/2 turns and due to the taper not changing & having a MJ of 170 I would get the same great power felt from 1/2 to wot as in test 3.

But< !> from reading this great flame discussion I now realize there is another factor - the PAJ. It would be great to think that the PAJ supplied in Australian sold bikes is 100 appose to the standard talked about of 75. If it is 75, would moving to a 65PAJ be better way to do things than getting a new needle as mentioned??

Some sub notes…

When I first changed the jets I noticed at the bottom stuck to the walls of the float bowl was dark brown sludge lines(worms). I believe this is from an over oiled air filter from the previous owner of the bike. It is possible he not only over oiled the filter but also used motor oil rather than proper air filter oil. I have since cleaned this but I have a concern that maybe the air jets may also be contaminated, and more so the filter above the float bowl pin may also have it. Cleaning the air jets seems simple enough by following the manuals diagram of blowing compressed air threw them, if not remove them first then do it. To get to the air jets I gather I need to remove the carby from it’s air intake boot and swivel it around, maybe also releasing it from the outtake boot as well. The real problem, and one if it’s worth worrying about, is getting to and cleaning the float needle seat filter. No instructions how to do that anywhere, plus I believe I would encounter messing with the float height setting to get to it. Something I wish to avoid, I don’t have a measuring tool of small quality measurements.

Next is I flew over a small meter jump two weekends ago to be greeted with 4WD railway tracks from a previous bog the persons got into. Held it together somewhat of the way till I stacked due to speed and the bike got some water in the air filter box. The overflow was full but on releasing it, there was no flow of water afterwards so I started the bike again after a pained rest. The bike at this point backfired like a high performance rally car on deceleration(multiple small continuous backfires). This was during test 4. Tried to figure what this could be, a wet filter causing a rich condition due to lack of air? Then later on I thought maybe water in the fuel making the flash point of the fuel lower and thus unburnt fuel was being dumped and exploded in the exhaust. At any rate it lasted for 10mins and then the bike was good again.

Another small point is when getting the bike(second hand) I pulled out grass seeds stuck in the two if the clear tubes running from the carby out to the ground. It’s clear these tubes suck rather than expel air or fuel. So it is possible one or both of these tubes may have one or two small grass seeds in the inner workings of the carby. Sigh.

So if your still with me I’m at the point now of the backfiring is caused by carby contamination(hopefully unlikely), to get a wider diameter needle, or what is the stock size of the PAJ & MAJ on Australian sold WR’s and if the PAJ is above 75 to change it to 75(or if it’s 75 is going lower a possibility)?


i thought i was long winded! phew!


MJ to 170 was good for revving out but you need a smaller MAJ. get a 160MAJ and then it will rev out AND a 165MJ or smaller will be fine.

(1) the 42/100 was lean and you need a PAScrew available from your keihin dealer. you also worked it out that ther surging is the needle too high.

(2)PS controls same area as PAS/PJ = 1/8-3/8 throttle.

(3)at low speeds you have deduced quite correctly that the bike was richened when you put the 170MJ in. this jet affects right down to your pilot circuit (PC). that's why your backfiring slowed.

(4)when you put the 45 in to richen it up it was the right idea but the wrong answer. given that the 42/100 balance was wrong a 75/42 would put it right. the difference being that although the ratio is corrected you want less of it all.

(5)the 170>165 loss of top revs is the MAJ story i have already explained on a post somewhere around THIS WEEK. go look for it. maybe Larry remembers where? Larry...?

(6)clip 3.5 might be better for now.

the needle straight is the third letter. so go get the EMM or EMN needle. or even DMM and DMN.



I believe there's a balance to be made to the upgrading of one's bike in terms of raw power and a rider's ability! When I first bought the bike with the stock DRR needle in it, I think this balance was definitely skewed towards the power I sucked as a rider (1st bike after all!)...and was only riding in the empty lot next to the old CO Powersports location on Federal. Now I'm much better (although you wont see me flying over 80' jumps!) and looking for some more power. I think the balance has once again skewed towards the power side with the addition of the E needle and the BK mod! Holy $&*$! I think I messed my shorts...and I was only riding in my neighborhood! Once I get this pilot circuit jetting down, I need to make a decision between the D series needle and the E series...given that I ride mostly tight trails at Rampart and I still pretty much suck! :)

I'll ride the EJQ#3 needle on Sunday morning at Rampart and see how it goes. The EKP, EKN, and DVP needles will come in next week (THANKS AGAIN TAFFY AND JD FOR THE PART NUMBERS!) and let you know how they seem to work.

One question for you Stefe9999 though...have you tried the 40PJ/75PAJ combo that Taffy's been recommending with that EKQ#3 needle yet? I'll have time to change my crappy 45PJ/75PAJ setup once before this weekend and wanted to see if the 40PJ would be better than the 42PJ out here in CO. Let me know...


The discussion of max torque/max revs in terms of main jet/MAJ was in "BK Mods - Net Jetting Impact - James Dean" 10th post. There's a reference in there to another thread, but start there.

I havent tried any jetting tests related to it yet, as I'm still working on the low/mid range..but plan on it sometime in the future.


now i've said nothing about the ACV because you may think i just spoil for an argument, but at least with the engine back-popping you could tell if you were in a lean condition. what are you going to do now?

you need all the help you can get to get the jetting right and you've just taken away about the only audible sound going.

Taffy, the whole reason for disabling the ACV IS to get it to pop on decel. With it intact the opening & closing of the valve PREVENTS "the pop" by artificially richening the circuit.

Now I know this has ZERO to do with 2000 & earlier WR's, mine is 2001. It is however a significant source of frustration when tuning the pilot circuit and forces the use of much richer jetting on the pilot due to the constant air leak running through this valve.


I'm seeing the same thing that you are on the ACV issue. Definitely seems to be a significant jetting parameter, at least on the pilot circuit. In my brief experince so far with the ACV, I've found that the PJ setting is dramatically effected with ACV disabled and also that there appears to be two different ways of disabling it. I tried first just removing the spring so that the "cutoff" is always active (i.e., the additional air circuit is always closed). With it this way I found that my 42 PJ (on 250F) seemed too rich. Since I didn't have a leaner PJ, I tried it the other way. I put the spring back in the ACV and disabled the vacuum pulse with a "gasket" in place of the o-ring (additional air circuit is now always open). In this configuration I ended up moving up on the PJ to a 45 and it seems to be running about right, but I haven't really got it dialed in yet on the fuel screw.

Point is, it seems like you would have to take into consideration whether your carb has an ACV and how it may have been modified to discuss pilot circuit settings.

Also, I'm still hoping to see more discussion about the neutral wire and how it effects the "run on" idle condition when in gear (remember the original question on this thread?). I haven't tried anything yet on this, but it is confusing given that a hanging idle is supposed to be a lean condition indication (and certainly an audible one). I have noticed that my bike after the ACV mod does not seem to exhibit this behavior, but wonder if it's just because I'm still a bit rich on the pilot. Note that I have not gotten any decel popping with the current settings and ACV disabled.

The only thing that I can really say for sure is that with the ACV mod, I can detect rpm changes when going in and back out on the fuel screw where I couldn't before.

Still a bit frustrated.

I removed the spring only and had lots of popping on decel. I'm running a 45p on a 01 WR 426 had to move the fuel screw out from 1 turn out to 2 1/4 turns to get rid of the popping. I'm running a ekp #4 on the needle. It was great to get the popping sound to be able to judge where I'm at. I am going to put in my ekn needle and see if it forces me down on the pilot.

I have tried the FHN needle on clips #3,4,5 very aggressive at 1/2 to wot seemed to run rich on the bottom. probably because of the _ _ n needle with a 45p PAS at 1 turn out 1 turn out on the PS this was done prior to the acv spring removal Clip #4 really ran very well Didn't spend enough time on it though. haven't had the time. I've been buried at work lately :)

Glad to see there's more tinkering to do. I love that as much as riden it.

NewRiver, just to clarify my ACV is completely shut off and even made a rubber plug to fit under the diaphram (no spring) to eliminate ANY air leakage on this circuit...again trying to get it as close to the YZ carb as possible. You're right, depending on how someone has disabled it, there is easily a 3-5 pilot jet size difference between the two.


Nothing went wrong other than I've only ridden the blasted thing once in the last month! I have some parts here and have 0 time to try them. I did have one change which was the 160MJ, up from 158. Good news is that its running quite well with the Euro pipe and current settings, so from here its only fine-tuning.

BTW, my ride of last weekend at Rampart(from 7K to 9k feet) was with the Euro pipe and it also ran great at altitiude. The knowledge gained on this thread will benefit you when you go to your stroker pipe if you have any trouble with rough running. If I ever switch out of the Euro, I would also use the Stroker, it's what I ran on my KLX and was great.


Still running the 42PJ and stock PAJ.

It would be good to meet sometime to compare bikes if you want to.


Scott, Yeah I think that matching YZF carb configuration with respect to the ACV might be smarter and would reduce the number of variables to deal with. Have you tried or thought about going closer to the YZF 100 PAJ (or maybe the adjustable PAS option)? I'm going to get a leaner PJ and a PAS and do some more fiddling on the ACV. I'm just thinking that the ACV changes may bear more on the PAJ (or PAS) side of the PJ/PAJ matching.

Also (and I know I sound like a broken record here, but) my experience has been that when my pilot jetting is good (i.e., bike pulls hard off the bottom) I get the hanging or run-on idle thing when stopped in gear (or reduced engine breaking on decel). Does anyone else have similar experience???


your second to last post.

you're either dialled in or you're not! you've either reached a point of satisfaction with the bottom end or you haven't. OK it's good to know which way you're heading but your test isn't CONCLUSIVE when you still haven't finished the job.

all you 426 boys are missing the point, you do what the bike needs and the popping will look after itself. i've been testing for nearly 15 months and it's only really started popping back in the last fortnight. but i don't intend letting it change the way i jet the bike one iota. i'll do what feels best on the way up and on the way down can take a flying **** .

i suggest you do the same. forget how it runs in neutral as well! again neutral will look after neutral will look after itself.

is your idle set low, i hope you didn't raise it to "stop stalling" mine ticks over like a greyhound coach. the closer the slide is to closed the more suck you get across the PC and the ET. (pilot circuit and emulsion tube).

FYI, the tickover had been the same all the way through until i put the 38PJ in at Besthorpe on 13/10/01 and the knob was rolled out a complete turn.

new river LAST POST

go look at my september 23rd post and about the 4th test and i had my only "run on".

i'm not telepathic but i did say yesterday that you(426 runners)were getting to the point where you will have to decide whether to run with no APJ and i think you're there.

i suggest you lower your "idle", go to a low PAJ # and disconnect the APJ. you want less fuel going in. and for goodness sake give yourself a chance to set it up. don't do it at 4.30pm.

lastly, don't forget there are two kinds of back popping. needle and PC. if as you close the throttle in top (flat out) and it starts immediately-it's your needle/MJ. if it does it near the bottom as you've just clogged 2/1st and entering a corner-it's your PC.

my bike had completely clogged radiators on sunday and of course i was in a race. it was running hotter than my tests of last wednesday.

the bike popped gently on closing the throttle and then it got worse entering the corner. what does that tell you?

well it's an indicator but that's all. the bike went like snot. the truth of it is, if you look at last wednesday's results; the MJ had leaned off the jetting right down to the PC and also that lifting the needle only half a clip MAY not have been enough to compensate. go see.

given that dropping the MJ from 160 to 155 has just kicked my jetting up the arse; don't you boys think you ought to be getting your main circuit sorted first?

what if YOU do all this and then get the 160MAJ only to find it wreaks havoc on your jetting?


you're misinterpreting your results i think.


you must stop saying that i'll learn from your experience (etc) when i go to an open pipe. i'll bet you a pound to a pinch of salt that i'll have it done in one day. and who is actually teaching me. the fact that i'm going to an open pipe for the first time on this bike is all that's new.

i road raced for 13 seasons and have helped many people during and since. i go on holiday every year with an ex GP mechanic who ran his own LC (as in yamaha 250LC's-maybe you're too young to remember them) race team for ten years when he wasn't at the GP's. i'm not learning off of anyone on this web site at the moment, it's all new stuff.

you're all watching me. waiting. seeing if i'm right. if i am, new river will probably say that he knew all along.

if i'm wrong? well i'm not wrong, everytime i ride my bike i'm right. that's the proof. the fact is i can't convert my figures and knowledge into you/your head, your bikes. they're all individual and this is a minefield, one size doesn't fit all. you've got to do that. notice how the "big guys" don't get involved even though they have dyno's and could tell us a thing or two?

if doug dubachs wife has been on the 250F site, then i bet doug has read this thread. interesting isn't it! none of them want to tell you how to run your bike's better. also they may face litigation or even ostrisation by the factory for recognising faults.

or is it because, like me, so little of DD's bike is the same as yours anymore? i don't read any of the comics but i was given one last year where they interviewed and rode DD's machine. in it he basically said that he took a std machine and perfected it!

that can be interpreted as "i'm on my own, i do all my own testing, i have a limited budget, the factory want me to help set up these standard machines for states air quality".

(lets put any responses to my DD meanderings on a new thread)


[ November 08, 2001: Message edited by: Taffy ]

hello; guys

i have not been jreading the posts. i am trying to catch up. in short, i am ging to try the Fxx needle this week end and we shall see what happens. i am taking all of my jetting bits and see if i can make it better. i will post the results for all to see.

i can relate to you guys having to use the hot start button for cold starts. i went to an EKQ needle and that went away( go figure) i went from ExP. i am going to try to lower my main jet to a 162 or a 160. then see what it will do the Fxx needle i am going to try is a FKN.

sorry for the scattered thoughts but it is how it cam out.


]all you 426 boys are missing the point

Taffy, I think you're the one missing the point here. You'll never admit it but there are significant differences in 400 & 426 not the least of which is the carb and the new fangled ACV and a neutral switch that keeps the rpm's up while in gear. Now until you get your hands on a US 426 I suggest you keep your comments about us "426 boys" to a minimum. See you're learning something now. Cheers.

[ November 08, 2001: Message edited by: Scott in KC ]

Now now...cant we all just get along? I'm not feeling the love...


Thanks for the Patrick Burns article link...VERY HELPFUL! Probably not a great read for someone just starting to learn about jetting, but once you've got atleast the basics down (which I think I have by reading the countless posts in here), it was very helpful.

The only shortcoming to the article (and probably understood why) is that he doesnt talk about what us normal people without dynamometers and gas analyzers should look for as far as symptoms of lean/rich conditions at each throttle position. I guess that's where all the experience in doing this comes in... :)

Anyway, thanks once again. It's an article that I'll be saving and referring to quite often in the months ahead...

With that, I'll ask a couple of outstanding questions in my head...

1. Given that I have a 39mm carb size (I think), what is the 39mm a measurement of? It's never been clear to me...

2. At approx. _M_ needle (middle of the road in terms of taper start), at approx what throttle position does the tapered part of the jet needle start to come out of the needle jet housing? 1/8 throttle? I'm assuming at some throttle position it does...and exposes the tapered part of the needle (assuming that's why he mentions that between 1/8 and 7/8 throttle, the clip position affects mixture evenly)...

Thanks in advance. The ol' mechanical engineer in me is starting to come out once again, having been suppressed over the past several years by this silly IT project mgmt. career I got myself into! Fun stuff!

OK, for those of you that think the ACV is a new thing, look HERE.

Pay extra attention to the DATE.

Also notice that the wisdom of the time was to change to a 100 PAJ.

Then, with that change the PC needed to be enriched. Why, all because they wanted to mimmick the YZ carb. If they'd not worried about the YZ, they would have run a leaner pilot instead of a bigger PAJ.

Now, Clark, JD, and now Taffy have spent alot of time to test and share their findings with us. Because of them we (I do anyway) can see that copying another bike just won't do.

Taff is showing us that that original 75 PAJ in those first WR's wasn't a bad thing.

Like Taffy said -give the bike what it needs, not what you think it should need.

Also, the original wr needle was a DTM, look at that 3rd letter. now look where Taffy is.


Learn what's happening and why. Then apply the knowledge to your machine.

1) 39m is the diameter of the bore of the carb from just outside the needle through the middle of the carb.

2) an EKP needle was measured in the first two major jetting write ups that are here for you to click on to. the first is called at last, "snap...."

and the second is called "more snap...."

these started out as projects to get rid of the APJ and to keep the brutal snap that i discovered one day. but from this thread you'll note that i let it go in the aim of jetting success throughout the rest of the carburation.

please note the close support that JD has given me and the fact that when i seem to ebb he seems to come up trumps.


[ November 08, 2001: Message edited by: Taffy ]


Thanks for the additional 20 hours of homework...digesting these two threads. I'll have my book report on your desk by Monday... :)

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