at last, snap off the bottom & 3/4 throttle response


crisp, cool, clear, high pressure in woods. that means figures would possibly have to be leaned off in average conditions.


wr99. lid & snorkel gone. std euro (fairly open) exhaust. soiled air filter. 14/50 teeth. taff timed cams. 10oz stealy flywheel weight. hydraulic clutch.

starting carb settings

172 MJ

200 MAJ

EKP needle on middle clip No4

75 PAJ

45 pj

1 1/2 turns on pilot screw

65 starter jet

std slide, float height, full throttle movement.

i'll run through this weekends test results.

test one

main jet checked went from 172 to 170.

test two

back to 100 PAJ & 48 pilot jet. 1 turn out Pilot screw.

result; poorer tickover but a 1/4 throttle dead spot is gone.

test three

a pilot air SCREW is fitted (i've had it since august) & turned out 1 1/2 turns, this is equal to a No 125 PAJ. PS = 1 turn still.

result; can really whack the throttle "wring it's neck" at 1/4 throttle.

test four

lift needle to it's richest setting No 7.

idea; get some snap at the 'tickover to go' & look at the lean spot at 3/4 throttle.

result; snap the throttle & the wheel tries to pop up, immediately the engine eight strokes & kills it. vibration from 3mm throttle to 5/8. then feeding it gently get through to 3/4 throttle & BAM wooooowah! hang on-big improvement! bike presently unusable but i'm onto something.

test five

remove the APJ (accelerator pump jet)

idea;can really start to jet the bike when i get rid of it. tuning will ALWAYS diminish the need for APJ.

result; snap at tickover, wheel over your head job, lift it over logs the lot. 1/4 throttle lean, the rest blows your mind, one big rush to flat out.

1/4 throttle lean

1/2 throttle rich

full throttle surging like a drunk!

test 6

50PJ with PAJ-screw out to 2 1/4 turns (= No 140). needle down one to 6th clip.

idea; get rid of 1/4 throttle leaness & stop throttle surge at flat out by dropping the needle. i think the needle was flapping about possibly.

result; log lifting front wheel, no hesitancy whatsoever. snap is there still but the acceleration is a little lean through 1/4 throttle. 3/8 to full throttle is brilliant. especially snapping the throttle at 3/4 where nothing used to happen, it now moves forward. no surging at flat out.

doesn't like steady throttle from 3/4, touch it & it rips your arms off. so it's a little rich.


if i'd had either; EFP (3rd clip)or EDP (4th clip) needles i wouldn't have had the surge at the top. possibly! coming down a clip for that last test made the bike rideable but even 3/4 throttle wasn't much fun. WOT or vibration.

i've got to lean off the top end now. the opposite to the problem i thought i had a week ago.

1/4 throttle is fun until you whack it. then it's good but you know it could be better. if the wr'00 & '01 rider's could really shorten the APJ to just 3mm throttle to 3/8 open it would be brilliant. however i don't believe in APJs at all.

i tried to disconnect my APJ once before & it was a disaster. this time i rode through trees left & right snapping forward & popping the front wheel. it's awesome.

what next;

well a needle that sits in the middle of all this & not on it's 7th clip. you've got to remember that the bottom 3mm of needle was missing!

to lean off the top end with some MAJ's & to possibly go to 52 pilot & three turns of the PAJ-SCREW (No 155) to give me the last Nth of 1/4 throttle.

started perfectly & ticked over perfectly all day.


didn't stall anymore than usual-i think!!!! i wish i had my old _ _ M needle back for tickover.

so thats EKP No 7

EHP No 6

EFP No 5

EDP No 4

EBM No 4 JD?

52 pilot jet

bunch of MAJ's



this is the way forward.

in most racing applications in the recent past they got rid of the APJ.

as i've mentioned before, if the air goes through the carb quick enough it nearly rips it out of the float chamber; it certainly doesn't need any help.

when we used to road race you just went & asked dell o'rto for the smallest APJ jet you could buy & turned the pump screw to it's minimum.

i'm on EKP No 6 & i want to get to the middle (No 4) setting.

i want the taper to start where it is or even was when it was on clip No7.

the richness through 1/4 throttle from having a higher needle setting would help eleviate going to 52PJ/155 PAJ which as i said to you is way outside the standard type of settings area i want to work in.

then we come to half throttle which is fine at the moment. it's a bit like chess i've got to work out these things three moves ahead (i'm good at chess!).

if for instance with an EDP @ No 4 & a new MAJ leaning off as low as 1/2 throttle, i can lift the needle again!! could end up with an EBP @No 4, pilot as is & working very very well & the 3/4 needle setting spot on.

am i right in saying that the EDP & EBM will have the tapers start at the same distance from the top?

i didn't do a plug chop to finish, wish i had, what if the MJ was working better? (i can't imagine why but there).

for racing applications the EKP is history JD.

do the following to your bike & help me please.

MJ = up to you. i'm at 170

MAJ = get one or two to lean off the top end

EDP clip No 4 (start by using EKP at No 6)

no APJ

pilot air jet - SCREW. set to 2 1/4 turns out.

PJ 50 (have 52 in hand)

pilot screw - as you like it

you've got virtually all that stuff in your box bar the PAJ-SCREW & some MAJ's.

your advantage over me is that you can plug your APJ back on if needs to help at 1/4 throttle, but i don't advise it IMHO.

in my dream world tomorrow morning allens are going to say they do a MAJ-SCREW & save us both a lot of money!!!



Very nice of you to share your results. (Very amatuer in your words last summer- :) ), Thanks.

This is great. The direction you are heading is a surprise after all the discussions about smaller pilots and such. The APJ is a consideration.

The combination you are searching for is a mix of DXM(tickover), EKP#4(3/8-1/2 throttle), and EKP#6(EKM#6)(1/2-3/4 throttle)- correct?

The only way to produce this is to go to a custom grind needle with multi-tapers. The closest single taper needle available is FHM or FHN in clip #3-#4. Your parts man may be able to get them for you.

Keihin P/N 017-394 (FHM) and 017-316 (FHN).


[This message has been edited by James Dean (edited 02-18-2001).]


how is it you have enough time to test jetting like you do? i barely have the time to wash the bike. how about taking your next vacation in ne pa and jet my bike!!


i did it all between 11 am arrival & leaving at 4pm. i also rode a DR400 trail version between runs 5 & 6 & a hopped up KLX300R after i'd finished. the suzook was dog slow, such a yawn compared to the great blue.

only did about four runs in each test & some trials riding in tests five & six.

i ran the bike up & down a dry alley in the woods. no sidepanels, no rad cowls, no tank bolts, only one seat bolt, no APJ cover. even the clip to the fuel pipe was dangling down.


[This message has been edited by Taffy (edited 02-19-2001).]

Originally posted by Taffy:

a pilot air SCREW is fitted (i've had it since august) & turned out 1 1/2 turns, this is equal to a No 125 PAJ.

It sounds like you are talking about replacing the fixed PAJ size with an adjustable system somehow. That sounds like a great idea to me, how come you always have all the good ideas Taffy?

Where’d you get this part? Will it work in the flat FCR on my ’00? How does it work? Does it replace the pilot air jet?

I’d love to try it…


i'll let you into a secret, & don't share it with anyone else.

i've recommended putting the hot start on the bars for 50 cents, too cheap, must be something wrong.

i've recommended taff cam timing, no takers, too technical.

i've recommended lowering the back of the frame, too radical, can't go back if it's wrong.

this PAJ-SCREW is £6 that's $10 bucks to you & you'll never need another one. it has a pencil tip & a hole across the end & up through the middle. it is tensioned by a spring. it replaces the PAJ. it cant be turned without taking off the 'belmouth'.

if you can't get them over there, i'll give you a Tel No over here.

test three alone will prove my theory of the PAJ being too small as was (No 100).

Hick, please, before i go mad, do me a favour, get a PAJ-SCREW, you see i'm starting to babble, it's been hard, that floor mop, you know the one in the corner with the long dreadlocks...

she thinks i'm mad!


[This message has been edited by Taffy (edited 02-19-2001).]


Disconnecting the accelerator pump is just too extreme for me.

What about this: Drill and tap the bottom of the accel pump housing and put a machine screw in from the bottom to make an adjustable stop to the accel pump. Seal it with an o-ring or fiber washer.

The standard stroke is 3mm. With a lock nut the machine screw in the bottom could be EXTERNALLY adjusted to any setting from 0-3mm. This is a step above the P-38 accel pump housing which is a fixed stop. (And it fits within the $10 budget :) )


EDP and EBM would have their tapers start at the same distance from the top. Problem is these are not available.

EFP and EFM are available. (**)

Think the --M will work better with minimum pump flow. EFM or FHM (**) both are in the running.

Rather than dwell on the high taper start and richer pilot jet, consider the --M and --L needles (2.705-2.715mm straight diameter). Last summer the EKL was tested and it was a candidate for eliminating the accel pump for sure! It runs just rich enough that it feels like a 2-stroke jetting, where ALL of those bikes have NO APJ.

I will look into the pilot air screw. Otherwise a #125 pilot air.

The biggest main air jet is #200, how will you lean the top end any more when you already have a 200 in your bike? I don't have a good wide open place to check full throttle settings so this will have to wait from my end.

When days get longer I can test in the evening after work.

Good luck convincing anyone that "for racing applications the EKP(EJP) is history". Yamaha took 4 years to get it this good and it's in both the YZ250 and 426. It took me a few days.


(**)revised portions - 2/19

[This message has been edited by James Dean (edited 02-19-2001).]


Was there a change to the '01YZ426 pump cover making it like a P-38?? Though someone was going to order one due to changed part number.

Well, I went and did it anyway. Took the accelerator pump cover and drilled and tapped the bottom of the cover with a 6 X 1mm screw. It's not very thick, so will have to be jentle on the threads when locking it in place. Thinking of cutting the stop bolt to length and using fiber washers as shims to set the pump stroke limit. If the threads would seal, a screw and spring like the idle screw would be ideal. Externally adjustable with 1 turn = 1mm in pump stroke. The stock total stroke is 3mm and the P-38 Lightning uses anywhere from 40-80% depending on the bike. Will be interesting. :)

If it strips out a replacement cover is $17.50 from Sudco.



I asked at Montclair and apparently the pump cover is the same PN for ’00 and ’01, but the diaphragms (all of them) are different. They are still not available.

I’m just worried about fuel leaks out the bottom of the cover as well as centering the hole properly. Too late for you apparently, but why can’t we just put some kind of shim under the diaphragm, between button and pump cover? What purpose will the shims you speak of serve?

At any rate let us know what length of pump stroke works best for you.

I’m too lazy to search for the post, but the guy I mentioned above (race mechanic) claimed a timed pump duration of only one second or so. That seems tons shorter than what I’ve observed stock (but I don’t have a stopwatch).


One day I WILL try the cam timing. Right now I’m just trying to keep the bike together for the upcoming season. Your hot-start idea won't work on flat FCRs w/ integrated circuit, plus I'd already overspent on the Ty Davis unit by the time I'd read your idea.

I’m a tall, skinny guy (how would you picture a guy named Hick, anyway) at 6’1” and 180 lb.s.

I think that is 1.85 metres and 81.5 kilos. At any rate, I don’t need to drop my subframe. I did break it and weld it though, so I have no fear of metal fab.

I will try Sudco or Carb Parts for the pilot air screw, I’m sure many road-racer types are aware of this feature and I’m guessing that is why you knew about it. As you describe it I would think the same PN would fit flat and slant FCRs.

All we need is a Yank ex-roadracer cum WR rider so we can read, in English, how to make our thumpers scream like a well-tuned road bike. :)

(only kidding about your English)


Someone claiming to be a one-time factory mech. on the YZ side used a different method to shorten the pump stroke, something about a set screw limiting travel at the upper arm (not rod or diaphragm). He hasn’t responded to my email yet but if he resurfaces w/ more details about his mod I will make sure the WR side sees it.

hick & jd

i noticed that for the first 3mm or 1/8th" throttle the bike was dead with the APJ on.

i lifted the needle about 3mm & left the APJ on. the wheel wanted to jolt up but the APJ was stopping it.

it would take you 1-hour to lift your needle to clip No 7 & remove the APJ. try it once, be shocked & amazed & then put it back to how it was.

1-hour hick! c'mon man!

good idea JD with the APJ mod. on reflection, we're talking a 400cc cylinder, maybe i will need the APJ.

but it will be the smallest jet on the shortest stroke.

yer man's 1 second of pump ties in with the 1/8-3/8 throttle area i've mentioned above.

in the meantime i've done a couple of WOT plug chops & my jetting is on 162 & rich.

i've ordered 155 & 158 MJ's. this don't forget, would be 162/5 on an open pipe.

i've also ordered a 52 PJ. essentially i'm enrichening the 1/4 throttle coz i don't want an APJ.

it appears that the needle position does effect the MJ. that's why the MJ needs to come down. we'll see pretty soon.

my man mr allen says that keihin do different length inlet stubs to help the power...

word is that even with the airbox lid off, the airbox is still very restrictive.

ordered an EFP as well which is only 1 1/2 clips richer. i will have to lift the needle to clip 4 or 5.


[This message has been edited by Taffy (edited 02-21-2001).]

Hick and Taffy,

To make a good seal, the bolt was cut to leave about 8mm of threads and the end was filed square to make a good stop for the diaphram. Went to the hardware store and bought 5 fiber washers .8mm thick and 1 brass washer 1.2mm thick. Using only 2 fiber washers on the bolt will stop the pump at .2mm stroke - basically zero stroke. By alternately adding in a 1.2mm brass or a .8mm fiber washer, each additional washer adds .4 or .8mm to the stroke. The stroke can be varied .2, .6, 1.0, 1.4, 1.8,... to 3.0mm (full stroke). A total of 7 settings. Investment: $1.50 and 1 1/2 hours.

Based on discussions with Sudco and Factory R&D, it will probably end up at about 2mm stroke. Last summer Sudco said they simply put a washer/shim under the diaphram to limit stroke. I don't like the idea of a washer rattling around under the diaphram. If you want to try it they said 1mm thick is what they use.

About the plug readings Taffy, be sure you do a "proper" plug chop. full throttle and hit the kill switch, rolling to a stop. Your needle is set sooo rich that anything less will be black from the needle clip.



Thanks for the second hand tip from Sudco, I’d rather have a shim rattling around than a hole in my carb bowl :)

I think I figured out where to put a pump stop screw that would act on the actuating arm. The slant FCR may be different…

But there is CLEARLY a tab built into my Flat CR oppossing the backside of the actuating arm for precisely this purpose (limiting the length of pump stroke with a spring loaded adjusting screw).

Another “ wish I had a Digicam” moment, but I’ll try to describe it.

There is a rim on the carb body around the pulley area that mates to the black plastic cover. On my ’00 the cover has a small tab that grabs a notch on the carb body outside this rim and to the rear of the carb. Just below this tab there is a small, round protrusion cast into the rim in the carb body, directly behind the pump actuating arm. This round protrusion is covered by a round extension on the edge of the plastic cover and is angled in such a way that a screw threaded perpendicularly through it would contact the back of the actuating arm perfectly. So the screw would thread in at about a 30 degree angle from horizontal, pointing up and to the front (right at the handlebars basically).

The actuating arm also appears to have a surface built into it to butt against the screw.

I’m thinking about ordering another pump timing adjusting screw and spring and drilling and tapping for that size (4 mm ?), the length looks just right.

Let us know how your bike runs with the shorter stroke, JD, and thanks for the info.


You turkey!! Why didn't you tell me this 2 days ago! :) Are you sure it will stop the pump, or does this set the "starting" point.

Another tip, Sudco has the pilot air screw and retainer spring. Part #021230(screw $6.25) and #021235(spring $5.50). Paul @Sudco said you just remove the pilot air jet and put it in it's place. This is confusing, how does the air get past when replacing a jet with a solid screw? Will the same setup work on the "00/'01 flat CR carb? They know very little about the flat CR because it is Yamaha's exclusive item.


[This message has been edited by James Dean (edited 02-21-2001).]

Originally posted by James Dean:


You turkey!! Why didn't you tell me this 2 days ago! :) Are you sure it will stop the pump, or does this set the "starting" point.

Sorry JD, I just figured this out last night!

It will stop the pump arm and therefore the pump. I already ordered an extra pump timing adjusting screw and spring to use here. More pseudo-visual aids:

Picture the black actuator arm. It rotates clockwise to operate the pump. The area on the arm to the rear of (or to the left of if you are looking at the carb) where the actuating rod fits in it will butt against the screw, which screws in from back to front (left to right). Screwing in will hold the bottom of the arm, via the screw, further forward (to the right), stopping it before the diaphragm bottoms on the pump cover.

I know it will work, and I’m betting this is what the factory mech. (BK) was referring to on the YZ side. I’m 100% convinced that this round tab cast into the body of the carb just below and to the left of the black pump arm was purpose-built for this function. Its face lines up perfectly with the arm just before the end of its stroke, it is round, and just big enough for a small adjusting screw.

If you could just have a quick glance at your carb (off the bike), you will see what I am referring to right away. If you didn’t insist on tuning things with the carb ON your bike ( :D ) you may have discovered this like I did (I just looked at it for a sec and there it was).

BTW I’ve ordered the air screw (that is what they call it) and spring. I’m also concerned about the Flat/Slant thing, but they do have the same air jet PNs…

…however the airbox joint IS different from slant to flat or ’99 to ’00 :D

Taffy noted in another post that his air screw came with instructions as to equivalent jet size per # of turns. I think. Sometimes I honestly can’t be sure what he is saying…


I pulled the black cover and sure 'nuf, there is a perfectly cast tab to mount a screw and in line to stop the stroke. Oh well, I already have a setup for now. Always good to have another pair of eyes.



Are you reffering to the illustrious Phil Allen?

Tell him Hi from Bill, formerley of Kibblewhite Precision Machine....

We spent alot of time chatting...

Not a bad guy.

Interesting discussion as a point of reference about a year ago I went through this with the Factory R&D guys and this is what I arrived at. The pump stroke as measured on my 99WR was 0.120" approx 3mm after much fiddling riding sending pump bodies back to F R&D and then modifying them I ended up with a stroke of 0.070" or about 1.78mm. Since I now looks like the world wide team is about to have a variable and rapidly adjustable stroke please publish your results.

I went from 3mm stock and too much which can lead to plug fouling under the correct conditions to 0.6mm to 1mm and ended up at 1.7mm.

My methods were time consuming and crude compared to your esteamed modern approach so I'm quite sure you will arrive at a more precise and accurate setting for pump stroke. I will be most interested in your results for 98-99 and 00 and above.

Good work All


[This message has been edited by Clark Mason (edited 02-21-2001).]


There is an in-depth tuning guide for the Keihin FCR on-line at . This article makes referance to the pilot air screw and relative comparisons to a fixed jet.

Thanks for the insights Clark. This will help give a range to target.

Hick, I've become very efficient at pulling the carb out the side and never pull the black cover off.

1)disconnect the throttle cables at the grip (2 screws)

2)remove the carb 2 clamp screws

3)disconnect the fuel line and TPS

4)pull the carb out by sliding the cable down under the tank only about 8 inches

(This much takes only about 5 minutes with the tools in-hand)

Leave the cables on the carb. Access the carb components with it hanging on the side of the bike. The '00 and '01 don't have the octopus or a hot start hoses to mess with, a big advantage when tuning. :)


[This message has been edited by James Dean (edited 02-21-2001).]

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