Jetting Qs

funktree, expect the idle to drop quickly or run rough if the WD40 starts to fill in the leaking areas. It sounds like a lean condition and I suspect you should use a #48 pilot jet/#100 pilot air to get the idle consistent.

Taffy, we did get sidetracked by the main jet/main air jet discussions. I remember your first few attempts were very favorable without the APJ. I'm still interested in hearing what you find out as variations are tried. I still have a book of notes on my jetting tests.

Something else I noticed the other day and forgot to mention. My idle problem is worse when the bike gets hot. Every time I change something i think I fixed it, then after a couple laps i have the same problem.


i revisited that 'patrick burns' write up on keihin tuning. he's added quite a bit more to his article.

i note that he agrees with you (or you read it!) that a SMALLER PAJ makes the low speed mixture richer. MMMMMM!

sounds like he knows his stuff. he puts a great deal of importance by the straight diameter of the needle. he ignores the fact that the taper point moves with the narrowing or broadening of the needle.

rich rohrich in another article agrees that there should be no plug colour and that the reading is at the bottom. at last someone can hear me. i stopped visiting the YZ side if for nothing else but this argument.

one of these days i'm going to practice and will blag a 37mm wr250 carb as well. i've tried to borrow one on the wr/yz250 site but there aren't too many east anglian owners of said model to choose from. no matter i will get my man.


my 48 pilot wont be here until early next week, but thanks for the choke tip Ill try that today


I remember when I bought my bike. The 42 pilot was sooooo lean, that the bike would not idle properly. I kept on messing around with the idle to get it right. I figured that maybe the butterfly valve in the carb bore was broken or didn't seat properly. It was very frustrating.

I brought it back to the dealer and they cranked the screw way out to like 3-4 turns and wha-la, the bike mysteriously idled again. Your symtoms sound very similar.

what if the choke makes it idle faster? this is what happened. I cant ride it right now as I loaned my truck out with my gear bag behind the seat, but I just rode in circles around the front yard and the choke made it idle faster. Maybe I didnt have it hot enough. Im gonna ride it tomorrow and try pullin the choke and check for air leaks. I have a hard time believing that a 45 pilot and 3 turns out on fuel screw with airbox lid back on is too lean.


Have you put a 48 pilot in yet?

This absolutely sounds like a lean condition that I had early on with my 2000.

When the idle races, try opening the choke, does this drop the idle?. If so you have a lean condition.

I knew what you meant about the choke but didnt explain very well. I tried pulling out choke while bike was having "the problem" and idle went even faster with choke. But it wasnt a perfect test because I couldnt actually ride the bike. I didnt have my helmet here. Gonna go do some testing right now, gonna check for air leaks and throttle cable setup. i will try the choke thing again with the bike fully warmed up. Ill post on here later tonight with the results.


The idea of using the choke on any kind of permanent basis was not the essense of what I was trying to communicate.

The concept of maybe only using a small amount of choke or only pulling in out and then pushing it back in, so that you could see that the idle would drop back down to normal instead of racing was more the point. I was only suggesting it as a way of very temporarily enrichening the circuit to verify a lean condition.

Yes a 45 pilot with 3 turns should atleast not be way too lean. So maybe check and make sure that the throttle linkage seats positively against the idle set screw. Who knows maybe there is something wrong with the cables that is keeping it from returning all the way.

JD i went and had a practice today for the first time since the foot 'n mouth and gearbox episode in Feb/Mar.

start position

EFP on clip 5 (EKP equivelant being clip 7)

165 MJ

45 PJ

100 PAJ

PS 1 1/2 turns out

235 MAJ

run one

dropped the needle to clip 4 (EKP = clip 6)

improvement with the bike no longer misfiring on 1/4 throttle but bad higher. some surging at an even 1/2 throttle. bike riding at tickover idle still snaps and pops the wheel, great for dead start wheelies which are no problem.

run two

dropped needle to clip 3 (EKP = clip 5)

ran a lot better with only the midrange misfiring. no surging. not as good at snap wheelies.

run three

swopped to the EKP needle on clip 4 (EFP =clip 2) so i've gone down one clip position if i were to have stuck to the EFP. ok!

bike ran as sweet as a nut everywhere, but snap wheelies are dead.

adjusted the PS up and down 1/4 turn and fluffy either way so went back to 1 1/2 turns.

run four

tried EKP on clip 3 (down one).

felt just as strong and seamless power everywhere. snap wheelies?-where's my hour glass gone!


you were right about the needle clip setting, so thanks and well done.

everything is a big improvement except the snap from dead which has completely gone. sure, after 2 yards it flies but you have to wait.

there was also a feeling on the last two runs of the engine revs having to build before it would spin. it seemed to be crisp enough but i may do a little tinkering with a 48 PJ just to be sure.

if we take it that EKP/clip 3 = EMP clip 4. what is your experience with _ _ M and also _ _ N off a closed throttle. i know i have a DXM in the box, but i can't remember 13 months ago-too many beers!

remember i used to think that too much of the "straight" part was blocking the ET? i still think so. my answer was to have a wider needle with a higher taper in a drilled out ET? you said i only need a thinner needle (even though the start of the taper drops down the needle).

i would now like to try an EMM or EMN. i want to get that snap back.

your thoughts mon ami?


I FIXED MY BIKE. The guy that worked on my carb put the slide in upside down. I discovered this today and flipped it over, it was like having a new bike. This is the first time I was really able to ride with the yz timing. Damn am I relieved , was free too.

funktree, I wondered about that because it had heppened to someone else before, but thought it couldn't occur twice. :)


Well done also, good of you to share all this.

The EMN had more testing than the EMM. Mainly because I didn't have the EMM until later. I prefer the EMN more than EMP also, but then ride near sea level and about 50F(cool) mostly. The EMP recommendation is aimed at the general population and warmer/higher climates with consideration to the success on the forum with DVP before. (Not likely to foul a plug.)

The EMN is close to EMP but rolls on smoother and will not hesitate as easily. This makes clip positions 4 or 5 both work great (EKP#3/4) and more flexibility on pilot screw settings, less sensitive. It might be pushing luck with clip #5 and a full APJ stroke though. With EMN#5 in slow/wet riding conditions I would use caution with the pilot screw (make it lean) to avoid fouling. Similar to your run 2, EKP#5. Reducing the pump was a different situation though. This became a favorite (EMN#5) with less than 50% pump flow.

EMM didn't get the full checkout. It is highlighted as an excellent setting with 1.0mm pump stroke. (EMM#5, Test run #28!! :D ) EMM#6 was slightly too rich at 1/4 throttle. Then we went off searching the main air jet issues and development stopped.

EML#4 was only run early on with full APJ. It was a 2-stroke MX type feeling. It was running too rich at the crack of the throttle, even on a slow roll-on. It also sputtered on the road in a very narrow range near 1/8 throttle. This wasn't much of a surprise as a 2.705mm diameter is common for a CR500 2-stroke. Jetting rich on a track is one thing, but sometimes I'm 50 miles from my truck off-road and can't afford to be fouling plugs.

Given the pile of testing, EMM#5(EKM#4) is close to the richest needle setting possible with no APJ that I can see for a reliable set-up (not to foul plugs). This would allow a minimal pump stroke with careful tuning of the pilot and screw. This is where I would look to pick up the development searching for the last thread in regards to minimum APJ. Maybe FHM/N#4-5 also.

Hope the discussion and insight is helpful to you and others.

James :D

[This message has been edited by James Dean (edited September 10, 2001).]


seems you gave up on that EMM needle too soon, considering that that EML was obviously well juiced up.

interesting how i had my EKP on clip 3 which will equate to an EMM on the middle clip when i get it.

yet again it looks like i'm leaner in the UK (how many times have i said it?)

i would have thought anyone here (especially with an '01 WR or '00/'01 YZ) could experiment with that lack of snap by lifting or dropping the needle. when the needle is rich (high) the bike over the first two metres/yards is awesome.

if you remember i did my initial test based on the observation that i had to open the throttle grip 3mm before anything happened. WE'RE BACK THERE AGAIN!

if i could buy a fatter needle straight that started it's taper 3/4mm higher, to go with an emulsion tube(ET) to suit i would get it today.

anyway i think i'll scratch the EMN and go straight for the EMM.

thanks again


Considering the two bikes have completely different carb bodies ('99 vs '01), the clip position results are surprisingly close.

Your test run three is a good reference, EKP#4 was "sweet as a nut everywhere". Add the little richness from the EMM and this will fill in the exact first 3mm of throttle you are looking for.

well i have tried the EMM needle this weekend and i'm delighted with the results.

i was on

165 MJ

EKP clip 3

125 PAJ

45 PJ

1.5 PS

no APJ

235 MAJ

std euro silencer with PB header

so tried the EMM on clip 4 so the only difference is the straight bit.

test one

above 1/8 throttle the same. but at tickover as you open the throttle nothing and then the revs rise gently but not in tune with your hand!

test two

PS down to 1 then .5 turns and it improves response.

test three

swop 45 to 42 PJ and PS back to 1.5 turns

bike is better still. no hint of leanness or hot running

test three

idle screw turned in to 1 then .5 again. again each improves low end response.

test four

PAJ SCREW down to 100. PS back to 1.5.

bike is better still!

i am going to work toward 40PJ and even the 75 PAJ.the needle may even come down one more clip as well but we'll have to see.

the richness at tickover has allowed me to turn the pilot system right down. i can't tell wether that slow build up of revs from a closed throttle is from being too rich with the _ _ M or the fact that the taper starts way too low (lean). i of course put my money on the low taper.

another experiment will be to see if by lifting the needle about 3 clips the snap wheelies come back.

let you know.



the euro silencer is the same as the canadians,aussies, etc get. we all get it except the usa. no screws on the end no vortip or whatever.

it's square ended and not particularly pretty.

i love it because it is so quiet. nobody does a pipe that quiet bar the manufacturer. crertainly mine isn't stainless nor is any other i've seen.

i'll have a think on your rough running, see if i can think of anything.


Interesting results Taffy. The stock '98/'99 WR's used straight diameter "__M" with the 45pilot/75pilot air jets. That was Yamaha's chioce. When you get down to a #75 on the PAJ, which pilot will it be for you- 40, 42, or 45 pilot? :)


You say stock Euro silencer, are you referring to the big stainless sealed type? I have one too, If that is the type you have, are you pretty happy with it? I really like how quiet it is.

I'm wondering if all my rough running problems might get better if I go back to it.

interestingly i started it up and until yesterday the bike used to spend 10 seconds thinking about it before launching itself to high revs-all this on choke of course. <P>yesterday i started it and it had had enough of choke after about a minute with the revs not really getting high nor the old "eight stroking, death warble". <P>i was aware that i'm getting back to nearly standard, it's quite funny how you go round the block and where do you find yourself!<P>i'm on the 42 PJ now and i can only see the bike getting leaner at these revs. i think it was a 45 std. <P>you said that going lower makes it richer which i can't understand because these results aren't born out by this. <P>oddly enough it was Hicks low pilot #'s and there is a bloke you may have read his article from those links that the administrator got together the other day. he said that he had always gone BELOW the manufacturers PJ and it was his top tip tune etc. <P>i've bought another ET. i will put the EMM right up and even the EKP again just to remind me of the brutal snap i had and that it really is just 'where the taper starts'. <P>as you know i said a long time ago that the needle hangs 3/4 mm too low in the ET. <P>what i'm going to do is drill 2mm out of the top of the ET, right where it's narrow, this will allow the fuel to get by earlier i hope. it's also something that the lads can do at home if it works. <P>regarding the pilot circuit, does it make it rich or lean. what about this idea. <P>i have a thin 'rich' needle and a lean pilot circuit so the pilot circuit is a leaning circuit. <P>if i were to fatten the needle and wack up the PJ it will be a richening circuit. in other words the pilot circuit is richening or leaning depending on our needle straight dimension. the needle straight and the pilot circuit posses the ability to actually overtake each other and work on the other side!<P>Taffy<P>PS. on the 9/9/01 i said i ran a 100PAJ but when i checked it yesterday it was still set at 125. <p>[This message has been edited by Taffy (edited September 16, 2001).]

The pilot and needle straight diameter are overlapping circuits. The pilot works best slightly lower than needle straight. Too much off the needle will be more than the pilot jet can compensate for. The result will be a good idle and at the crack of the throttle will be excessive sputter. As the throttle opens more it immediately stops sputtering. Long downhills can be a problem for this setup because more fuel is drawn off throttle or at slight throttle. Then plug fouling starts.

What diameter will you drill the 2mm step to? If more than 2.95mm, the result may be nothing more than equal to changing the clip position richer.

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