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


can you try talking WITH me, & not AT me or ABOUT me. i exist, i'm not a syborg!

i challenged you to try an hours labour & you've ducked the issue. chicken!

if you don't understand, ask!!!

i do, i'm never too proud to eat my words, hell, like all the brits you take the piss out of i say "sorry!".

anon, apparently you can't get a smaller APJ 'jet'. IMO, there is TOO MUCH of it & for too long.

does the p38 lightning limit the length &, more importantly the amount (via a smaller 'jet') of fuel?

i get my new MJ's tomorrow & will try the bike on saturday. i'm very confident about the results i'm going to get.

i've got the bike 95% right & it's easier to go forward than to go back.

you two are piddling about with the APJ & my bike is pawing the air in the first three gears!

JD you ask how the PAJ screw works. the answer is in my 2nd or 3rd post on this thread.

thanks for all the help with plug chops. you are such a tease!! now i know your american sense of humour as well!

it was a joke

it was a joke!!


..wasn't it JD?

on another thread one of you already abreviated the PAJ-SCREW to AJ & straight away one of our more junior members had a 'misunderstanding'. for their sake please keep it clear.

to finish hick;

EKP needle on clip 6 or 7

unbolt the APJ at the hinge & pull the rod plunger around to the side

50 PJ (go to a 52 if you have it)

2 1/4 turns on the PAJ-SCREW (= No 140)

PS out 1 1/4

MJ down by 12 or 15 (mine was a 170 & i'm going to 155 or 158)

the real difference between our bikes isn't the cam timing for this but the open exhaust you have.

one hours work man!!!


[This message has been edited by Taffy (edited 02-22-2001).]

[This message has been edited by Taffy (edited 02-26-2001).]


You know what you're doing, on-on (the hashers say).

The P38 Lightning just limits stroke. It does not replace the nozzle in the venturi. They make such claims that appear unfounded.

The pilot screw has "a pencil tip & a hole across the end & up through the middle". I'll have to see it.

Right now I have an EKN#4 with an FKN#6 taper ground up higher (this is like EKN#7 at 5/8 throttle). Across the range it will be close to 0-1/4= EKN#4, 3/8= EKN#5, 1/2= EKN#6, 5/8= EKN#7, 3/4= EKN #8. I will back out the pilot (fuel) screw with a #48 PJ and see how far it can go this weekend with little to no accel pump.

As far as your presence in syborg land, no syborg could match your character, no even Hal. Things are just out of phase on your side. :)



On the Flat CR you cannot disable the pump in the manner you describe, you must remove the actuating rod (or the diaphragm I suppose).

I’ve been convinced for some time now that the pump gives too much for too long and I’ve been trying to find a way around this ever since, and I’m not one to change horses in the stretch run, especially since it appears I’ve finally found a simple way to adjust this (since I haven’t been able to get any optional pump diaphragms).

JD unknowingly talked me out of the Factory R&D cover when he discovered, by simply asking its designers, what it does. Simply put it shortens the pump stroke because the bottom is raised over stock (so the diaphragm contacts it sooner, ending the stroke). They also claim better atomization but I don’t see how this is done without changing the APJ or rate/leverage of pump.

I think tuning the bike to run well with good throttle response w/out the pump would be a good exercise, and maybe one I should try, but I barely have the time to keep my scoot running as it is. I seem to be particularly hard on my equipment for some reason, although I am very strict about maintenance.

I have run it without the pump before and was startled at how awful it did run. Remembering what that was like I can easily see how radically raising the needle would help things. But when you run at steady throttle won’t you be far too rich just off the taper?

I’m thinking that the racing applications you speak of where they removed the pump were not off-road or motocross. I would think a steady hand works when dragging a knee, but my right wrist is all over the place during a race or just a simple trail ride.

I think I’m very close to having it perfectly carbureted for me, and since I appear to be going the way of Tim Ferry (Factory Yamaha rider in the US) with the pump stop I’m thinking I’m on the right track. If I end up with a ridiculously short pump stroke you will only be more vindicated. :)


i've just figured that you have looked at your spare 75 PAJ & it has a flat bottom, correct?

if you look in the hole in the carb it has a tapered bottom to suit the screw. the drilled hole exits the side of the screw. as you screw it in more of the hole shows, letting more air into the slow circuit.

anyway, it's in the bike now & you've got me rocking & rolling. (that means trying hard to remember)


it's quite apparent from your last comment of 'it's running well, i'm almost there' that you DON'T believe in a whopping leap in power! fine, somebody, somewhere will have a go.

i can think of two brits on sunday who will be pleased to ride the bike.


i just can't think of needle numbers the way you are.

i'll tell you how i think.

i'd like a needle on clip 3, 4, or 5

i want the taper to start at least 2/3 clips (or four/six letters) 'higher' than the EKP. so the EFP on clip No 5 will be similar at 5 letters down.EFP No 5 = EKP No 7 clip.

i'll deal with the rest by feel & when i hit a problem i'll look at it. there are so many overlapping circuits that it's more important to get out there & answer a few questions & not just suck on the theories.

last weekend for me was a case in point. you & i discussed many aspects privately & we helped each other 'draw the picture'.

i have a slight 'hover' at around 1/4 throttle. the other settings are working brilliantly.

for you to know how much APJ to give your bike, you should run w/o APJ & guage the area required. it may be an idea to go to my settings hick & introduce the APJ, get it started right, stopped right & lower the PAJ/PJ back.

like weaning the bike off something.

the APJ must not start IMO until at least 1/8 throttle. why? coz the bike is **** hot to 1/8+ & as you can see from my test (4)

the APJ killed the bike after 2 yards in this test.

try to sort it by starting with the APJ & you condemn yourself to a very long sentence IMHO.

if turning the pilot out to 1 1/2 or 2 turns doesn't work then a 52 probably will. don't forget that my final setting was only 1 turn of the PS.

the bike will be getting the fuel beautifully atomized through the tubes & galley's of the pilot circuit. what's the other choice? the APJ? nop! not for me.

the modern two stroke roadrace carb runs a powerjet & years ago you had to jet only 75% for the bike & 25% for the powerjet. it was murder to sort & i don't know whether they got better but at the time riders just junked them. too much hassle. KISS.


my 1/4 throttle is far better than your 1/4 throttle. go back to the first post & read it again the bike was pawing the air at 1/8, 3/8, 1/2, 3/4; & on the last run WOT.

when i say there is a 'hover at 1/4 throttle it's relative to the new higher plain that the bike has hit. that's why i say the above statement.

1 hour hick. 1 hour!!!!!



You really REALLY REALLY MUST try an EFM#4-#5 needle for the 1/4 throttle lean condition based on tests 5 and 6.


So I am having trouble with my '99 carb, and was wondering if one of you guys could help me...??? :)

When all of this $hit is over, and the ULTIMATE FINAL OPTIMUM JETTING IS COMPLETE, someone HAS to forward this stuff to Yamaha who will hopefully jump all over this like a fly on $hit, and put out some tech/part updates for these pathetically OEM jetted bikes of ours.

Just my 7 cents.


I once said that the only way I'd try this was if Yamaha or KTM would pay me to do it. The results should be worth something, how about a new bike to test ride for a year? :)

WR250, YZ250, KTM ?? Honda??

The results aren't posted yet-



With work and all I have precious little time for tuning, although I’m no motorcycle guru IMO I’m skilled enough in general to fake it quite well. I assure you all the (very promising and innovative) mods you have come up with are well within my abilities, it is my schedule that is the problem.

Perhaps it is just me, my lack of patience or insensitivity to the subtleties, but with the pump disconnected (makes it loads easier to distinguish between clip settings) my bike was definitely rich off the taper at #5, in fact I hovered between #3 and #4. I imagine once the summer temps hit I’ll be on #3 again (last summer I was #3 EKQ and #42 I think).

At any rate it seems that the best method would be to jet for static throttle positions with the pump off, then try and manage your throttle response with timing and pump stroke (though I’ve not yet thought how to arrive at each of the two). I would think you would agree on this. In fact I think you said as much, but, honestly I sometimes have a hard time following your posts. My English ain’t so good (the language of choice in my area is Spanish, anyway).

As soon as I get the parts (air screw and adjuster for the pump) I’ll sacrifice a day of riding and put on my tuning cap and get back to you.

But the cam timing will have to wait…

NH Kev,

I second that!!

I honestly think Yamaha should add the pump stop to the carb (they added the timing adjuster, why not a stop?), all they have to do is drill and tap a hole in the existing FCR. Maybe this feature will make it from the track to the showroom on their rumored “new” motor.

ok hick & jd. i'm on my own right?

jd, you're the only one along with clarke that people might listen too (note; i didn't include myself).

1 hours testing that's all i asked for, jeez! i guess i just don't know how to ask nicely anymore!

i hope that i get everything done for sunday & one of these two brits have the energy to thrap my bike at 4pm after a straight 3 hour race.

i really despair lads i'm sorry. it took me 20 years of piddling about to make last sunday look so easy, & when i get there what do i get? fence siters & hand wringers. pha!!


i've got a chart & i'm sure you have of the keihin says N427-OC at the top & then





needles are charted. a month ago even that chart was dutch, now i understand it completely.

EFM is good. which is more important though jd; start the taper at the right moment? or have a thinner needle straight & later start?

it's got to be a thinner needle & a later start; because you have gotten rid of metal & won't have a lack of fuel. agreed?

so read on my thoughts are changing by the hour...

the WIDER the needle straight i can accept the better my chances of an early taper start WITHOUT the needle becoming too thin along the taper.

talking to you jd it has become clear that the '_ _M' was what yamaha engineers were told would be THEE correct straight diameter & the RIGHT needle for the job the first year. yamaha said "ok we'll take it".

they then realised that the taper started too low on the needle & the only way they could simply (as opposed to radically) change things for '99 was to squeeze a bit more on the straight portion & prey they kept as good a tickover as possible. this way the taper has moved up (earlier).

they've gone from '_ _M' to '_ _P' to '_ _ Q' & did i see it's an '_ _ R' this year?

they're getting the taper to start higher & higher. but they'll be &%$#@!ed eventually because of the APJ chucking in fuel like a retard playing with water & a bucket.

so i guess to some extent i've got to do the same jd. without a doubt a larger emulsion tube is required but you, jd, said that they don't make one. i bet yamaha are really pissed at this. my old mikuni flat slide has about 5 sizes of ET in about 18 letters. that's 90 ET's to choose from!

keihin do only one! wankers!

you are wrong about the CSA jd. you get a larger fanny, she's got a larger CSA!!! as i've said before yamaha want a larger ET to go with a larger 'straight of needle'. have i got that wrong?

thats the only way to start a taper early WITHOUT AFFECTING THE REST OF THE NEEDLE TAPER.

in england we call that "being stuck between a rock & a hard place". do you have that expression?

going back to that chart where the needles are laid out i note that the EKP isn't even mentioned. so can you give me a 'UBB' to a FULL list anywhere jd please?

going back to your needle profiles i've come up with a beauty to try.

don't forget i'm looking for a needle taper start at EKP clip No 6 or 7. but i want it on the middle clip.

EKP - clip 7

EHP - clip 6

EFP - clip 5 that's what's coming in post

EDP - clip 4

EBM - clip 3 would give me a later taper start but same needle taper as EDP.

EBM - clip 4 would give me the same taper start but would be one clip richer than EDP No 4. at least with the EBM i would be given this interesting choice.

like everything in life i don't want either or, i want both!!! (greed is good!)

i've got the EFP due & we'll see how it goes here first. one day jd you'll be able to say "well an EDP off the shelf or go to town with....."

i note an interesting needle on the right of that chart called an EBR!!

yes, you know what i'm thinking already...

so how easy is it to hold a needle still in a model lathe & turn it down?

my engineer has only ever turned down two jobs in 21 years & this was one of them. "can't hold it anywhere. i would simply use a file". gulp, er no simon i think i'll give this one a miss!

i walk in during work hours & get a huge smile from his boss even though he's openly moonlighting.


12 years ago i asked simon if he would try making titanium con rods out of solid billet. he said yes but when i saw him three weeks later he admitted to throwing 5 shagged cutting tools in the back of the cupboard worth £200 each, a total of £1,000 or $1,500. this is 1990 don't forget.

one day when eric (the boss) looks in the cupboard he see's the cutters & goes apeshit. simon comes clean " ye i did it while moonlighting for Taffy."

a month later eric gets a call, "can you do £50,000 ($75,000) of titanium machining as i can't get it done anywhere". "one moment sir".

"simon, are you any good with titanium or not"

"yep, no problem!"

they've been specialising in it ever since, thus the smile! turns out you gotta cut it very very fast & use an old (fashioned?) cutting fluid!

ps just got the needle OCEFP. it's brassy looking is that normal jd?


[This message has been edited by Taffy (edited 02-23-2001).]

[This message has been edited by Taffy (edited 02-23-2001).]

Taffy, I've been trying to follow this discussion, and I'm not to proud to admit that I don't understand it 100% but I think I've got this much: The accelerator pump is more of a hinderance than a help as it usually delivers more fuel than required (from just off the idle circuit). Hence you have disconnected the pump and are now having trouble finding a needle to compensate for this adjustment. From the tests you have done the power gains have been dramatic (does the pump work from off idle to wide open throttle?). Are we all better off waiting for the correct needle for this application? and what would be the easiest way to stop the pump, could we just disconnect the pump arm or would it be better to block the jet?

I thought Yanks were supposed to be the trail blazing types, but then again Britain was always the home of the excentric, genius type. I'll keep following until the penny finally drops. :)


More yanks would listen to you if you'd just admit we kicked your arse during the Revolutionary War!! :):D

Even w/ your dry, Brit humor you have to have a smile on your face, right?

BTW, I AM a direct desendent of Sir Francis Drake, the infamous cicumnavigator. My Grandfather was from England.

Now, can you fly over to the U.S. to work on my bike...??? :D

I thought the acc pump was supposed to help with snapping the throttle from the closed position. I have a lot of value for the testing you guys are doing but I don't want to sacrifice a worse bog when you come to a short steep jump and almost end up on your face. Maybe you guys have already been watching for this.. Thanks for the information!!!

Ion H.


You've come a long way in the last month. You have the "chart" and understand!

The chart is all we have along with Yamaha needle options which are a dead end.

Now study the graph I posted before on needle profiles. We don't need a larger emulsion tube (ET) because a smaller straight diameter AND a richer clip position create the same result. You hit the nail on the head with the EDP#4 and EBM#4 having an identical taper start point. The EBM#4 will have nearly .02mm difference on the cross section area (CSA) for the full length. Same increase in CSA as a .02mm bigger ET using the EDP#4!!

Smaller needle full length or larger ET full length, the same result either way.

The needle you have is brass. All the OC--- needles are ready made to turn down. What do you think I have been talkng about with double/triple taper needles. Enter JD's toolbox and what it is littered with is a boneyard of ground up brass needles. EBK is begging to be in your toolbox too, modified as an EBM. But first look at EHM and FBM in the chart.

BTW - 3:55 to take the needle out, 8:25 to change the clip and put it back together.

hart, the bog is everything here. Any bog is unacceptable within the range of what the "best" accel pump can do. Hick said it best, jetting for the richest acceptable static throttle settings along with an accel pump just adequate to keep the bog from occurring. A tailored needle that fits like a glove. Taffy is on the edge of fouling a plug IMHO :) .


[This message has been edited by James Dean (edited 02-23-2001).]


After reading all the info you have supplied it sounds like you may need the accel pump to fix the 1/4 throttle lean condition. Maybe you should experiment with accel pump stroke start/stop and fuel volumes to get a richer 1/4 throttle alowing you to drop the needle down to clip #4 or #5 to lean out the midrange. you may have already tried this but it's just a thaught!





it's only my opinion that the APJ is a hindrance so i will have to get it right to prove i'm right. don't buy a needle until the final results are in.

i've got overlapping circuits everywhere & what i thought was a small problem is proving hard to go away.

since the first thread i have thrashed the bike in the dark, on the road & had a heavy misfire from 1/2 throttle on. like a two second cough. this didn't happen at thetford.

it appears to have been running rich & i was confident the 162 MJ (from 170) would cure it but infact the 165 was better.

today i did more tests & they are as follows

clear & crisp. roads near house. not in woods. air pressure is 'normal'.

test 1

162 MJ down to 158. needle is an EFP on clip No 5 (equal to EKP clip No 7). PS from 1 to 1 1/2 turns.

idea; get a light colour in the plug plus sort the misfire, with the MJ change. PS to help 1/4 throttle.

result; horrible misfire just past 1/2 throttle still, acceleration to here is now excellent.

test 2

158 to 155 MJ. lean the needle to clip No 4. (EKP = clip No 6)

result;lighter plug, horrible misfire from dead on 1/2 throttle. terrible in 5th gear**.

test 3

lift needle through to clip No 6 (EKP + clip No 8)

idea; kill or cure!

result; slight misfire is down to 1/4 throttle unless WOT. terrible in fifth**.


those highly oxygenated woods & high air pressure at Thetford are gone & i'm struggling! i need less fuel from 1/2 throttle. so look at the needle.

test 4

DVP needle fitted at clip No 7. i estimate that the taper start is still lower than the EKP on clip 6 but it will have to do. 170MJ fitted. gearing goes 14/50 to 15/50.

idea; see what happens in first four gears.

result; goes from 1st to 4th in one hit beautifully. violent jolting in fifth (see another thread). slight loss of snap.

test 5

needle up to 8th clip using washers, pilot screw in to 1 turn.

idea; get the snap at throttle back off tickover.

result; goes like snot (that's very quick hick!)snap wheelies not quite as good as E needles were.

conclusion; how can i compare a road with woods & bumps? felt lovely with the 'D_ _' needle. i'm on the right road. more experiments to come & some will have to be back to back on the same day to get a feel.

DVP on 9th clip to get the taper started early?

JD could you tell me the height at which the taper of the DVP & EKP on it's 6th clip would start?

i told you i would tell you everything warts & all. win some lose some.


[This message has been edited by Taffy (edited 02-23-2001).]

DVP#6 and EKP#6 have the same taper start. (within .5mm)

For those who are struggling to follow this thread -


There are several ways to richen the jetting at or below 1/4 throttle.

1)Using a higher taper start and less taper will make more fuel below 1/4 and will still have less fuel at 1/2 throttle and above.

2)Using a smaller 2.71mm(richer) straight diameter makes the taper start lower but still gives more fuel below 1/4.

3)A richer clip position shifts the sloping lines left, so it also adds fuel at 1/4, but mostly effects 1/4-1/2.

4)Bigger pilot jets and larger pilot air jets add fuel and crossover this area also.

5)Less slide cutaway can be used to richen.

6)The accelerator pump can be timed to start or stop earlier or later.


When you snap open the throttle, the fuel/air mixture delivered is a combination of whats in the intake port already along with what all the circuits can provide. The problem is getting good atomizing when the vacuum drops (pressure increases) and fuel doesn't get drawn from the circuits temporarily. Richening the clip helps the acceleration but is overkill in a steady(static) mid-throttle condition like on roads. The accel pump squirts fuel like a garden hose which doesn't burn clean. We are searching for the balance point of each circuit.

The advantage will be better torque at all throttle settings, rude acceleration, and none of this up-and-die bogging.


[This message has been edited by James Dean (edited 02-23-2001).]

thank you all.

aussie andy read my last post & you'll see that the snap is still there with this D needle. but it's hard to quantify when it's nine tests & six days since i was propelled through thetford forest.

i have got the snap.

i have seamless power to the rev limiter.

i have a broken fifth gear!!!!

even with the D_ _ needle fitted the 1/4 throttle is now fine. my problem is now that the D series is too straight & the E series is too steep an angle.

the jigsaw is coming together.

a D _M needle would make the needle narrower at 3/4 throttle.

playing with MAJ's with the E needle is still an option to reduce the 1/2 to WOT mixture.

i've got lots to worry about for now.

James Dean

Any progress in finding a better pump stroke setup?? Please post your stroke length recomendation after you get some testing accomplished.

Kinda sounds like Taffy is heading for that tripple taper needle--have not we already been there?


[This message has been edited by Clark Mason (edited 02-24-2001).]


the jetting at the moment could be compared by many on this site. a DTM needle on clip 5. PAJ-SCREW at 2 1/4 turns, 50 PJ & PS out 1 1/2 turns. your main jet to suit.

even though i'd changed the sprockets the bike ran beautifully from zero to WOT with the DVP. to slot a needle in off a hunch & have it go so well is really pleasing.

so which way do i go now. the DVP can be changed to a middle clip needle like a DPM.

i need to test my standard set up, my 'E' needle set up & try some more idea's on the 'D' series.

the 'D' series is already for the road. I.E. the figures above are OK.

but the real set up is in taming the 'E' series needle. next, i'm going to try lowering the EFP needle until i no longer get the snap & pop of the taper change.

when you think that these machines run lean at 3/4 throttle on EKP clip No 4 & yet at clip 6/7 the needle is woefully rich.

i will then see if i can clear the rest up.

the 1/2 throttle setting will be the fulcrum for all testing. the rest can be adapted with air jet's etc.

get that WZ back out & scrap that red peril.


Clark, Taffy, etc.

Your perspectives are invaluable. Today I spent as much time making a needle as was spent testing it. The plan was to find the minimum accel pump (AP) stroke to get a hard snap with no bog and increased torque in the upper mid-range. More than a D or E taper can provide as a single taper needle.

The straight diameters least likely to hesitate on medium accelerations with zero pump are 2.715(--M) and 2.725(--N).

1)With EKN#5 and second taper FK-#6 with zero accel pump there wasn't enough juice to wick it hard. It falls off and backfires.(48 pilot, 1 1/3 turns,100 PAJ, 175 main, 200 MAJ)

2)adding .7mm pump stroke makes it start to feel like a Husaberg 501 with a DelOrto carb. It pulls with a smooth speedup just when a normal Yamie drops off. It feels like the fuel keeps burning, just a little weak.(165 main)

3)The "newest" DPM#6 with second taper EKM#5 and third GK#6 pulls hard with a more noticeable sputter just off idle. Ran .7mm pump stroke which ain't much of a squirt. It accelerates hard always. Takes a hard twist of the throttle with no hesitation and again feels like the non accel pump bikes. It just doesn't spin up as fast as an AP with full stroke. This was a disappointment because alot of time was spent on this needle. It can't be set richer for general use because it's on the edge of fouling under lighter loads and static throttle. The 8-stroke feel is ever present and choke is not required.

(.7mm pump, 165 main)

4)The next solution was back off some richness and use the AP to keep the hard acceleration. Increased the AP to 1.0mm stroke and lower the pilot screw to 1 turn (#48). This pulled harder with the front coming up quicker. Still raspy rich at times 1/4 throttle.

5)Shifting the clip 1 step leaner, DPM#5/EK-#4/GK-#5 on this triple taper needle hit paydirt. The bike ran clean with hard acceleration and minimum AP. This is a spooky feeling. It's a 2-stroke feeling. The impression at 2/3 throttle is that of a bike that wants to rev hard and not stop. I'm a veteran 2-stroke rider used to this on KX's and CR's both 250's and 500's. It's addictive to come out of a corner with the front lifting so hard. If it doesn't hook up it revs hard. Then on uphills there is no lean feeling of waiting for something to happen or wanting to roll open the throttle more.

More refinement will follow as time permits.

This testing differs from Taffy's observations at 2/3 throttle from my interpretation. Clearly EKP#7+ works great at 2/3 throttle and minimum AP on my bike. The temp was 45F, no airbox cover, '99YZ exhaust, YZ rear fender with it's extra opening to feed the airbox. This is also on a '00WR with the updated flat CR Keihin as opposed to Taffy's '99 FCR. Some of this is a factor.

The big question is whether one highly refined setting can carry over to a large population in varied states of tune. Probably not. This is too bad because I'm having a blast now! :)


[This message has been edited by James Dean (edited 02-25-2001).]

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