James Dean – Accelerator Pump


In your reply to the “bogging” thread on the YZ side you said your pump was set one full turn in from “zero clearance.” This made perfect sense when I read it but I became confused as soon as I got home and looked at my carb. A few questions then:

By “zero clearance” you mean the point at which the actuating rod begins to protrude below the carb body, make contact with the diaphragm and thus begin to pump fuel?

Did you check this with slide fully closed (i.e. idle all the way out)?

Either way, how many turns in is your idle screw set from full closed?

And finally how does the rod detach from the plastic arm (so I can see if it is dirty or corroded, the action doesn’t seem very smooth)?



Zero clearance is referring to the rod making direct contact with the diaphram at idle. Not backing off the idle to a fully closed position. My carb had the pump adjuster screw set 1 turn in from this "zero freeplay" position. The pump action with this setting starts between 1/8 and 1/4 throttle. If this is reset to 1/2 turn the pump action will start at only 1/8 throttle and nearly squirts the fuel on the face of the slide, but just under it. It appears that a range between 1/4 and 1 turn on the adjuster screw is available to pick the best response from. Depends on how your wrist action is :) and what your other low speed jetting is rich/lean.

Will check the turns from fully closed later today. I did not detach the rod from the plastic piece. My rod has smooth action. :D

How many turns in is the accel pump screw set on yours when measured at idle??


[This message has been edited by James Dean (edited 10-02-2000).]


how do you check for zero free play and

what do you mean 1 turn from zero?

is that turning the screw all the way

in and backing it out?


00 WR400


Remove the black cover on the right side of the carb so the accel pump linkage can be seen. Push on the linkage with your finger to feel the light spring tension holding linkage to the throttle cam. There will be a small amount of movement in the linkage from where the cam rides to the position where the rod pushes the diaphram. Check it by feel. Turning the screw out will lower the rod towards the diaphram and reduce this free movement. Turning in the screw adds to the gap and allows more rotation when pushing the linkage with your finger. From no movement, the screw was 1 turn in.


The idle adjustment was about 4 turns from the slide bottomed. This was checked by listening to the sound of the slide bottoming on the screw and backing the idle screw out till the sound changed. Didn't take the carb off, hopefully didn't flood it checking.


[This message has been edited by James Dean (edited 10-02-2000).]

Thanks for the response James.

My idle is set five turns in, but my bike is apart so I’m not sure if this is correct. I may have left it turned up a bit because I last rode it in a race, but I doubt I increased it by a whole turn.

The reason I am curious is because my stock pump setting was not nearly a full turn like yours is, more like a half or 3/4. But the extra turn on the idle speed would change this. My hunch is that they set the idle speed and mixture at the factory and then set the pump screw (mine does have a dab of green paint on it to mark its position just like my TPS).

Assuming five turns is correct, would higher altitudes necessarily mean a larger slide opening to achieve good idle speed (I ride at 4,500+)? 426 vs. WR ignition timing? Different pilot air jet from WR to 426 (counterintuitive though because YZ is bigger)? Flat versus slant FCR (unless you have an ’00 of course).

At any rate I think we can be certain that I did not increase the amount, velocity or duration of fuel delivery but only advanced its timing a bit relative to throttle position. But this did not leave a dead spot at initial throttle openings. Maybe the higher initial slide position works better with the same (or close) amount of delay as a lower initial position.

BTW I currently have a 40 pilot (one size leaner) and idle mix is back at stock with PC pipe (was ¼ turn leaner with stock pipe). I just realized that WR has smaller pilot air jet combined with larger pilot jet relative to the 426 (unless I’m mistaken). What do you make of that?

I am always interested in learning new things about bikes and motors so thanks for all the information; I’ve gotten a good amount from your posts already and will continue to appreciate any advice or insight you care to offer, I doubt I have the patience to ever mimic your carb tuning observations.

PS if anyone has done this I would still like some advice on removing the actuating rod from the little lever. I don’t want to force it and break something (again).


Mine is of course a '00, the screw is not on the '98/99 models. The setting may be pretty much a nominal 1/2 to 1 turn. Currently my '00 has a #100 pilot air and a #48 with YZ cam timing. If your idle speed is up, the accel pump turns would be lowered. This is what you found. Are you saying that after it was tweaked from the stock setting you now run it at 1/2 turn or less in?

(The #40 sounds like a pretty small pilot jet.)



It was a half turn stock then I tweaked it an additional quarter turn. At the stock setting the fuel stream does not contact the slide. At four tuns of idle speed the stock position is very close to a full turn of delay. I was mainly curious about the extra turn on my idle compared to yours, that seems like a lot.

As for my pilot maybe I am biased from past experiences with 2 strokes ALWAYS being way too rich here (high and hot). But when I came back from Glamis last year I left it one richer than stock for a few rides and it was too rich. Maybe I didn’t give the stock pilot enough of a chance at that time (when I arrived at the jetting I’ve got now).

After all the jetting discussions I have read here I now feel a bit insecure with my leaner jetting. I am going to Glamis again and when I get back I will try a more thorough and scientific jetting exercise. Ordinarily I just make a few passes down the ditch bank at different throttle openings. How is it that you can determine all the little nuances between all those darn needles you are always trying?

Of course I’m the guy who went almost an entire loop (~20 miles) with a flat front tire and had no idea, despite the fact that I fell twice, until the last check when the scorer pointed out my mangled rim. There are more big hammers in my tool box that precise little screwdrivers.

I assume you changed your air jet to better match the bigger pilot. Is this a popular mod for YZ timing only or for all WRs?

It seems to me that if I understood why the WR and YZ differ so much I would understand the FCR that much better. When compared to a WR, why is the YZ400 richer on the main but the 426 leaner (on pilot as well)? How will the WR426 be jetted? Is the displacement or ignition/cam timing the bigger factor?

I’m just overly curious I guess.


I went back and forth on the pilot air and decided on the 48/100 combination. The needle selection is a case of ride and feel the difference. Always know what throttle position to expect the change before riding. Then make a judgement as to too rich/lean and whether roll-on/accel/torque/rev etc. is better or worse. The accelerator pump is a wild card. It may be the source of your ability to use a smaller pilot than most. It still looks like 1/2 to 1 turns IN is a standard range for this adjustment.

The WR jetting versus your YZ426 is a long list of compromises for the quiet exhausts, throttle restrictors, California sales, exhaust cam timing, slow speed riders and everything else. Some of the jetting difference is for the targeted buyer. The 426 has a 1 degree taper carb needle so the main jet is smaller, otherwise the jetting should be very close to an uncorked WR or YZ400. That would be assuming Yamaha took the time to sort out all the possible avenues for the 426 jetting. Looking at 2-strokes and how they are usually rich, they probably got cautious. In this case they went LEAN to avoid fouling mishaps and frustrating riders who have to tear everything off to change a plug. The needles ending in R and S are VERY lean down low. This why my push has been to use DVP and EKP.


Thanks for the info and jetting tips James. What you said about Yamaha being on the safe (re: lean) side makes a lot of sense.

You have inspired me to order some new needles to try at Glamis (EKQ, EKP) and when I rejet for the winter (I just reread your YZ needle post).

The reason I bring it up is that the guy I talked to at Sudco looked up the accelerator pump adjustment (in a Keihin manual) and it said the pump delay should be .6 mm (with measurement taken at the arm). I’ll bet that this is roughly equivalent to what you have found. So there is a base setting (according to Keihin anyway) for pump delay. He also said that street bike owners have long known that advancing the timing would cure the FCRs initial “hiccup,” which is nothing new to them. I always thought the “hiccup” that everybody was talking about had to be a lean spot. Now I think maybe its an overly anxious accelerator pump (at least that is what many street bike tuners say according to this one guy at Sudco). This also agrees with the latest product test of the P-38 pump housing (in Oct MXA?). The article stated that Yamaha had cured the “YZ stumble” for ’01 by delivering less fuel initially. But it seems to me that MXA editors (and others) often print questionable information.

Having said that I’ve gotten plenty of just plain wrong information from strangers over the phone as well. But it does make sense to me and the two corroborate each other as well as my own (very limited) experience.

Another interesting thing, that you are probably already aware of, is that Sudco only listed one PN for the diaphragm (guy said the word “all” appeared next to the PN in the application column).

I also asked him how to properly remove the actuating rod and he said the same manual indicated that the throttle shaft needed to be removed to disconnect the rod from the arm.


Interesting bits of information you have found. When you said street bike riders advance the timing did you mean the ignition timing? (not accel pump timing-)


Pump timing.

When I asked the guy how to properly remove the pump actuating rod he ran across the .6mm of delay spec and then mentioned that, in order to improve response at sudden throttle openings, many street bike owners were advancing the pump timing. He offered this without my asking about it, although I was asking several questions about the FCR pump. I think the guy’s name was Tim, he said he was waiting on a YZ250F, which would explain his interest in the FCR.

I called back today to ask about buying a manual specific to the FCR but they don’t sell one.

BTW the latest P-38 test I read was in Dirt Rider, not MXA. It will be interesting to see what parts of the pump will be changed for ’01 and how these parts will work on older bikes. It kind of irritates me (but doesn’t surprise me) how the mags are quick to tout a product at the expense of what could be valuable info for its readers. The article stated that the P-38 would be a cheap alternative to buying an entire ’01 FCR. How insightful. :) But I’m sure it will be loads cheaper to retrofit parts from the ’01 since I doubt they redesigned the whole carb (OEM pump cover is less than $30 for ‘00).

After reading the article in Dirt Rider, I called Factory R&D about the P-38 Lighting:

1)The pump cover they make has a taller post in the base directly under the diaphram to limit travel and accelerator pump flow. This avoids wasting extra fuel when the pump is taking too much time or flowing too much. They speculate the bog is caused by to much fuel. -IMO the bog part seems unlikely but the saving fuel part sounds ok.

2)The inlet hole is bigger so it refills the pump quicker. -no big deal.

3)The outlet is smaller so it will flow fuel at a faster speed to the nozzle for a better "spray" into the engine. -This sounds like nonsense, the nozzle is the smallest restriction in the fuel path. Making passages smaller upstream only slows the fuel down.


After this I called Sudco:

Their perspective on reducing a bog is that good jetting of the pilot, pilot screw, pilot air jet, and careful adjustment on the pump timing are the keys to best response. They have resigned themselves that rolling on the throttle is required, get used to it, adapt. I feel the needle straight diameter and clip are also factors. ( basically EVERYTHING)

-From what I have learned the P-38 Lighting is not a cure but more like a patch.



this dead spot off of tickover? is anybody any nearer getting rid of it? i think the bike isn't short of fuel so is it rich?

have you tried disconnecting the APJ altogether & tried various roll on throttle tests. my old duke ran 41mm malossi's on each 300cc cylinder. it was only two-valve aircooled, yet it snapped your arms off. it didn't have an APJ at all.

another point is that if you make the jet nozzle smaller the juice continues to come out higher up the revs where it wasn't originally meant to. this gives it less time to refill so they restricted it's volume & allowed it to refill quicker using larger holes.

sounds to me very much like the thing is overjuiced what with the recommendation that it starts later as well!

let me see if i've got this right. it's recommended that it squirts LATER, LESS of it, what should have been a longer squirt has a restrictor to make it FINISH EARLIER/SAME TIME.

the quicker you make the air go through the engine (more torque) the less need to have an APJ. we did survive without them.

just putting some idea's down.


[This message has been edited by Taffy (edited 10-06-2000).]


By APJ do you mean Air Pilot Jet? Also, if you take the carb off you can see that it takes a while for the pump to expend itself, it continues to spray fuel well after the slide stops moving. I agree that it sounds like the P-38 might extend this even more.

I feel sort of guilty for starting this whole pump thread since I didn’t have any problems with dead spots on my bike. If I purposely tried to make it cough it would, (I had to really whack it open from below idle) but this never happened to me ordinarily. When I turned the screw in a quarter turn I didn’t even know what I was doing (oh sure, I THOUGHT I did) but I could no longer make my bike cough. I rode the bike many times (once in a race) with this setting and noticed no ill effects.

But, as the saying goes, I know just enough about tuning a carb to be dangerous and my jetting job may not be the best.

But I certainly learned something.


That is good info on the P-38. I wondered exactly what the difference was, I’m glad someone thought to pick up a phone and ask.

I trust you didn’t tell them you were going to pan their product here on thumpertalk, and that we would all listen to you and not buy it? :)

Seriously though, thanks for the insight, I would imagine I am not the only one who found the info useful and interesting, like always.


What is tickover? Idle or zero throttle?

I have found that richening low speed circuits within reason allow hard throttle twisting at low revs under load. For each component richen until it has a dull response or sputter and then back off. Hick has improved his response by only adjusting back the accel pump timing.

Many times I have considered disconnecting the accel pump but this would lead to richening elsewhere to compensate. Right now it seems better to utilize it rather than disconnect it.

As far as the jet nozzle is concerned, they aren't changing it. They are changing a passage upstream which will have little effect. The bigger source passage in the pump may refill quicker only if there was a restriction there to begin with and if refilling was a problem. -don't think it is.

The basic problem is getting well vaporized fuel to the chamber when hard wrist action makes the venturi pressure jump up to atmospheric pressure. Good jetting on all the circuits and the right pump delay brings out the best response possible.

The toughest condition that I've found is tying to whack the throttle open coming out of a tight corner and into an uphill at low revs. It's worse with a restrictive exhaust. The bike will pull hard with the right combination. :)


The Factory R&D guy was good about returning my call. Even though he didn't sell me on the P-38... :D



APJ is the accelerator pump jet.


how could i possibly argue!! you're right of course if we get every circuit right we will have the correct jetting.

but there is a correct way to jet the bike AND in the right order.

1st main jet

2nd roll on tests on 1/4-3/4 throttle-no APJ

3rd the whole low speed circuit.

4th & LAST the APJ

as you improve the performance of the engine the speed of the air through it goes up (more air) which means that the carb becomes more responsive to any throttle change-i won't quite say snap-& the petrol is instantly "there".

this means that all the other parts of the carb will be jetted to the bikes needs but that the APJ will always, ALWAYS come down in either, quantity or time squirting.

the APJ is there to make up the delay between throttle opening & arrival of the fuel on a snapped throttle. if these bikes ran a 33mm carb there wouldn't be an APJ on it at all. the air would be going through so bloody quick it would rip the fuel out the floatbowl!!

anyone having a problem percieving all this should think of flying a power kite on a calm day. how much control/response do you have over it? now think of a windy day. now how much control/response do you have? that's what's happening at the top of your emulsion tube

other variances come in as well, such as

1) ignition timing/TPS (throttle position sensor)at tickover-1/8th throttle

2) exhaust system

3) the cam timing

4) size of carb, ports, valves

you can winde the APJ right off i should think.

finally if the petrol chamber for the APJ is still emptying it's balls on full throttle how can it possibly refill in time for the next squirt? for a start there is absolutely no reason why the APJ should still be squirting after, say, 1/2 throttle. so all the rest of that fuel is being pumped in needlessly.


[This message has been edited by Taffy (edited 10-06-2000).]


It always seems to take 90% of the time to get that last 10% of the jetting right. Even using your "correct way" of jetting takes several cycles through because of the overlap of circuits.

I agree with you on just about everything. On the APJ, it is slow to empty and we can only speculate how quickly it refills, how often you need it to reset quickly, and if the P-38 would make a noticeable difference in that regards.

It's a good feature to able to limit the pump duration with the P-38, but when So Cal Erik got his P-38 they tried 5-6 different pump bodies before they were satisfied. He was lucky enough to live near Factory R&D. Diaphram heights can vary even if the part numbers are the same. Clark had to have his P-38 sent back to get the correct pump travel.


BTW - The 3 optional Yamaha pump diaphrams are now on order from the local dealer. Hopefully next week some feedback will be available as to what difference there is between them and stock... :) Just another twist when you thought you had seen it all.



[This message has been edited by James Dean (edited 10-06-2000).]


Do you think the primary difference in the diaphrams is the lenght of the bottom mounted stop?? ie a longer center mounted stop would limit travel and squirt duration??

Let us know what the difference is.


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