factory r and d p-38

i told all of you that i would post my results of the p-38 after installing it on my 99 yz400 so here it is. as soon as i installed the p-38 my slight hestation just off idle turned into a big time bog even at mid rpm while riding, i was just about to send it back for a refund when i disided to try riching the mixture because the bike seamed lean as soon as i went down two clips on the needle all the hestation and boging was gone unless i really wacked the throtle open hard (a big improvement over the stock setup) it still needs fine tuning but it is much improved from before the p-38 i think with a little more tuning i might get perfect throttle response. was it worth the $74 yes . some bikes might not need tuning but if it does not work try a few different settings before getting discouraged it worked for me.


Thanks for the post and review on the P-38.

If others want to try it, something to keep in mind when replacing the pump cover is that there will be air in the pump initially. This may take a little run time to get it to fully prime. You may find a very BIG bog and backfire until it does finally prime itself.

Using a richer clip position with the P-38's reduced pump stroke is a good idea. Careful adjusment of the pilot screw and needle clip are important ways to reduce bogging.


I have installed th P-38 also and had no problems at all ! Did you make sure that the pump rod had alittle bit of play ?? I also found that when i run 91 octane the bike is sluggish and just dosen't run 100% !

As a P-38 user myself, I found that using race fuel(VP C-12) and being patient in fine tuning my jetting was well worth the expense and effort. C-12 has a tendency to make the jetting rich so counteracting with a slightly leaner main jet and getting the fuel screw set correctly were the keys. I can whack my throttle open as hard and as suddenly as I want from right off idle to anywhere above and get instantaneous response. I agree that the P-38 was money well spent.

Can anyone explain why some bikes seem to need this and some don't? I have a 99' 400 that started bogging when I added an E-series pipe. I added a 48 pilot and set the fuel screw to 1 1/2 turns and it ran better than ever. I could not get it to bog no matter how hard I tried. I whacked the throttle open as fast as I could and it just motored. Any answers?



99'YZ-400 (Mine)

00' TT-R 125 (Hers)

91' KX-125 (Son #1)

93' CR-80 (Just sold it. Looking for another TTR125 for Son #2)

99' PW-80 (Son #3)


The P-38 is not necessarily needed but allows jetting without the problem of extra fuel being pumped when it's not wanted. The P-38 is not a cure-all. Other adjustments still need to be made. In your case the bigger #48 pilot jet was the solution. Another might need the clip moved (to #5) or a smaller needle diameter (DVP). Too slow of an idle speed can be a problem along with unrealistic expectations as to how hard the throttle can be wicked open. After a time, riders adjust as much as bikes get adjusted until they're satisfied. :)


[This message has been edited by James Dean (edited 04-10-2001).]

James, we own a 250F and I saw what you wrote about the P-38 cover. What does it do exactly for a YZF426 and would it be a good idea or even benefit a 250F. Also you mentioned that the one for the 426 might fit. Is it R&D racing that sells this part and do you know their phone number? Thanks in advance for your feedback.

Factory R&D can be reached at (949)206-0894.


like a kid again!

00 YZ426F

01 TT-R125L (my son's)

91 CR125

83 YZ490

74 Hodaka Super Combat(gone but not forgotten!)

Originally posted by blakemx124:

What does it do exactly for a YZF426 and would it be a good idea or even benefit a 250F.

It shortens the pump stroke. The Factory R&D part replaces the stock accelerator pump cover on the bottom of the float bowl on the FCR.

The accelerator pump works via a spring-loaded plastic arm that, when the throttle is opened, operates a long rod which pushes down on a diaphragm inside the pump cover. The diaphragm displaces fuel which is pushed through the accelerator pump circuit or jet.

The R&D cover has a thicker base so the rod/diaphragm bottoms out on the pump cover sooner, creating a shorter pump stroke vs. stock (less fuel is delivered over the shorter duration). Very simple really, and it isn’t necessary to spend $70 to accomplish the same thing.

But quite a few guys here have had success with this part and really like what it did to their 400/426. You may try the 250 side of Thumpertalk and ask if anybody has tried one in the baby thumper.

Yamaha supposedly shortened the stroke a bit from ’00 to ’01 by changing the diaphragm. When I rejet my buddy’s ’01 WR I’ll measure it so we’ll all know the exact difference (I tried to order one a few months back and they weren’t avail.)

Good answers!

I'm really curious to hear if the '01WR had a change in pump stroke Hick. The diaphram center post on my '00WR measured 8.1mm tall. This would allow as much as 3.4mm stroke IF the cam would stroke that far. It doesn't, the cam pushes it about 3mm. To get a stroke limit of 2mm would take a diaphram post height of 9.5mm (8.1 + 3.4 - 2 = 9.5). The other way is if the cam alone is changed.

The WR426's are still using a D-- needle, whereas the WR/YZ250F's and '01YZ426 have E-- needles. An engineer who only looks on paper might not make the change to a shorter stroke pump using the D-- (lean mid-range) needle.

Have any '01YZ426 owners measured the accel pump stroke? (probably Factory R&D would know)



I’ve already had the carb out of my friend’s ’01 WR when I cut the throttle stop for him. I was showing him how the pump worked and it didn’t seem to spray as much fuel as my ’00 YZ did when stock, but that’s hardly a scientific observation.

Once it is broken in I promised him I’d help alleviate the “Yamahiccup” he found. I’ve ridden it and it isn’t bad but it is still there. Anyway I’ll measure the diaphragm post and compare it w/ the figures you posted. I suspect the cam is unchanged but I didn’t have my carb there to compare it to.

As for jetting on my bud’s WR, does the cam timing really effect the jetting that much? I notice that most guys on the WR side who have YZ (or any “E” taper) needles usually have changed to YZ timing. Should I just stick with the stock needle if my friend is staying WR timed?

We’re in the high dez down here in NM, 4,500 ft. elev., low humidity. It doesn’t run bad at all with the stock jetting but my first impression is that it may actually be a little rich, but, believe it or not, I couldn’t talk him into doing without the airbox lid. How big of a factor do you think that would be? He did uncork the exhaust, but the guy is so anal he didn’t even want to try my stock YZ can, so I may have trouble convincing him to drill holes in his carb :)


Just tell him you know a fix it and get the drill out to see how big his eyes get! Now, was that 7/64?... or 3/32?... :D


Before changing cam timing I ran my WR with about a dozen needle settings using the E-- and F-- taper needles. The EKN and EKP worked great and I would recommend them with WR timing.

I usually recommend guys switch to the EKP needle when going to YZ timing because it is an indication that they want more power and are willing to work a little to get it. In most power hungry riders minds the timing is a must. The irony is that I was mostly aware of the lean needle mid-range after riding a stock timed WR with the air box cover on and a filter that was nowhere near clean! It should have been rich at mid to upper-throttle, but was lean on uphills in this range.

Yamaha started off using richer jetting on the WR's (pre-California models used DTM or DXM) and EKN#3 is a perfect fit for this at your altitude. The airbox cover should be OK with a smaller main jet, maybe stock #162? :)

The stock DRR#(?) in his bike may be running rich just at the crack of the throttle or the pilot screw may be out extra. Did you check it?

You might point out to him that nearly ALL the dirt lands on a small area of the filter

when the lid and inlet tube is used. Not an ideal situation.


[This message has been edited by James Dean (edited 04-11-2001).]

Hick or James, does the shortened stroke on the accelerator pump improve the initial hit off the bottom when you first crack the throttle or is this mod used to improve throttle response or horsepower? I don't want to sound stupid but I was trying to figure out the advantage you guys are recieving by doing this mod in terms of performance besides eiminating "the hiccup" that is experienced on the initial crack of the throttle on a 426. You both seem very knowledgeable and have put some serious R&D time on this P-38 mod. I'm very interested. Thanks again!


Thanks for the tips, I didn’t even check his pilot screw, and I’m not putting too much stock in my initial impression of how it ran, I was only on the bike for a few minutes and I had just hopped off of mine (the two bear zero resemblance to each other). I have an EKQ we can try now, meanwhile I’ll order the EKN.

As for the airbox lid, that is CO-RRECT (sorry, I like that commercial), it did result in the filter being very dirty all in one spot. Unfortunately he was out of town last night when I was over there fixing a flat, changing the oil and cleaning the filter (don’t y’all wish you had friends like me?) so he didn’t get to see that.

I went ahead and left the lid off and then decided if I was doing that I should up the main (168) and remove the MAJ. Heck, my friend’s lucky I didn’t go ahead and YZ time it, drop-kick that 739 for my extra 756, lose the coolant catch, and put my YZ exhaust on there (he’s also lucky I left my drill at home :) ).


The “Yamahiccup” is a common complaint thumper neophytes have with this bike, witness the Factory R&D part and the (as yet not totally confirmed) shorter pump stroke for the YZ/WR in ’01. The obvious conclusion is “the stock pump squirts too much fuel for too long” and creates the hiccup.

So we all sort of arrived at the need for changing up the accelerator pump independently but at the same time. James is some kind of jetting savant and wanted to duplicate the effects of the R&D part on his own, about that time a guy named BK showed up here and explained how Tim Ferry’s bike has an adjustable pump.

The idea is you turn the pump off completely, jet your bike (it is much easier this way, really), and then slowly turn the pump back on, stopping wherever it gives you the best throttle response. I think everybody who has tried this ended up with a MUCH shorter effective pump stroke. I’m not done playing with my settings but James and I are both somewhere around 1mm of stroke (you can measure the movement of the rod with the pump cover or float bowl off).

With the pump stroke shortened most guys’ jetting ends up a little richer, maybe one or two pilot jet sizes and a clip (or three) position on the needle.

Do a search on “accelerator pump” and “pilot air screw” on the WR & YZ side and you can follow along.


Pretty cool of you to wrench on your friend's bike.

You described the effects well on the accelerator pump. The easiest way to get a quick throttle response seems to be with a richer needle clip and secondly from the pilot circuit, but too much fuel from the pump in an extended stream can make for over rich and stumbling conditions. Shortening the pump stroke allows resetting the other circuits for better response and overall power. The P-38 alone doesn't work miracles, its the careful tuning that makes it happen (unless you get lucky :) ).


When I opted for the P-38, it was to attempt to dial in my throttle response and rid my '00 YZ426 of that annoying and dangerous hiccup. I totally agree that it's not a bolt-on and go type of cure. You must go beyond this design and carefully setup the carb. When I decided to go with the P-38, the BK recommended mod was not yet common knowledge. Also, what has become apparent is that the P-38 does not necessarily cure the hiccup for every 426. No doubt that I have been extremely fortunate in that I bought a "good" one. I'm still waiting for a 4th gear explosion...counter balancer shaft key wear....etc.

[This message has been edited by Boit (edited 04-13-2001).]

Thanks guys for your info. Now it's time to experiment!!!

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