James Dean ????

James, just wondering if I go to the 37mm FCR off a 250F will the same jetting apply as to the 39mm? Or would I be too rich with the smaller carb.Oh yeah, it's going on the "yellar" 400. Thanks

This will be a first. Jetting will need to be by trial and error. That said, I would start with the similar jetting to a WR250F and a bigger pilot jet/ smaller main.

#45 pilot

1 3/4 turns

OBELP needle, #4 clip

#160 main

(Stock WR250F needle)

This carb should be a great choice for it.

great, you beat me to it!

i would have thought that your jetting would have been the same, smaller main as well.

you really must try and do back to back testing with a mate.

it may be best to fit the 37mm carb and test and test before asking a mate if you can do some back to backs. then put the original carb on knowing that it fits right.



2nd gear tickover to go

2nd gear roll-on

top gear roll-on

you must try and ride against a buddy with both carbs in one session. get his opinion as well.

i'm hopeful of this. you'll either stomp on in the first 2 seconds or it won't be an improvemen IMHO.


If you do any back to back testing be sure to use identical pilot jets and needles.

no JD

you jet the bike according to it' needs.

sorry mate.



I aggree with your statement but was hinting at getting what it' (a stock DRZ) needs. The presumption is made that the stock bike with the 39mm carb has perfect jetting. Stock DR jetting will not come close to the stock WR250 jetting. I made the statement to at least get it in the ballpark. If you don't, it's not a valid comparison. DXP and ELP needles are night and day apart whereas the YZ426's EJP and the YZ250F's ELP are nothing more than 1 clip position.

Honestly, you must spend equal time jetting with both carbs or don't do the comparison at all.


It is comforting in a "carburation way" knowing you are keeping close tabs on this websight. Your insight is invaluable.

FYI: I have put the FHP needle back in my WR. I am starting w/ a 168MJ. I have the 100 YZ air jet installed as well.

A mod I am looking at is to cut and thread my AP actuating rod, then tap out a coupling to re-attach the two together. One end will either be loctited/jam nutted on, the other end will just use the jam nut. By lengthening the rod, I can set the AP timing.

Either tapping out the AP cover, or going w/ a P38, I can then set the stroke of the AP diaphragm. This may get a little tricky, since altering the length of the rod to alter the timing will have a direct effect on the AP diaphragm stroke. I do not want to unnecessarily put excessive tension on my (P38), and will shorten, as necessary, the AP rod.

I am open to all comments (or, "Kevin you are AFU"... :) )


'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, DSP Doug Henry airbox w/ velocity stack, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

ooohhhhhhh! i g-e-t i-i-t!

he's on a bloody suzuki (the penny has just dropped).

Taffy (he of the big blue one!)


I didn't have a '99 but according to Clark you just need to bend a forked arm to adjust the timing start. The stop then is controlled by the separate P-38 stop or spacer/bolt on the pump bottom. The linkage is spring loaded so excessive rod tension should not be a problem.

I like the idea of using the FHP and 162-168 main.

Hey James,

I just tried your recommended WR 400 jetting and my bike has a sumble at partial throttle (about 1/4 to 1/2). Here is what I tried:

165 Main

48 pilot

EKN needle in 4th position

1 turn then 1.5 turns out on the air screw.

My bike is a '00 model; stock exhaust with Vor tip; YZ timing, grey wire mod.

My jetting before was 170 main, 45 pilot, 1.75 turns out on air screw.

I did the WZ timing and grey wire mod, and experienced the similar hesitation (about 1/4 throttle).

I ride at 1,000 to 3000 feet elevation.

Think I should try going back to the 45 pilot or dropping the needle?

James, I'm gonna do the BK mod to this carb before trying it on the 'zuki. I would think if the "stock" jetting is o.k. on the 250F that it should be within reason for the 400. The only thing I could think of is that a smaller carb throat would not flow as much air and would make the jetting richer than what I'm running now. I know the smaller carb will give more bottom end which is why I'm trying it. Don't really need all the top end that I've got now.I've followed all the post about jetting on this side because they do apply to the 'zuk also. The motors are really more alike on the DRZ and YZ/WR than most people give credit to.With all that said I believe I will try the stock jetting of the 250F carb first and then try what has been working good on the 39mm.By the way I've got a selection of jets and some needles to try. Will keep everyone posted on results but it will take a little time to do all this with me buying another house and having to move. Oh yeah, if I'm not satisfied or get weary of all the wrenching, since it's a bear to work on the carb on the E, the carb will be for sale. Thanks for the present and future input.


I can't even begin to tell you how many different inputs went into the suggested jetting above. It is not stock WR nor stock DRZ. The suggestion was made specifically for your bike and the 37mm carb. I hope you will try it in whole.

Have you run your bike with stock 39mm FCR carb using an E-- tapered needle? This is necessary for a good comparison.


you're wrong about the flowing of air.

the engine cries for air and the carb lets it in. simple.

that means that the air will be going through your 37mm carb even faster than a 39mm carb. this means the vacuum will be greater and the fuel will be drawn up even faster!

because we all jet our bikes so that a certain amount of extra fuel gets drawn in to compensate for the slow reactions; your jetting should consiquentially get LEANER especially at high revs but possibly even at lower one's as well!

JD knows that i've rattled on about this for some time. i believe you'll get a gain in first and second gear and i don't think you'll spot the 0.5hp loss at 15,500 revs (if you know what i'm saying!).



Did you buy this YZF250 carb...how much was it?

PSI has introduced their "Big Air" carb that somehow flows high velocity at low and high engine vaccums, much like a variable inlet venturi carb. I am curious how it works.



what year is your bike, and how much for your old carb?


'99 WZ/YR (you choose!) with ALL YZ mods, de-octopused, FMF PowerBomb header, Stroker SX-1 silencer, SS front brake line, OEM YZ tank, IMS YZ seat.

[This message has been edited by NH Kevin (edited September 15, 2001).]

James, why do you suggest a 160 main for my 400 and a 172 for a 250f. I can't figure why I should be leaner on my 400 when I try the 250f carb. Please explain. Oh yeah I'm running a 165 now.

On your bike, you answered the question yourself! :)

I was tempted to suggest a #155 because of the combined effects of needle taper AND smaller carb bore. :D

The 250F comes with a 170-175 main and stock 37mm carb. Those little motors don't suck air very well, let alone fuel.

Whereas a KTM 520 will be rich on top with a #165 main and an E taper needle while using the bigger 39mm FCR carb.

Like I said, the suggested jetting was for YOUR bike. :D

[This message has been edited by James Dean (edited September 17, 2001).]


there's nowt as blind as those that will not see...

ever had the feeling you're talking to yourself.


OK heres the deal. The 37mm will bearly fit on my Z.Everything is tighter than with the stock 39mm. If you have ever worked on one or have a buddy with a Z you know it ain't no fun.So I'm going to put the 39 back on and go from there. I appreciate everyones input but I am going to sell the 37mm off the 250F. Got carb,cables,throttle housing with a few scratches and throttle tube but it will need to be replaced. I have done the BK mod but it will need a spring on the screw. I would like to get $350.00 plus shipping for all of it. I will not break this stuff up. It's all or none. Also am interested in a P-38 to go on my 39mm. Thanks everybody!!

My pleasure Partsman! :)

Yes, David!

Taffy and NH Kevin, and JD, you KNOW you were talking to many of us, and we were listening :)


Rick Fuller

'01 yz426

'00 yz426 destroyed by fire in desert race 4/22/01

'81 490 Maico

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