Jetting Qs

244 replies to this topic
  • James_Dean

Posted October 08, 2001 - 07:48 PM


You have covered such a broad range on the pilot jet and air screw, needles, and mains, the exhaust differences are still considerable.
When will you try the stroker pipe?

[This message has been edited by James Dean (edited October 08, 2001).]

  • Taffy

Posted October 08, 2001 - 11:38 PM


yep i've been everywhere.

i felt down sunday night, it was the first time i'd not found considerable gains.

then i sat down and thought about it and went ey up! (that's northern speak!) it's not that bad, you've got it revving out in top, juiced up the midrange (via MJ)and got some more OBVIOUS testing to do.

you'll agree with me that changing the tailpipe is a sizemic (sp?) change and therefore it would be only sensible to continue right down this road until it's all done on the standard euro pipe.

i've got three tests left to do.

they are;
180/160 MAJ tests and MJ drop when i get it right.
38PJ and settings
drop the needle a clip again.

interestingly i've got a 13 mile loop enduro on sunday with generous 'clock-in' times so i might use that to my advantage!

i think stefe has made a mistake by not sticking to the euro pipe, he should have come along for the ride....


  • Jekel

Posted October 09, 2001 - 12:03 PM


What is the BK mod and has anyone used the Factory R&D accelerator pump replacement part?

  • MN_Kevin

Posted October 09, 2001 - 12:23 PM


The P38 from R&D is especially for the 98/99 YZ/WR. It allows adjustability of the accel pump stroke, and therefore the amount of fuel pumped into the carb throat from the accelerator pump.

The BK mod is for 2000/2001 carbs which use a differently designed Accel pump. It gives control over the timing of the stroke of the accel pump relative to the position of the slide valve. It also allows you to control the stroke of the diaphragm (like the P38). Apparently, factory settings (2000/2001) has the AP squirting fuel onto the slide valve before the valve is open, causing the fuel to drain off the slide to the bottom of the venturi in the carb.

'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, Mobil 1 15W-50.

  • Jekel

Posted October 10, 2001 - 04:57 AM


NH Kevin, Thanks for the info. I will probably get the R&D for my '99. No major problem with the bike, just the annoying bobble.

  • Jekel

Posted October 10, 2001 - 05:53 AM


OK. Reading all the info on carbs is making me realize how much I don't know. I have a Question: '99wr400 am going to YZ timing,remove the octopus, install power bomb header, and R&D accelerater pump fix. Have a Supertrapp exhaust(like to be different) which I run with 15 disks - open at times. Any ideas on the jetting I should install with all these combos???? I believe I have a #48 pilot, 172MJ, and the rest stosk. Also airbox lid removed and Twin Air filter.

  • Taffy

Posted October 10, 2001 - 10:06 AM


hit a motoX track today with a mate who hasn't a bike anymore so i gave him the bike a few times.

did however fit the 38PJ and it didn't need the PS adjusting at all. it seemed great from the off and so i didn't try the PAS down from it's present #85.

excellent again. my friend who is a roadracer and had a '98WR this time last year enjoyed the ride and although he's "steady" he did say how responsive the engine was.

tried the EKT with the 2.95mm ET and it was bad i've got one or two idea's in the pipe line to set it working but with a race tomorrow this wasn't the time!

may try the #160 MAJ during the race tomorrow.

so 165MJ
EMM clip 2
85 PAJ
no APJ


[This message has been edited by Taffy (edited October 13, 2001).]

  • Taffy

Posted October 14, 2001 - 08:55 AM


this bike just gets better and better!

i did a pine forest enduro today in sandy soil which was like a soufle(y) i was slaughtered and didn't do the 4th lap but the bike was superb.

it didn't stall all day, pulled hard from everywhere seamlessly and without missing one single beat.

since i raced three weeks ago i have gone from a 230MAJ to a 180MAJ and the top speed has gone up by 5mph (a guess) simply by reving out.

the 38PJ fitted yesterday got me into trouble today as i found the front wheel had lost contact with the ground as i tried to accelerate through a left-right in a few places.

this bodes the obvious question-what will a 35PJ (the last and smallest) do for it?

other tests
needle down one more? (to EMM-1). it was a stroke of good luck JD getting this _ _M needle. i didn't know whether to get the N or M, but as we agreed the other day; if you shove a needle far enough into a whole so as to block it, it's size goes from slightly rich to correct (in this case).

because the needle has been rich enough not to need any help from the pilot system, i've now gone down to a leaner PJ.

DON'T MISTAKEN THE WORD LEANER FOR LEAN. if it runs better, as long as i haven't over or under compensated from another circuit it is the CORRECT jetting.

the MAJ may come down again when i try the #160.

the only thing left that isn't how i want it is from idling along snap the throttle and have the wheel sit infront of my face. this however isn't a feature out and about as i wouldn't do a snap drag start (no cluch fanning) anywhere except the start line.


  • James_Dean

Posted October 14, 2001 - 01:03 PM



Very good to hear that progress is being made. The --M needle diameter is allowing a distant clip position from the testing early on. I always want to snap the throttle and make it more of a consideration in the jetting. I have seen EMN#2 used in a 520EXC but never any EM-#1. Posted Image There is no disputing results when they work good.

I have sent an email for your evaluation, hope it is useful.


  • Taffy

Posted October 14, 2001 - 10:28 PM


when i first joined up i said that the more power a bike produced the more the jetting would come DOWN. this i believe you concurred with.

the exception is the opening of the airbox and a freer flowing exhaust.

is it a coincidence that my bike and a 520 katoom have this in common. i'm not saying that it is as powerful as a KTM just that what with the euro pipe 'suppressing' the jetting they're about even.


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  • kevan

Posted October 15, 2001 - 10:27 AM


Taffy you seem to be da man on jetting!!!!
can you advise....
i,ve got a y2k wr400 standard apart from
white bros "e" silencer..standard jetting
much more lowdown than standard...BUT
where's the topend "hit" everybody talks about?????read lot's on here but none seems
to be in the U. K.
can you advise on a good safe starting point
base setup

  • Taffy

Posted October 15, 2001 - 11:04 AM



JD is excellent at this kind of thing. swopping pipes and jetting for altitude is his area i hope he'll oblige.

there isn't a single standard item in my carb anymore except the slide and even that will be swopped out soon!

where in the UK are you?


  • John_in_Long_Beach

Posted October 15, 2001 - 02:21 PM



I will give it a try, these settings pretty much come courtesy of JD anyway.

I am going off my settings, and slightly tweeking them leaner since you have the original header.

1) Change to YZ timing
2) Use EKP #3 or #4 needle (EKP #4 or #5 if you do the BK mod - reduce accelerator pump squirt from 3 seconds to .3 seconds
3) Use a 168-172 main jet
4) Use a 48pj / 100 maj
5) 3/4 - 1 1/4 turns out on screw.

2000WR - EKN#5, 172MJ, 226MAJ, MAJ Passage drilled to 278, 48PJ, 100PAJ, YZ timing, BK mod, 3/4 turn pilot screw, DSP tapered header and pipe, IMS tank & YZ seat, Scotts dampener & triple clamp & Renthal Fatbar 971, White Bros suspension, Dunlop 756's.

  • James_Dean

Posted October 15, 2001 - 08:07 PM



See also the "Taffy and JD" thread with Brett Green.

With your "e" silencer on the '00WR it should be close to that discussion and John's post here. Get the same needle OBEKN (p/n 5JG-14916-EN) from Yamaha. As John said, start in the middle clip #4, 170 main, 48 pilot jet, 100 pilot air jet (75 is stock), 3/4 - 1 1/4 turn out on the pilot screw. Keep the cover off the top of the airbox. This will awaken the mid-range and top end from stock, whichever cam timing you use.


  • kevan

Posted October 16, 2001 - 09:35 PM


Thankyou guy's!!!!!
taffy.....I,m a shandy drinkin southern'er
by the kent coast....

  • kevan

Posted October 16, 2001 - 09:38 PM


how the hell do you guys manage too tell the
differance between all these small changes
you make to carb!!!!!!!
this science beats the **** outa me....

  • James_Dean

Posted October 16, 2001 - 02:04 PM


The front wheel comes up! And it fires off first kick. Didn't take a rocket scientist. Posted Image

  • partsman

Posted October 16, 2001 - 04:04 PM


Taffy i've got a 37mm FCR off a 2001 yz250f for sale. Comes with cables and housing. Will take $200.00 and shipping. Anyone interested can e-me at

  • slashman

Posted October 16, 2001 - 04:35 PM



i tried the EKQ needle @~5800' and worked good all except the mid range. off the bottom was awsum. top end was good. mid range poping was back. i looks like i will have to find a happy medium. between low end snap(jerk you head off snap) or no poping in the mid range.

would like your thoughts. just for more in put.

BTW got the sus dialed in way better for rocks. it is a weird setting though.but it works

yz timed now
some mods

  • James_Dean

Posted October 16, 2001 - 06:55 PM



You mentioned your jetting has gone down to a point closer to the 520EXC's. The different exhaust and special cam timing may both be entering into the differences from some of the other WR's. It would be interesting to compare your modified bike to another well jetted '99WR with a stock cam. Do you have anyone nearby to compare with? The trend would hint at a better performing bike.



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