FCR carb tuning

Just for reference a #44 drill will provide the equivalent of a #220 main air jet (2.2mm).

After enlarging the passage behind the MAJ there may not be a need to jump much bigger for a first attempt. The stock passage looks to have been a significant restriction by itself.


Ok you guys got me to jump into the frey and modify my carb MAJ etc. I realise my bike is heavily modified so my results may not apply in general. Here is what I did and my results to date. Drilled out the MAJ shaft to the center of the carb to 2.75mm drilled out my MAJ to 3/32, 2.35mm.

Started the machine immediate high idle had to turn out the idel a full turn to get it to calm down. Ran three exhaust configurations and several main jets. A 178 was too big would stumble and lug at 3/4 throttle medium to high rpms. 172 and 170 ok.

I ended up with a 48Pj, Fuel screw at 7/8 turn out, 172 or 170 main jet, EKP needle #4, and the FMF powerbomb header with the Stroker SX-1 tail section--its quieter and I like a little quieter.

I revised my gearing to 14/49 and topped out at 91mph could hit the Vortex rev limiter at 63mph in third but not in 4th or 5th. As before the bike has fantastic low end and mid range. I may just turn the bike over to Curtis Sparks and let him work his magic but then I would probably blow out the bottom end.

The beat goes on


[This message has been edited by Clark Mason (edited 03-10-2001).]

[This message has been edited by Clark Mason (edited 03-10-2001).]


post what it was you ran before you changed please.

i'm off to get the engine right now.

i'm right behind you, you buggers!!!

i've had a 2.1, 2.1, 2.3, 2.4 (3/32"), 2.5mm & 2.8 (7/64") drill bits sat here three weeks & you've nicked all my theories! i shall return!!!

i seek revenge!


[This message has been edited by Taffy (edited 03-11-2001).]

I tried the 2.5mm drilled MAJ and 3mm drilled air passage, first with the #170 MJ, EKN#4 there was a slight drop in power from about 2/3 throttle to WOT and pickup was not as good as a 3/32(2.38mm)MAJ/"passage",It took a little longer to hit 11,800rpm rev limit.I then tried a #178 MJ to see if it helped top end but found it was even worse, I could not hit rev limiter above 2nd gear and it felt terible when aproaching it in 1st & 2nd, so unless a #172 or #175 MJ work with a #250 MAJ we are looking at a #220(2.2mm)- #238(2.38mm) as the best options. With the 2.38mm passage & #170 MJ the bike felt like it could have reved another 1,500- 2,000 rpm, next I'd like to see how a 2.2-2.3mm MAJ goes compared to the 2.38mm.


What cdi/timing curve are U using, if you haven't already done so try a vortex or YZ cdi, As you said "now it feels like a supercharged h/berg 501" mine does too in traction mode(wr curve) but switch to race mode(YZ curve) and it feels like someone also threw in a bunch of steroids and a 5 stage nitrous kit :):D




YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust& header......

Somebody stop me !!!

To the Tuning and tinkering Croud

Before I drilled out the MAJ passage to 7/64 and the MAJ to 3/32 my jetting was as follows

PJ = 48

fuel scrw = 1 1/8 turns out

Pilot air Jet = 100

MAJ = 200

MJ = 168

Needle EKP #4 clip from top

I did the drilling and started with the following

PJ = 48

fuel Scrw = 1 1/8

Pilot air jet = 100

MAJ drilled to 3/32

MAJ passage drilled to 7/64

MJ = 172

Needle EKP #4 clip from top

Conditions 60deg F altitude 800 to 1200 feet.

On start up it ideled real fast probably over 3000 rpm had to turn out the idle adjusting screw at least one full turn to get it to calm down to a normal idle level.

I concentrated on MJ tuing range investigated 178 to 165. 178 too rich stumbles and will not pull smoothly from mid RPM in top gear it was obviously too rich. 165 felt whimpy probably lean a 170 or 172 seemed best. I ended up turning the Fuel screw in to 7/8 turn out seemed a little more responsive but did not experiment enough with the FS to be conclusive.

I now feel confident in the main jet selection at 170 to 172 and probably should investigate running the EKP at #5 clip and using a EKN #4, I have the EKN on order.

I'll be running the big XR650 all day today so the Yamaha experiments will be on hold until at least next weekend.

I may have taken too much out of the MAJ at 3/32 but don't know how to get between stock 2mm and a 3/32 drill at 2.35+mm.

Cheers all


that's all we need isn't it!

i thought the MAJ was only going to affect the 3/4, WOT & red line area.

it's obviously helping at tickover as well. this needn't be a bad thing & it should be encompassed.

the trouble is it's going to affect two or three circuits & we've got to guess which way to go.

in the meantime, "back at the ranch" i checked all my rear linkage bearings & they're all shagged. major investment time.

so i'll be off the road another fortnight i should think. i've spayed the subframe & will niw do the frame peacemeal.

i intend putting grease nipples on various parts of the rear end.

aussie andy, go across to the YZ side & there is only one jetting question up at present. on it hick gives the yamaha part No.



I have found the 3/32(2.38mm)MAJ to work extremely well,although i've tried a 2.5mm MAJ and was dissapointed,James has used a #44drill bit to get a 2.2mm, I think taffy has found some drill bits that go up in .1mm sizes.So were looking at a 2.2mm to 2.4mm MAJ #170 to #172 MJ and needle(posibly EKN) in clip#4 or #5 for ultimate riding pleasure. I'ts pissing down rain here ,they got 6inches in 1/2 an hour up north the other day I'm hoping we dont get the same here, so till it stops or at least slows down enough to be able to ride I'm stuck inside un able to do any testing.perfect oportunity for a grease & oil change though :)




YZ timing,EKN needle, VORTEX dualcurve ignition

SF triple platinum s/plug

open box -unifilter

98oct fuel

Staintune s/s open exhaust& header......

Somebody stop me !!!

Let me think about this....


#172 main,

2.20mm MAJ (passage drilled to 2.8mm),

EKN#5 (slightly modified),

#48 pilot,

~1 1/6 turns on pilot screw,

~1 1/4 turns on pilot air screw,

accel pump at 1.0mm stroke,

1 1/2 turns delay on pump start

WR ignition

YZ timing and exhaust

40F. 500ft elev.


This ran great with a new plug :) and looking at everyone's testing, not sure what direction to go looking for more.

Clark, thanks for joining in on this. Great to get your involvement in testing. The detailed information helps.

Andrew, if you have an extra VORTEX ignition laying around I would be happy to try it. For now it will have to be WR ignition.

The accel pump can be turned on or off with a simple turn of the stop screw. With clip #5 the pump is optional (Taffy- :D ). When riding on the roads I would rather leave it off. Out in the dirt approching jump takeoffs and roosting out of corners I want the little extra snap with the pump on.

Any less pump stroke than 1.0mm is not enough to notice.

A careful evaluation of my setup will show it to be slightly richer than a stock '01YZ426 in every area except the APJ. This is a result of testing both richer and leaner on each circuit. It was not designed to copy Yamaha's jetting, it just worked out that way.

What now? The adjustable PAJ settings are being evaluated at varied turns and re-adjusting the pilot screw to match. A #48 pilot jet uses in the range of 3/4 - 1 1/2 turns on the pilot screw with adjustable PAJ settings 5/8 - 2 1/4. This adjustable PAJ is nice Taffy. Thanks for pushing us to get it.


[This message has been edited by James Dean (edited 03-11-2001).]

TO all,

I may have a solution to MAJ sizing but could use some help if any one can,This afternoon I looked at using a shortened re-drilled MJ as I have no spare MAJ's "yet". Before shortening or drilling i checked for thread size and seating of jet & the hole in the jet, the thread and seating was great but the drilled/jet sizing part was only in the top 5mm of the hex end of the jet which would have to be cut away it then opened out to aprox 2.45mm (TOO LARGE for what I need).SO I had a look at carb parts warehouse jet list to see if there were any jets which would have the right thread,length & required drill size.As they don't list the thread sizes I became STUK but there are a couple I did notice which could be likely candidates they were Keihin MJ series P/N: N424-27-*** up to #250 or Mikuni N.102.221 *** up to #220 in increments of 5 then a #230 & #240. If any one might have these jets or similar or access/info could you please check them for fit and report findings, I'll do what I can to find out tomorrow but any help would be very much appreciated! :)



i think that you've got to work from the top down. forget the snap tests for a while.

when the MAJ is larger it is leaning off the top end. the torque peak is at 8-9,000 revs. so you've improved the bike from there to near the redline. now do some more tests with the MJ going up & down with the No230 MAJ in. why?

you know that 220 was an improvement with the original MJ. now you put a 230 in & try the MJ's again. because you MAY have 'over corrected' by under main jetting the bike the chances of any improvement come in the potential form of No230 MAJ with 175MJ.

if this is no improvement then the dramatic changes that "rhinodyno" talks of aren't totally true.

after all that, the needle is the middle connection.

you know i said that the 'E' needle was too rich & the 'D' was too lean? another avenue that would be far more valid to the 'average joe' on this site would be to fit the 'D' series needle & possibly end up richening the top end via No 150-190 MAJ.

my latest setting that worked beautifully was;

MJ 170

dvp clip No7

no APJ


PAS out 2 1/4

PS out 1 1/2

std pipe

i'm a racer & i only want to use what works best. My hero of all time is KR & he used to detune his 130bhp TZ500 for some tracks & today they haven't got enough power with 200bhp. i know who the realist was!


Went to a couple of bike shops today and rumaged through some boxes of MJ's & MAJ's, the mikuni jet thread is different could be forced in but this would damage carb thread.what we did find though was a #250 MJ this is too lean but a good sign that there is a #230 out there somewhere,what bike and carb this came from is unknown (could even have came from a motorveicle and ended up in the box) but I'll keep trying,I did get some spares to drill to different sizes to try in the meantime . any help/ideas apreciated thanks.

[This message has been edited by Andrew in OZ (edited 03-12-2001).]

Hey gang,

I finally had a chance to get out and try the EKN this weekend.

Settings, 2000WR, EKN#4, 48PJ, 100PAJ, 1 screw out, 168MJ, 200MAJ, DSP tapered header and pipe, lid off.

Yes it was my observation that it was a little bit rich. So a larger MAJ should fix this.

Here was my unexpected pleasure. The bike now seems geared better. I used to somewhat regret not having bought a YZ, because of tighter spread on the grearing. The WR has always seemed like there was never a good gear choice. Many times 2nd was too low and 3rd often was too high. Frustrating.

This time out with the EKN I didn't feel at a loss for the right gear. I think the usable RPM range is much better. Maybe this is because I ran my DVP at #4 and this was too lean? Or maybe the EKN makes the RPM range more usable.

Either way this small difference in jetting has made the bike MUCH easier to drive. Anybody else have this weird observation?

I think I will make the MAJ modification as soon as possible.

Thanks for your work Andrew. I look forward to seeing what you think is the best MAJ / MJ combination.


ask your local engineering (toolmaker) shop where you can buy drill bits in .1mm increments. they cost me just 60 pence each! or 90 USA cents.

any other MAJ that's the same shape would do. spend a couple of aussie dollars on drill bits.

it sounds like you're all already settling on No 220-230.

the MJ's seem to have hardly moved so therefore you've leaned off the WOT (& NOT the redline!).

what's the needles relationship from 3/4 to WOT like i ask?

try getting the right MAJ/MJ first (& you're very close)THEN run some needle height checks. you should only be interested in the bottom of the needles area as it blends into WOT.

clark & andy

you both found your tickover shot up! could it be that you were running very rich at tickover? & that the introduction of more air via the MAJ was badly needed?

JD any idea's to throw in? your's didn't do that did it? if you've looked at your slides they sit 4/6 mm off the bottom of the carb. i think it's one turn of the idle screw = 1mm. this must be due to this modern propensity to have a square slide in a round hole.

good luck.


[This message has been edited by Taffy (edited 03-13-2001).]


That change to EKN#4 was anything BUT a small change in jetting. Thanks for posting your impression. Your DVP maybe should have been in #5. The EKN makes a big difference in the midrange and its no surprise the '01YZ426, YZ250, and WR250 are all using slight variations of it. Try turning in the pilot screw to 3/4, that richness may be down lower.


Changing to the #220 MAJ had very little if any effect on the idle. I've run lengthy tests in the past on #162-#175 mains and keep going to 170/172. The runs by Clark and Andrew point to this also. Temperatures, climates and state of tune aside, this seems to still be the case. I don't have a good measure to compare a #210 vs #230 vs #250, can't hold it open that long to quanify. This REALLY needs a dyno run to be validated.

Last summer I was raging up a 5000ft peak with a '99WR400 and a '00400SX in hot pursuit with a #172 main/EKP#3 in my WR and running 3/4 to full throttle most of the way. The power was nearly too much to hang on. It never missed a beat, never feeling rich. More peak power is not much of an issue in reality. Unfortunately, those 2 guys are BOTH now riding 520EXC's!! :)


[This message has been edited by James Dean (edited 03-13-2001).]

well no luck with the MAJ's the bloke at the shop we found the #250 jet could'nt track down where it came from nor find a compatible jet to use, anyway it was worth a try!I did pick up 2.2mm & 2.3mm drillbits from the metal center though,Thanks taffy.although I 'm not sure if I'll have time to test them till the weekend, I was thinking the best way to test results would be to do 5 or 6 runs with each MAJ by bringing the bike up to a steady 100kph, roll the throttle open and time with a stopwatch 100kph to red line and then again with richer / leaner MJ's,which ever is pulling fastest times I imagine would be the best jetting, IDEAS?.Anyone have any news/new findings James, Clark,anyone.? "Oh yea taffy"

I found and ordered the PAJ screw's (X-2),down in Adelaide au $36.00ea +delivery what are they made from ? GOLD? :) I'll be reporting results a.s.a.p, stay tuned! :D ha ha!



why did you order two PA(J) Screw's?

IMO you won't have a perfect running bike on all your test runs. it'll run like **** on one jet then get better then run like **** again on the third jet. you can put an hour glass on the difference. in the meantime who is going to click the watch at the start line & 1/4 mile away?

can you gaurantee starting your roll-on at the same revs everytime.

go by the seat of your pants.

did you order another MAJ that you can drill out letting them overlap each other?

try & fidgit in that area where the needle gives way to the MJ/MAJ combination.


With the AP disconnected, wouldn't an exhaust gas analyzer be the way to go??

I heard this bike couldn't use a EGA because of the AP...


You're THE MAN! Show us the way.

No other news to add. Will you be running with a #172 or #170 and at what temp & alt. ?

Just ask your girlfriend to sit on the back of the seat and run the stopwatch. :)


Taffy ones for my mates bike!

james #170 to start off with, do runs with #220, #230 & #240 MAJ's then try a #172 & #168 or #175 depending on what the #172 feels like. temp ? 20- 30 C (68-86 F) alt 600-800 Ft. I dont think the missus would agree, any thing over 60 kph and shes SCREAMI'N I'll strap the stop watch to my left grip so it can be stoped/started with thumb.


The results of testing are as follows;

"00" WR 400

YZ timing

open exhaust

EKN #4

#48 PJ

PAJ(Screw)1 1/4 turns out

PS 1 1/4 turns out

AP 1.25mm stroke

VORTEX ignition on Traction/WR ignition curve

11,800 rpm rev limit

temp 25 C (77 F),altitude 700 ft

tests were run in 4th gear, 14/50 (std) gearing,on a slight up hill run from 100 kph to red line (144kph approx)5 runs on each setting.

jets used .........approx time in sec

#170MJ/#220MAJ .......6-7 sec **

" " /#230 " .......6.5-8 sec

" " /#240 " .......7.5-9 sec

#172MJ/#220MAJ .........6-7 sec **

" " /#230 " ......5.5-6.5 sec * :)

" " /#240 " ...... 6-7.5 sec

I dont have a #175 MJ to compare but previous tests with a #178 proved to be too rich even with a #250 MAJ so I would imagine if the #175MJ is used it would need to be run with a #240 - #250 MAJ with times around the 7-8 sec range.when run on race (YZ ignition curve) all times were reduced by at least 2 seconds at every setting, with #172MJ & #230MAJ 100kph to rev limit was fastest at 3.24 sec's.

Hope this helps, please let me know if any one finds a better combination, and of your own opinions of these settings. Remember these are approximate times worked out from the average times of 5 runs each.



[This message has been edited by Andrew in OZ (edited 03-18-2001).]

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