Jetting EKP vs DVP

Originally posted by James Dean:

Did the #175 main accelerate better to a point where the #170/#165 would maybe rev out better? :)

You know the feeling when the bike accelerates hard through the gears and then just keeps reving with authority - this feeling is missing with either the 165 and 170. There is no stumbling and it revs, but just not the way the YZ/WR is so famous for. Topspeed may be the same (I lost my speedo at the last enduro), but with the 175 it is there way earlier. To me this is a lean condition.

Last year I was testing main jets with the DVP needle on a long uphill street section, and while the top speed was identical with main jets from 170 to 185, it just took way longer for the 170 to get to the same speed as with the 180 and 185. Acceleration was also much better with the 180+. This is where a dyno run really would help!

This is a 1998 WR, YZ timing, a little head work, YZ base gasket, WP tork valve (yes, it works) and a modified WP e-series s-bend pipe. Altitude is around 250m.



Them wheels, they are 'a turnin now....


Got some literature on Dyno testing with the FCR carb and how to tune WOT--- :)

And the beat goes on


[This message has been edited by James Dean (edited 08-10-2000).]


I was refering to needles at the same clip position at WOT.



Oh,... Can I take my foot out of my mouth now... You're right.


This is getting to be tougher than I thought. Today early as it hit 100 by noon I tried two jetting configuraitons.

Config 1)


Fuel Screw 3/4

100 start air jet

EMP #4 with a slight mod to richen the transition from the straigth section to the taper.

165 MJ

On my WZ 420 it would stumble between 1/8 and 1/4 throttle position and the response was too rich in the mid to top end. I tried the fuel screw at 1.5 no help.

Second run


fuel screw at 3/4

100 start air jet

EMP#2 same taper as above


It worked better but still had the exibited the stumble not as pronounced around 3/16 throttle.

I have now carefully further motified the EMP taper to richen up the transition from the straight section onto the taper. It is now vary close the the DVP#4 in this area of the needle taper.

will try again tommarow.

Once I get the bottom 3/4 of throttle working, if it can be done, then I will work on WOT

And the beat goes on--I think I'm wearing out the screws on my carb top!!


Originally posted by Clark Mason:

This is getting to be tougher than I


And the beat goes on--I think I'm wearing out the screws on my carb top!!


Same here. Found that I like the EMN needle on the 2nd clip position very much. Clip pos 3 already is a little rich, although you can hardly feel it - but with pos 2 my bike accelerates harder. Also I found that with the EMN needle the setting of the fuel screw get much more responsive than with the **P needles. Unfortunately I wasted the EMN needle when trying to modify the starting taper like on the DVP, so I modified my EMP instead but haven't tried it yet.

Actuall jetting (before killing the EMN):

main 165

neddle EMN/2

pilot 48

pilot screw 1 1/4

pilot air 100

It is very hard to tell, if modifying the needle would have been necessary at all (no stumble), but if you don't try you won't find out ;-)

BTW, I also modified my accelerator pump to inject 20% less fuel.


The temperatures are never that high here so I can't say exactly what would work on my bike. Can only estimate that the change in settings would be something like this:

60-70deg F/500'elev_____100deg F/3000'elev



FS= 3/4 turns_______________FS= 2 turns

pilot air=#100______________#100

EMP#4 (EKP#3)_______________EMP#3 (EKP#2)

#168-#172 main______________#162 main

An EMN#3 or EKQ#2 also would be possibilities with the #45 pilot. The newer FCR2 carb appears close to what you're running here. After reviewing my testing notes I ran EMP or EMN in clips #1,#3,#4,#5, and #6 before selecting EMP#4. Also ran EVQ and several D-- and F-- needles in varied positions.

If you want a custom grind (DEF)MP needle let me know. Pick your best D-- clip setting and we can put the E and F tapers on in any relative position. This should keep low end changes to a minimum. Got some extra ones this week and have not wasted any needles in a long time, just need to work it slowly.

Hope you're keeping good notes. :)


[This message has been edited by James Dean (edited 08-12-2000).]

Ran my modified (modification #2) EMP needle. Used the following jetting

50 PJ

EMP (mod) #2

100 Start air jet

Fuel Screw = 3/4 turn

165 main jet

Only change from yesterday was my custom grind on the EMP needle to further richen up the 1/8 to 1/4 throttle response. This combo worked excellent my stumble is almost gone like the DVP #4. This set up is a little harder hitting from 1/2 to 3/4 throttle. I like the action and will not attemp any more needle mods now.

For tomarrow I will try the same EMP but in the #3 a 48Pj with the fuel screw at 2.0 and a 162 main jet, and then perhaps a little main jet tuning if time permits.

I would like to get everything running with a 48 PJ so I don't have to change it when I go altitlude in the mountains if they have not gone up in smoke.

James once I complete this needle I would like to sent it to you so you can evaluate it and then measure it and perhaps recommend the closest available off the shelf needle and clip position so others can gain from these efforts. Still may have to custom make them.



That would be great of you! I can graph and compare our modified needles for comparison. Looking for the best choices off the shelf for S/A and open exhaust setups.

-Are there any YZ426 owners that would like to comment on how putting a spark arrestor affected their jetting? What needle/clip/pilot/screw setting they were and are at? This will benefit you guys too.


Just returned the e-mail with address.

Todays jetting settings worked excellent and except for a little fiddling with the main jet I'm done. This only took three weekends, a total of eight days and about 400 miles of riding!

PJ 48

Fuel Screw 2.0

EMP modifide profile #3

100 start air jet

162 main jet

This combo worked excellent on my test course it hit 93 MPH top and work excellent over the test run. Altitude varies from 1200 to 2800 feet and the temperatuure was between 72 and 86 deg F.

James Dean I e-mailed you to get your shipping address as I want to send you this needle overnight tomarrow.

The Ty Davis gas tank was outstanding as expected, but did have a interference problem with the DSP CF air box on the left side. I will have to get a DSP air box only and use the stock rubber boot. With the stock boot no problem.


Clark and James,

What spark plug are you guys doing your carb testing with, the OEM plug or a plug that works with a wider heat range, i.e. Nippondenso Hot-U or a Splitfire?

I have found, without a doubt, My Hot-U did clean up some minor jetting problems with my '77 Yamaha DT250, '75 Kawasaki KS125, and my '91 CR250.

Maybe it would be worth a shot? I'll reimburse both of you the cost of the plug(s) if you want to stick another variable into your carb work.


99 WR, all YZ mods, de-octopused, OEM YZ tank and IMS seat, jetting by Clark/James, got forked by Pro Action, DSP airbox, PB Header, Stroker SX-1 SA, Thumper Rad Guards, Acerbis Hand Guards. Moab, HERE I COME!!

Clark and James,

Any comments on the EMN needle in comparisson to the EMP?



During my needle testing with WR cam timing the EMN was run in #3,#4, and #5. 60degF/500'

- #5 was rich

- #4 was very good running, pulled hard and my preferred choice with the EMN needle and #48 pilot/pilot screw 1 turn.

- #3 was cleaner running and would have been a good choice for warmer temps or altitude. Noted that it "feels like less flywheel".


No hesitation was noted with any clip setting.


EMN will run much like the EMP with an initial "V" taper. This will probably be closest off the shelf to Clark's modified needle.


(Clark, did you send the modified needle? didn't get it today.)

James Dean

Servus James,

During my needle testing with WR cam timing the EMN was run in #3,#4, and #5. 60degF/500'

Very interesting! I liked the EMN best at clip #2 at 30°C/300m (86°F/1000') - this is maybe due to the silent-insert in my WB e-series muffler, which restricts the exhaust area by about 40%.

- #3 was cleaner running and would have been a good choice for warmer temps or altitude. Noted that it "feels like less flywheel".

Yeah, this is also a very good way to tell if the jetting is in the ballpark. If you feel like you have to much engine braking, the jetting is off.

Still haven't had time to test my modified EMP needle - any measurements on Clarks mod needle?



Received the modified needle yesterday from Clark and did a close comparison to other off the shelf needles.

The EMN#3 or EKN#2 are the closest available from Keihin and Yamaha. The straight portion and start of the taper were reduced in size mostly where the original taper started. The needle started out as an EMP, was polished down and retested to find his best performance.

Hopefully Clark can add to this when he gets the graphed profiles and needle back next week.

It is apparent that nothing but EMN or EMP come close in this range. (same as EKN or EKP only 1 clip richer) These differ only in the straight diameter, with EMP being a little leaner below 1/4 throttle. This has been my recommendation with the '00WR FCR2 carb. The 426 uses a EKR which is 2 steps leaner yet on the straight diameter, but also is set with a richer clip position. A pattern starts to emerge when looking at the settings here... EMN#3 to EMP#4 to EMR#5(EKR#4) , leaner straight and richer clip. This helps avoid the lean hesitation at 1/4.

Still wondering if any 426 owners want to try an EMN or EMP. :D

James Dean :)

[This message has been edited by James Dean (edited 08-18-2000).]


Did you measure further down the needle as I also modified the needle as you approach WOT for a little more fuel flow on top so I could really jet the main jet?



No, the measurements further down did not look any smaller within a range that I thought would be effective. How much did you think you took off?? I measured down to about 2.40mm diameter...


I'll be on vacation next week and will not be able to follow up on jetting questions here till Sun 8/27. Hope your riding goes good!!!

See ya,

James Dean :)

[This message has been edited by James Dean (edited 08-18-2000).]


The needle was measured down to a range of 2.3-2.4mm at which point it appeared that you hadn't modified it below. You must have been very good about making it smooth and polished. I'm pretty sure you didn't take enough off the needle towards the tip to make any difference near full throttle. When you get the graph and needle back it will become more clear. See where the main jet kicks in. It takes much more off the needle near full throttle to make a difference in jetting with the "E" taper. Sometimes a picture tells much more.

It's raining in Seattle, I'm outa here!! (back on 27th) :)


[This message has been edited by James Dean (edited 08-19-2000).]

James Dean

I got my modified needle back and your graphs made from detailed measurments of my needle. Reading your graphs it looks to me like takng a EMN needle and blending the transition from straight to tapered probably about 2.5mm above to 2mm below should be vary close to what my modified EMP needle came out to be. However, I feel to duplicate the exact taper of my modified EMP needle would be extreamily difficult.

Tomarrow I will order a couple of EMN needles and do a little more experimenting and see if I can come up with a easy mod to the EMN that is an excellent performer.

I must be doing to much of the mod stuff because tonight when I tried to tighten up the front clamp on the carburetor boot the threads inside the stock clamp captive nut have warn completely smoothe making the clamp useless, so installed a aftermarket clamp.

And the beat goes on but I'm vary close


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