What are '01YZ426/250F Jetting Specs?

Would someone mind looking in the manuals of their '01 bikes to let us know whats new?

What are the revised jetting settings for the YZ426 and the new YZ250F?

Pilot jet

Pilot screw setting

Pilot air jet

Needle code

Needle clip

Main jet

Main air jet


James Dean

Is this too difficult? Open the book and read?


Don't worry, we won't pass you up after the latest secrets are out. :)

James Dean,the manual doesn't seem to state what the main air jet and pilot air jet is in the 01 426 manual.

Here's what I can find though...

Fuel screw: 1.25 turns out.

Pilot jet: 42

Needle jet: OBEJP 4th clip.

Main jet: 162

Isn't an EJP just a clip (or half clip) richer than an EKP?

I have the latter in my '00 and its working well (4th clip pos) in the winter here at 4,000 ft. (162, 42, 1.5 turns). I think this needle is a few steps richer on the straight diameter vs. the stocker (EKR).

So does EJP #4 = EKP #5 ??

Thanks for the spec.s Tom and Max...


Thanks for the 426 inputs.

How about the YZ250F?????

(EJP will be 1/2 clip richer, like EKP 4 1/2)

I'll get the YZ250F spec to night and post them herer. If anyone has any info on the

jetting pblms and solutions pls pass them on.

I just picked up the 250F last night and all

seemed ok but I really haven't riden it much.


Just finished breaking in the 250F and after some getting used to (not lugging it like the

400) I was able to ride it faster(maybe just a perception) than my '99 400. It's lighter

pulls really well and is just a blast to ride,it also does not tire me out like the

400 would after 2-3hrs of fast trail riding.

Temp: 50-62 F (northern CA)

Stock exhaust with 12 E-Series disks.

Jetting here is what I have:

Pilot: 42 (stk)


Leak Jet:#105

Main: 182

Fuel Screw: 2 1/2 turns out.

It will bog if you have wick the throttle open too quickly, I think that opening up

fuelscrew to 2.5 turns may have helped.

It will sometimes start hard if you run it hard and let it sit for a few miuntes.

Plug is a little too white for my liking but

the main jet made a BIG difference in how well it pulls on top(I may go to a 185 for fun, the 180 wasn't enough).

I still think that it is lean on the bottom you still need to open the throttle a little

to get it to start.

Thanks for the replies,

The FMF charts were wrong though, they are being lazy and showing old (99) and incorrect needle codes.

The 250F may need a richer pilot and remove the check valve off the tank vent if it has one. This may be the source of hard starting after letting it sit, pressure could build in the tank flooding out the carb.

What needle clip position on the 250F? -I just read that it was ELP#4

The YZ426 EJP is the same as the YZ250 ELP only 1 clip richer - EJP#4 = ELP#5. Last summer I was recommending EMP#4 for all 400 owners which is 1/2 clip leaner than the stock 250F, basically identical.

You may want to speed up the idle for improved starting and better response from idle too.


[This message has been edited by James Dean (edited 12-18-2000).]

i just got my 250F on friday and broke it in this weekend. i have the off idle stumble when the throttle is really opened fast too.

so what are the suggestions to improve this?

more pump shot, less pump shot, more pilot jet, less pilot jet, more clip, less clip???

this is my first 4stroke so i don't have a clue on this bike.


Steve Unruh - Texas Championship Hare Scramble Series - T14

James Dean: factory clip position is 4. I did turn up the idle. Although the manual suggest to run it as low as possible. Regardless, turning it up did help it to not stall as much.

SUnruh: Here's what I've tried to get the cought out. Move the clip one up and one position down. It did nothing. Also adjusted the pilot screw 1/2 turn each way with no change.

I would like to start trying some different jets, but unfortunately I can't find any here in town. ( I did not get any with the bike) If you can believe that. When I do finally get them in. I will start with leaner pilot, richer main. Then go from there. If anyone who has the stumble and has experimented with some jet changes, could you please post what you get?


[This message has been edited by Duece Exmachina (edited 12-18-2000).]


i got the 2 main jets with my bike. have not looked at them to see what size they are though.

what fuel are you using? i'm using Howell 002 (like VP C12).


Steve Unruh - Texas Championship Hare Scramble Series - T14


I am using premium pump/Sunoco race. The bike runs very good with a tank full of strictly high(race) octane.


Duece and Steve,

Don't expect to whack the throttle real hard on a thumper. It can be made very good though.


Try backing out the pilot (fuel) screw much more. You can go up to about 3 turns before going bigger on the pilot jet. A #45 or #48 pilot may be a reasonable choice. A #45 might use as much as 2 1/2 turns out on the pilot screw and a #48 might use 1 to 1 1/2 turns out. Beware, it could foul a plug, but it will sputter some when you get close to too rich.

The accel pump can be increased by backing off the screw very slightly. (1/4-1/2 turn) This is a sensitive area too so take note of where you start at. It will squirt just as the slide clears.

The richer clip usually has an impact too. I'm surprised you did not notice any difference.

A fast idle helps starting usually also.

Don't expect the main jet to make any difference on the low speed performance. It will only have an affect at near full throttle.



your fuel sounds ok, but not race gas.

you should have been able to adjust the stumble out via the pilot screw on the underside of the carb (so i'm told) by going a 1/4 or so OUT.

what altitude/temps are you in?

i'm 700 feet, 50+ degrees F.

Originally posted by SUnruh:


1. your fuel sounds ok, but not race gas.

2. what altitude/temps are you in?

1. ?

2. 1,117 ft. a.s.l./65-75 degrees right now.



1) because any mix of pump and race is not race. you are still getting that crap that goes in a car. there is MORE to race gas than just super octane ratings.

2) not that much different than me.

james dean,

yeah, i've been told by one of 4stroke hotshots (Lightspeed to be exact) to quit riding it like my 2smoke and roll it on and not whack it open. keep it percolating and it will scream.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now