Patchy mid range power due to jetting?

Hello chaps and lasses....

I have an 01 WR426, and have noticed when riding on the open road trying to keep a set speed, that the engine seems to have occasional patches of 'difference' in power delivery..... almost like a small flat spot, but without a change in speed or throttle position..... I have since replaced the air filter for an aftermarket type and oiled it, and left off the airbox lid. although there is a noticable increase in power, the phenomena still occurs (as well as the extra induction noise).

I wondered whether this maybe a 'normal feature' with this engine, as I understand that it is technically a race engine and is not meant for cruising on the motorway, (if the sun is shining its a better way to get to work!).

Also, on the subject of jetting, does anyone have any tips about the jetting for this year WR... the bike is standard UK spec, with just the airbox lid removed. (throttle stop?? just a socet cap screw!, slide opens fully from new).

I've read a lot about jetting on this site.....amazing info... but not been around from the start.... and I've not managed to pick up the info I'm asking about.

Thanks in advance guys.... I know you will deliver....

P.S. UK conditions, sea level, shite weather.



What is the "standard" UK jetting shown in your manual? Have you changed it?

Is this correct????

pilot - #42

pilot screw - 1 1/2 turns out

pilot air jet - #75

needle - DRR #3 clip

main jet - #162

main air jet - #200

Next, What throttle position would you estimate the hesitation is at? 1/8-1/4-3/8-1/2??


Hi James,

Thanks for your assistance.

I left my manual at work , so can't check, but will tomorrow. I haven't changed anything at all apart from previously mentioned. the speed of the bike when it happens is about 50 - 60 mph, approximately mid range revs..


sorry James.... estimate at 1/2 - 3/4 throttle position

On my YZ timed WR 400 I had the identical situation and it was due to the bike being a little lean and 1/16 to 1/4 throttle while crusing. If you trail ride you will never notice it its just while crusing on the street I was able to jet it pretty much out through a combination of fuel screw adjustments and a needle change to richen this part of the thtrottle opening.

Follow James Dean on his suggestions and you will be fine.



I have the exact same problem w/my 01 426 at about 50mph or half throttle. I never worried about it. Derek explained it well, it's not flat but patchy. If I give it a little more gas it has great response and the patchyness goes away. The throttle response is excellent with no other problems. I don't want to cause another problem trying to fix this very minor one.

I have also noticed this patchiness. Its kind of annoying. I have had to run a few longer stretches of road at a consistant throttle. Doesnt matter what gear you are in, its all about throttle position. Between half and 3/4 throttle. It stops with a throttle twist for further acceleration, but returns when I back off a smidge. Im almost thinking I need to move my needle up 1 position to improve richness at that portion of the Rpm. It seems more like its "leaning" at that speed. Thing is, I dont have my baffle in either. Did you guys have your bikes with the stock baffle? I know that it runs a bit "richer" because of the reduced exhaust output, therefore combustion chamber is always charged with a bit of "spent, but incomplete burned fuel" particularly around 1/2 throttle and above. Comments?..

This is a range where the needle taper and main jet have overlapping effect. This would also be lean by the general jetting that Yamaha has been using (D-- needle).

Have you guys tried a bigger main jet? Suggest you try a #168-#175 main jet or the EKN needle.



I have the same patchy symptoms on my 01 426 while riding on the road at consistent speed. Airbox lid removed w Vortip insert. I thought maybe turning out the fuel screw might improve this. Anyone tried this?


if your bike improves by clicking open the throttle a BIT you are running RICH. try dropping the needle & dealing with your other problems from there.


Redbird, I have the exact problem, at constant throttle I get a hiccup sometimes, I was running a 165 main so I decided to try a 170 and the bike ran really bad. Dan


Is your problem at 1/2 to 3/4 throttle? Or could it be down lower?

James (or anyone else Inrtested), checked my manual today, and here is the jetting my model should be running:-

pilot - #42

pilot screw - 7/8 turn out

pilot air jet - #75

needle - OBDQR #4 clip

main jet - #165

main air jet - #200

Does that throw any more light on the subject?

I haven't removed the baffle from my pipe, as it's not removable on my bike, it seems the UK/Euro spec bikes are a fair bit different to US models. Also, there's no throttle stop other than the small socket cap screw in the carb body, certainly no restiction on the throttle opening.



My problem occurs at less than half throttle and seems to happen when I am maintaining the same speed/thottle postion on the road. Dont notice anything off road. Should I turn fuel screw out a bit?


drop the needle one.


I aggree with Taffy, drop the needle 1 step from the descriptions. You may need to adjust the pilot screw in the process.

WR426 owners. Thanks for all the great information. I'm also experiencing this problem and have already tried going from a #42 pilot to a #48 and, from a 165 main to a 168. Both moves have not improved the problem. Also, the pilot screw is 1/2 out from all the way in. This move seemed to improve it a little. I will now go back to stock and lower the needle one. Thanks again. Sonny.


where did you get your figures from!!!

no wonder it ran like **** !

those of us running a 48PJ do so because of the year of our bikes & the set up yamaha installed. it's called horses for courses.

42pj should have become 45pj. one step at a time.

why did you go up on the MJ. was anything wrong?

crazy. calm down. all other settings on your bike are mechanical & pre-set. the carb is set to each bikes needs. that's why we spend 50% of our time talking about jetting.



James, Thanks for the reply. My problem is 1/2 to 3/4 throttle. Let's say im cruising down a fireroad at 60 mph in 5th gear sometimes the bike will hiccup a little, not alot but just a bit, also if I were to twist the throttle really quick from idle it will cut out a little bit. Any suggestions, Dont mean to beat a dead horse here, but my 98 WR400 ran perfect I just that I havnt gotten the 01 WR426 right. Thanks, Dan

WR426 WR timing,(T4)pipe low elevation

Taffy, Thanks for the tip!!!

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