Jetting Virgin !!

6 replies to this topic
  • Brett

Posted July 12, 2000 - 12:14 PM


I've looked through many many previous posts, and I have to say I'm even more confused now.

I've got a '99 UK spec WR, and have just fitted a CRD pipe and header (pretty quiet!).
Took the bike for a long blast around Wales over the weekend, and all seems fine - but:
- a little hesitant on initial pick up, even in low gears
- plug well and truly oiled up
- excellent when wound up, just hesitant

Its currently running standard timing and jetting (168 main), I want to get it to run sweeter, but I have to admit I don't know what all these various jets really do, and need you all to help.

All suggestions eagerly awaited !!

  • Taffy

Posted July 12, 2000 - 12:55 PM



i'm from the UK, ride a 99 WR & have had a pro-circuit 304 that came with the bike.

i went up to a 180 main & left it alone. it was great. you don't have to pull the carb off to do this just turn off the fuel & undo the nut under the carb. get on your knees & you'll spot a little brass jet. you'll need your 1/4" drive set to undo this. put the new jet in your socket & feed it in by hand. trouble screwing the new jet in? open the throttle & try.

you may need to raise the needle to sit higher in the slide by one notch.

let us know if you have anymore problems.

don't look at the plug for colour, not until you know what you're doing. there's a whole story in a plug so don't look at it.

go by the exhaust. do three or four runs with as little fiffing & faffing as possible. give it a good belting up & down a piece of long flat land.

with the throttle wide open-hit the kill button.

undo the silencer & look in the back & the front. ivory white is lean & do something about it. a very light cup of milky tea is what you want. darker is ok. but go by the feel & use the colour to confirm what you think is happening.

[This message has been edited by Taffy (edited 07-12-2000).]

  • James_Dean

Posted July 12, 2000 - 12:58 PM


The quick and easy way to start is try adjusting your pilot screw out a little at a time for low speed jetting and for top end a bigger main jet. Perhaps a #175-180.
If you're not satisfied there, maybe a #48 pilot jet.
Then for more boost in the midrange read the posting on the YZ426 carb needle for YZ/WR400's.
James Dean

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  • mcarp

Posted July 13, 2000 - 12:01 PM


"you may need to raise the needle to sit higher in the slide by one notch."

This solved the hesitation problem on my US '00 WR. I ended up with a 45PJ, 172 Main w/ YZ pipe, airbox lid removed. However if you are fouling plugs, you may want to change the plug, do what James said and adjust the pilot screw counterclockwise a little, and give it another test ride.

Maybe the wrong needle was installed or it is in the wrong position? Clip # 4 is where mine is now.

Also some folks have had this problem and it was solved by rotating the TPS clockwise 1/16". Don't remove the screws, just loosen SLIGHTLY, then turn SLIGHTLY, then tighten. Try this last.

Good luck. The main jet will have no effect below 1/2 throttle.

Think of the jets and needle as fuel metering devices each doing it's own thing based on throttle position (not necessarily engine RPM's). The manual will help determining what jet's are doing what at various throttle openings.

  • Hick

Posted July 13, 2000 - 12:27 PM


Most YZ owners have also richened up pilot jet a size or two to cure low rpm hesitation. U.S. YZs are likely jetted different than your bike but do share same carb.


That is the first I have heard of fiddling with the TPS. Interesting. Have any 426 owners messed w/ this?

(Throttle Position Sensor is black electrical sensor on left side of carb. The timing curve is altered by the ignition according to input from this sensor. So I would imagine rotating clockwise fools ignition into retarding timing or delaying timing advance? Or is it other way around? I'm too chicken to take it off and see which way is which, but factory setting is marked w/ dab of yellow paint.)

[This message has been edited by Hick (edited 07-13-2000).]

  • RodH

Posted July 13, 2000 - 07:21 PM


I know a guy who is a very, very good mechanic, and he set his using a multi metre, and the difference it made was amazing. At the moment it's not set at the factory setting, which does vary from country/model as the are different depending on the octane of the fuel in that particular country. In the manual for the Oz WR, it isreccomended that you use (unleaded 92 octane), but they are sold with stickers on the tank that say "Premium Unleaded" (96 octane)
So with all the variations from country to country, we have to be carefull with the advice given or taken on the forum.

RodH Canberra, Australia<A HREF="" TARGET=_blank>
400 Thumpers Australia</A>
2000 WR400F See Photo's and Modifications

  • Guest_Guest_*

Posted July 18, 2000 - 01:19 PM


ROdH from australia

hey rodH from pto. vallarta again, last saturday use my wr 400 and fells exelent, went for montains and some high ways but on hig way in cruising speed betwen 70, 80 ,90 km/h the motor hesitate if I open full torttle rips fast no problem, only a cruising speed, the model is wr 4oo 2000 from california the mod´s snorkel remove, bafle tip remove, the torttle open until 1/2, i just change oil and oil filter too the stokc main jets is 165 I think. i have a 172 main jet do I change?, but all mys trips go up the montains about 1000 mts,I live at sea level, the plug looks dark I think its ok. What do you recomend me
what can hapen i I change the MJ to 172 when I go for montains?
Thanks for you time.
Renato Gomez Pto. vallarta Mexico.


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