newbie jetting questions

9 replies to this topic
  • MrTwister

Posted March 01, 2003 - 02:15 PM


ok team, hunted through all the old posts but cant quite get the answers im looking for. i've got a '98 WR400, runs a twin air filter and YZ pipe, no other mods. main jet is 168, starter jet is 62, pilot jet is 45, these all seem to be stock for the Aust model. i have had the original exhaust on it and it wheelies like a monster and fouls plugs every 50kms, with the YZ pipe and the lid on its ok but feels a little sluggish at the bottom compared to when its got the original pipe. take the lid off and its like 250 at the bottom but the top is fine. seems to use plugs at a rate of 1 every 300kms. im new at this so some directons here would be great. all my riding is basically at sea level. thanks in anticipation

  • Snapper

Posted March 01, 2003 - 03:57 PM


people don't seem to answer jetting questions anymore?

whats going on?

assuming your carby is functioning correctly check out the details in peoples signatures - judging by my own experiance and most Aussie's my bet is both your main and your pilot could be reduced - gotta also think about the position of your fuel mix screw.

the ratio of your PJ:PAJ and MJ:MAJ is important (see ratios in "jetting Q's" but I would start by reducing your main

put the airbox lid on the shelf unless doing lots of marine work

i am currently running a 160

  • michaeltrundle

Posted March 01, 2003 - 06:58 PM


Another thing to check is the accelerator pump squirt duration. In standard form the older models would squirt for up to 5 seconds. My 2000 was about 3.5 - 4 seconds.
Do a search for the 'taffy mod' or the 'kl mod', I think thats what they called it for the 98 and 99 models, and 'BK mod" for 2000 and onwards.(someone correct me if I'm wrong)
You want to get the accelerator pump squirt back down to about half or one second long, or so, and these mods allow you to control this squirt duration via a few different methods, depending on the carby/year of manufacture.
Once you have done this and stopped all the gallons of unwanted extra fuel from pouring in you can then concentrate on the jetting and what needs to be done with that.
Your plug fouling will probably go away as a result of this mod too, and then you may find you dont need to rejet.
In my opinion the accelerator pump squirt has got to be regulated before you can really accurately jet these bikes.
hope this helps you.

  • MrTwister

Posted March 01, 2003 - 08:15 PM


thanks guys, i'll have to check the accel. pump, i didn't even look at it, so i'll follow that up first and move to jetting from there. i'll have to play to get it right with no airbox lid(if its wet i get lazy and watch telly usually!)
Snapper- if you need work or advise on the wave blaster talk to Mark at Coast Yamaha as he owned the jetski hire business at Goolwa for 3 years and did all the spannering himself.

  • MrTwister

Posted March 01, 2003 - 08:34 PM


just had a look in the tech articles this is what it says "This Modification is only for the 00'-02' YZ426, 01'-02' YZ/WR250F, 01'-02' WR426, 00' WR400 ...this modification WILL NOT work on 98-99 YZ400's and 99 WR400's"
so does this mean there is a different mod for my '98 or im stuck with the way it is?
Lachy :)

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • tctrailrider

Posted March 02, 2003 - 04:08 AM


The BK mod is for 00 and newer and for the 98-99 its the Taffy mod. The procedures have the same result but the process is very different. Taffy mod uses a collar on the AP rod.

  • Hick

Posted March 02, 2003 - 10:46 AM


When you take off the air box lid and run the YZ exhaust.... now have a YZ motor as far as jetting is concerned. If it were me the first thing I'd try is stock YZ 400 jetting and then take it from there...

...unless you want to run the air box lid, but not many people do that.

Hope this helps.

  • MrTwister

Posted March 02, 2003 - 11:36 AM


TCC, cheers i'll hunt that mod down then, thanks for pointing that one out.
Hick, i probably should of realised that shouldnt i?! cheers thats a good baseline to go form. hopefully now i can get my WR roosting like it should!
Lachy :)

  • KiwiThumper

Posted March 02, 2003 - 01:57 PM



What do you mean by Opened Exhaust on your WR?
If the Aussi ones are like ours, there is no removable end cap and cutting and rewelding the stainless can sounds a little drastic :) ....
Is this what you did?, did it make a big difference?

  • Snapper

Posted March 03, 2003 - 01:09 AM


As far as i know the NZ pipe is the sames can as the Aus. I had planned removing the centre of the can and weldingin a 40mm end piece (as per Roohunters mod) however in order to remove the centre drilled holes around the outside (16X 3/8th holes). This "opened" pipe made ahuge differance and is well worth the 10 minutes - may need some jetting adjustments.

Overall its free, reversile and the pipe is still very quiet. I have seen several others who have done the same thing.

Will keep coast yamaha in mind - touch wood the Blaster (which belongs to wife) doesn't need anything done - very clean unit.


Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.