JamesDean, Taffy, Hick or whoever else.....

9 replies to this topic
  • BrandonW

Posted August 13, 2002 - 09:57 PM


I recently timed my '98 to the YZ specs, and I am ready to start dialing in the carb.

I cannot do the BK mod ('98 carb, right?) so it will all be jetting.

I wanted to start with:
Main Circuit:
158 Fuel Jet (currently have 172)
??? Air Jet (currently have 200)
Pilot Circuit:
38 Fuel Jet (currently have 45)
60 Air Jet (currently have 100)

Don't know

I went down to the local shop, and all that they have is the 158 Main Fuel Jet and the 38 Pilot Fuel Jet; so it looks like I need to place a phone call into Sudco.

The only YZ needle that they (local shop) was the ODBV-Q needle. Is that what I need, or shall I ask for something else from Sudco?

Also, what air jet shall I ask for to correspond with the 160 main fuel jet?


  • Bill

Posted August 13, 2002 - 02:04 PM


I'm not the jetting expert that you asked for but, the jetting in my signature was given to me by taffy and it works great.


  • tk421

Posted August 13, 2002 - 02:49 PM


Brandon: Your numbers look common, I would think they are good depending on your elevation and temperature.
You are right about the BK mod, can’t be done on a 98. I think taffy had some sort of AP mod, try the search but definitely take care of the accelerator pump.
I don’t think you need to mess with the maj until you see how it runs. For now leave the 200 maj. For needles try an EMN clip #5 or EMM clip #4.

James Dean: What symptoms will the bike give when it’s rich on top? Same way if it’s lean on top, what symptoms will the bike have?

[ August 13, 2002: Message edited by: tk421 ]

  • Hick

Posted August 13, 2002 - 03:00 PM


I have never jetted a 400.

I’m a big fan of E taper needles in the 426, as is JD (KTM thumpers also come stock w/ Ds). Taffy used to run them in his 400 too, then he had some sort of epiphany and I thought he switched back to D needles.

But right there in Bill’s sig is an EMM.

What’s in the sig above looks fine to me. The 60 PAJ seems a little small for a 38 pilot, but if it works it works...

In my 426 I run a 40 and about 85 on the PAJ. It seems relative exhaust restriction makes the biggest difference here. With the stock 100 PAJ my bike pops like crazy on decel w/ a T4 or R series silencer. With the stocker it is still lean, but not as bad. I just went smaller until I had good trailing and neutral throttle and the popping went away also.

So while you’re on the phone w/ Sudco then make sure you order the adjustable Pilot Air Jet, also called an air screw. Two PNs, 021-230 & 021-235, one is the spring and one is the actual screw. One turn out equals a 100 PAJ, ¾ is an 85 I believe.

Click HERE for a good jetting guide, there is also a table that gives the full range of air screw settings and corresponding sizes.

If you want to do a BK-like mod to your slant FCR, just buy a wheel collar (for RC cars etc., I imagine) from a hobby shop.

Click Here

Apparently you want the 3/32” size.

Click HERE for a TTalk post on this subject.

The idea is you slide this collar over the actuating rod and use the set screw to lock it into place on the rod at whatever distance from the carb body that gives the amount of pump operation that you want. It just acts as a stop on the pump mechanism, like the BK mod, but in a different way.

I’m not sure, but that EMM needle may appear on the 250F parts fiche...

Hope this helps.

  • MN_Kevin

Posted August 13, 2002 - 04:46 PM


you cannot do the BK mod, but there is the "KL" mod (my own) and the Taffy mod.

Taffy's is easier than mine, but both work great.

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

Posted August 13, 2002 - 07:46 PM


Hick (and others) have about covered everything, Taffy Mod or Kevin's are the accel pump tuning options on '98/'99's. Bill has the complete "Taffy" setting in his signature.

Brandon, don't reduce the main by that much (158) unless you are using an EMM or EMN needle. The Exx needle interacts with the main to need a lower size. The Dxx needle will use a main jet in the range of 172-178. The 158 main jet may need a main air jet near 160 using EMM#4-5. A 165 main jet will likely work with a 200 main air jet and EMM#4.

tk421, if the bike is rich on top it will either sputter or just not rev out. It may seem like the rev limiter is cutting out too early when you try to scream it. When the bike is lean it will not pull with the authority of the next richer main, just revs less quickly.


  • BrandonW

Posted August 14, 2002 - 05:28 AM


Thank you guys very much. I will get on this ASAP, as a good riding buddy of mine just got a 2000 XR650R, and that thing SCREAMS! (I call it the BURP: Big Ugly Red Pig)

Anything I can get to stack the deck in my favor, I will do!

Side note: Last weekend, I overheated my bike for the first time (was a very extreme hillclimb that was approximately 2.5 miles long with many switchbacks, logs, switchbacks, , ruts, switchbacks, rocks and did I mention switchbacks?, there just was no speed about it so there was also no airflow). Will this leaner jetting cause the bike to run at higher temperatures?

Thanks for all of your help Gentlemen! I will attempt to do you proud.

*Just got off the phone with Sudco, and I have an adjustable pilot screw on the way, as well as a EMM YZ needle. Couple that with the 160 main and 38 pilot jet that I got from the local yahoo, I should be ready to go. Right? *

[ August 14, 2002: Message edited by: BrandonW ]

  • Hick

Posted August 14, 2002 - 06:27 AM


Actually, the Exx needle will make the bike richer vs. a Dxx needle over most of the throttle range despite the smaller main.

I wouldn't worry about overheating.

  • BrandonW

Posted August 15, 2002 - 08:18 AM


Why didn't you guys tell me to do the grey wire mod earlier?

I pulled that thing out of the connector last night, and I am throughly impressed!

This thing is just about where it needs to be.....

Smiling ear to ear,


  • tk421

Posted August 15, 2002 - 11:23 AM


Hey Brandon.
The grey wire mod was the last mod I did but did not notice any performance gain. I did notice that the mid engine control improved but that was all. From what I’ve read here, the pre 02 WR’s seem to really benefit from the grey wire mod.
Glad to hear your where you want it… happy trails!

James Dean:
Thanks for the info on the lean/rich conditions up high. The maj is still confusing to me, still learning this jetting thing. It’s been 16 years since I’ve ridden, the IT175 I used to ride worked well. Barley new where the carb was let alone jetting it. Just put premix in and go!


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.