Jetting Chart .....Acurate?

15 replies to this topic
  • Yak

Posted August 01, 2002 - 06:08 AM


Based on your own jetting experiences, would you say this jetting chart is pretty acurate?

Im looking to re-jet, but its such a pain I want to get it right the first time. Can I trust these numbers?

  • John_Lorenz

Posted August 01, 2002 - 06:18 AM


To answer you question

My yz is ata 168 main the needle pos is setting 1 down from stock (this richens the needle riasing it up)

The chart to me is now where near accurate but a suggested starting point.

I am at about 800ft ASL (Abouve Sea Level) the hils raise to 2000. Heat can be from 60 degrees to 100 + in the hills now its about 85 - 90 daily

If I go by the chart I would be in the following range on the main
60 70 80 90 100 Defrees F
163.8 162.0 160.2 158.4 156.6
162.1 160.3 158.5 156.7 154.9
160.5 158.7 156.9 155.1 153.3

[ August 01, 2002: Message edited by: E.G.O.**** ]

  • AZ918MAN

Posted August 01, 2002 - 08:04 AM


The chart to me is now where near accurate but a suggested starting point.

Hey EGO go back to school and figure out how to spell NOWHERE!!!!!!!! HA HA bro just had to give you crap no big deal

  • John_Lorenz

Posted August 01, 2002 - 08:27 AM


HAHAHA Posted Image

  • Kirtwell

Posted August 01, 2002 - 09:26 PM


I think you're on to something Yak,
I rejetted my bike just this week to compensate for the high heat/humidity in the North Texas area. I went by the chart and got pretty darn close...but I had to go a bit lower with the main to get it just right. Therefore the book was off slightly....saying I was more into the lean side.
I'm sure these numbers are computer generated and in the perfect world of perfect tolerances however the numbers will get you to a ballpark area.

  • motoman393

Posted August 01, 2002 - 04:20 PM


ask mikeolichney...he is the one that made the spreadsheet (if my memory serves me correctly) the chart has worked well for me and 2 of my other 426 friends! Good Luck,


  • Shawn_Mc

Posted August 02, 2002 - 07:31 AM


Depending on what batch of gas you get out of the pump, your jetting specs could and usually go out the window.

The worst part of jetting something in is the variables are endless. It makes you have to trek out to the desert, usually with a carload of brass, and wrench on the thing for hours. Oh well, it is what it is. Yamaha ought make a giant hole in the tank so you can get the plug out, with a reasonable amount of cursing, as opposed to pulling the freakin tank, to do it with ease. At least on the 450 you'll be able to rotate the carb far enough to get to everything easliy, since the coil isnt there to get in your road.
Yak my suggestion would be go back to the 42 pilot, and try that. Then just change one thing at a time.
The problem with what you did is, you did too many things at once and you dont know which thing caused what your issue is now. The main jet wont have anything to do with the way it starts. But dont underestimate how much the pilot, needle and main jets overlap. Its more than most guys think.
If youd like to try something without a bunch of wrenching, try turning the pilot screw out to 2 1/2 turns out and see how it starts then. If its better, go to the 42, and only 1 out and go from there. I hope this helps out. Good luck

  • gonzo

Posted August 05, 2002 - 08:54 AM


Wow it says that I should be running a 158 right now. Thats crazy. I am running a 180 now and the bike is fine. Is it really accurate?

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  • Shawn_Mc

Posted August 05, 2002 - 01:49 PM


Gonzo, it kinda depends on what ya got going on. Temps, the PAJ, all sorts of varibles there.

  • motoman393

Posted August 05, 2002 - 02:14 PM


The chart is also for a stock bike (pipes, air filters, etc change the results) If you enter the jetting you are running currently at the top (change the red #'s to your current jetting specs) it will show predicted jet sizes for diff. temps etc. It has worked well for me and my bike is far from stock! Later,


  • mikeolichney

Posted August 07, 2002 - 12:21 PM


Yeah, I wrote that spreadsheet. I put a disclaimer in it that says it doesn't tell you how to JET a bike. All it does is predict how to REJET a properly jetted bike for different Temperatures and Altitudes. If you change something on the bike, (pipe, fuel, etc.) this chart wont help predict what the new brass should be. You have to know the right jetting at some T and A for your setup.

If you have a STOCK bike, and the manufacturer did a good job with the jetting setup, then enter those numbers in for the numbers in red. Then you should be in the ballpark with the chart if you go to some other T and A.

  • gonzo

Posted August 08, 2002 - 04:44 AM


Yea I do have alot of mods. Yoshi exhaust notoil filter carbon box high compression piston. All the goodies. Rigth now I am running a 180 and it seems a little rich for this heat. My plug seems to be ok.

  • sigar

Posted August 09, 2002 - 09:35 PM


what is the temperature that the base line setting is based on? I know it is for sea level altitude but what about temperature? For example I may run a 175 main at sea level on a 50 degree day but a 172 main at sea level on a 100 degree day (as an example only). Is there any reason this chart would not work for a CRF? I can't think of any. Thanks for going through the trouble of preparing this spreadsheet.

  • sigar

Posted August 09, 2002 - 09:37 PM


Nevermind, I just figured it out, 70 degrees. I'm a putz.

  • Boit

Posted August 09, 2002 - 01:38 PM


Gonzo; Is your bike a '98 or '99 400? Jetting for the 400 and 426 is radically different.

After looking over the chart and checking the notes in my tuning journal, I find the chart to be fairly accurate. Keep in mind that the chart is meant to project a baseline starting point. What I find to work well for me is when I am concentrating on one specific area of the jetting is to enrichen that jet or needle by at least two increments and then test. Why? Because I want to force an abnormally rich condition in this area and then work my way leaner gradually. This is especially effective when dialing in the main jet but takes more testing and patience to dial in other areas such as needle clip position.

  • mikeolichney

Posted August 12, 2002 - 08:06 AM


Sigar, there is no reason that this wont work for a CRF (I have one too now). The 2002 CRF manual has a similar but coarser chart to mine inside. I compared the tow predictions and they were pretty close. If I remember, there was an area of the CRF chart which predicted richer jetting than mine did. Take a look for yourself.

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