1998-99 YZ400F User Jetting Database

READ BEFORE POSTING!: This thread is intended to be used as a reference collection of jetting used by members ONLY. Please use the provided template below, and post only the jetting you currently use, along with any pertinent comments. Post only in the correct thread for your model bike.

DO NOT ask questions or discuss the information posted here in this thread. If you want to talk about anything here, please do so in a new or current thread on the topic and use a link to the particular post you wish to discuss. Posts not complying with this format will be removed without comment.

NOTICE:The people who post here are of all skill levels. This information is only intended to let you see what others are using, and cannot be taken to mean that the information is accurate, will apply to your use correctly, or even that the poster's bike runs right with the posted setup. Use it as a guide/starting point only, and look at as many relevant postings as you can before proceeding.


Post your jetting using the template below (copy and paste to your post) and please be as complete as you can. In the Comments section please include any modifications you feel will have a bearing on the jetting, such as cams, accelerator pump mods, bored carbs, etc.



Edit Revision Date:

Main Jet:

Pilot Jet:

Starter Jet:

Leak Jet (if any):

Fuel screw/Setting:




Altitude Range:

Temp Range:

Humidity Range:



OEM 1998 YZ400F(K) and 1999 YZ400F(L)Jetting:

Main Jet: 175

Pilot Jet: 45

Starter Jet: 65

Leak Jet (if any): N/A

Fuel screw/Setting: 1.5 - 2.5 turns out

Needle/Position: OBDVR/4

Model: 1998 YZ400f

Edit Revision Date:

Main Jet: 170

Pilot Jet: 45

Starter Jet: 65

Leak Jet (if any): N/A

Fuel screw/Setting: 1.75

Needle/Position: JD Jetting Red Needle/2

Filter: Stock

Pipe/muffler: White Bros E2

Altitude Range: Sea level to 100m

Temp Range: 20 - 50 deg C

Humidity Range: 80-95%

Comments: The bike is ridden in the UAE desert. Summer temperatures range 35-50 deg C, winter 20 - 35 deg C.


Edit Revision Date:

Main Jet: -stock

Pilot Jet: -stock

Starter Jet:???(what is this) Im a honda ATV man and im not sure what this is.

Leak Jet (if any): (???)

Fuel screw/Setting:2.5turns out

Needle/Position: stock


Pipe/muffler:FMF slip on

Altitude Range:516 above sea level

Temp Range: - 30F-100F

Humidity Range:40-100

Comments: friend of mine got this bike for the right price- and im not very sure about the CR carbs. I was wanting to know what everyone is jetting there carbs to around the midwest area or east coast. i need jet sizes needle position and air-fuel mixture.

any info would be great guys. Thank you very much.

mark knox

centralia illinois


Edit Revision Date:

Main Jet:185

Pilot Jet:45

Starter Jet:65

Leak Jet (if any):

Fuel screw/Setting:1.5-2


Filter:twin air

Pipe/muffler:fmf powercore 4 slip on

Altitude Range:850-950

Temp Range:-10f-90f

Humidity Range:40-100

Comments:Also running hotcams, and 94 fuel, live in ohio so the weather is never the same and allways seems to work

Model: 1998 YZ400F

Edit Revision Date: ?

Main Jet: 160 ran (should have been 162 - read comments below)

Pilot Jet: stock

Starter Jet: stock

Leak Jet (if any):

Fuel screw/Setting: 1.5 ish

Needle/Position: middle

Filter: Pod Filter

Pipe/muffler: White Bros w/ USFS Sparky

Altitude Range: 7800 - 9100

Temp Range: 62 - 80 (ambient)

Humidity Range: 30% - 100% ( thunder storms, rain, hot sun all in one day )

comments : I ended up running a 160 main jet. This was just marginally lean, but i didn't have a 162 with me. The plug looked good after 1hr and 10hrs of riding on it, so it wasn't a danger, but there was a small amount of popping on heavy decell

Also : Running CA "May Contain Ethanol up to 10%" 91 octane pump gas (Shell and Cheveron )

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