Hey everyone-

I know this varies from bike to bike, but I'm just wondering what size is your Main Jet? I'm trying to find the sweet spot on my 400ex. I have the White Bro's slip on- K&N air filter... and trying to find the right size jet. I'm at 165 right now... but i'm thinking I need to go a little higher by the look of the spark plug.
Anyone have the same setup as me? and what size are you using?

 

Dirt Wheels did a pipe shoot out and they used a 155 main jet with a K&N filter and no airbox lid, the 155 should still be ok with the lid on.

 

Unless they're at a really high altitude, that seems pretty lean. I believe they just used the jets the pipe maker recommended. I believe that jetting recommendation was for simply adding a pipe only. Air box mods seem to have a more profound affect on jetting that pipes.

With simply a modified airbox, mine ran best with a 165 main.

I am running a 170 now, but it's kinda irrelevent since I am running an FCR carb and a bunch of other go fast goodies.

 

I run 150 and that's what LRD told me to use and so far, so good!!

 

Are you talking a Dynojet jet?

If your looking at a #165 Kiehin jet, it'd be so rich it'd barely run!! With just a slip-on you shouldn't need more than a #155. Even with a K&N filter. I've seen some with a #152.

 

The dyno jet kit comes with an adapter then you screw the main jet onto it, and the Kiehins are just a big one sized jet, SO is a Kiehin 165 main the same thing as a Dyno jet 165 on the adapter? Right now i have a t-4 pipe, K&N, and no lid with the dyno jet 170 main, with the adapter, and it seems to have more low end then with a Kiehin 155 main and lid on, i am only backfiring a little. Yes, it seems once you take the lid off you need to make a bigger jump in main jet size, especially Dynojet recommends a larger increase in main jet size with lid off.

 

No, they are not the same a 165 dyno jet and a 165 Kiehins are different. So this whole question on what size main jet do you use is irrelevant seeing how everyone seems to be running different setups and I also agree with 400exrydr a 165 kiehins jet would be way rich especially with just a slipon and a k&n Im running a k&n no mods to airbox and a 152 kiehin and a 40 pilot jet at about sea level. and it runs great

 

I RUN THE LID WIH A K&N AND PREFILTER AND FULL YOSHIMURA SYSTEM AND I HAVE 155 MAIN AND 40 PILOT ..HOPE THIS HELPS

 

Before I went to Utah I ran a 155 Keihin at low elevation only mods being a Uni filter and Sparks X-4 exhaust. When I went to Richfield, Utah I switched to a 152 Kehin and it ran alright, but far from as well as it ran with the 155 in Virginia at low elevation.

When I got back to Virginia I tried the 152 and the throttle response was awesome couldnt keep the front wheels on the ground in almost every gear. So now my brother's 400EX is set up the same way. One note is that I have not modified the airbox on either machine.

Adam Smith
99 400EX

 

THat might ba a bit to rich. YOu could run that jet with a cam, porting, higher comp. piston and a 39mm. I would jet down a little if I was you.