does anyone know what the stock main jet is on a 400ex and if my friend has a K&N airfilter and a whitebros slip on what should the main jet be any help.

 

stock is a 148 main, you should go up a couple of sizes and do a plug reading

 

I am running a modded airbox with the White Bros E-Series as well. (stock 12 discs). I am currently running a 165 main with a 42 pilot. I would suggest buying a range of jets from 160-170 instead of buying the jet kits. ($17)

I tried a 155 and 158 and it was stumbling hard. I would use the 160 as a starting point though. Every machine is different, so a plug reading is definitely in order.

 

I'm running with no air-box lid, WB slip-on (18 disk) and a K&N filter with their needle.

Main Jet = 165.
Pilot Jet = 40.
Needle clip on 4th position from top.

 

Stunz

I have just about the same setup but, I have the clip on the 3rd notch and a 170 main. I can't run with the airbox lid on at all it bog's at about half thottle so I have a prefilter and prefilter lid on does yours do the same with the lid on are do you think I have to big of jet?

 

I also have a Outerwares prefilter on the K&N, but no air-box prefilter, "yet".

If I put the lid on, it does not bog, it just does not make as much power.
I modified the lid which helped a lot, but I get the most power with the lid off.

To run with the lid on, I would have to go back to 12 disk on the WB, but leave everything else the same.

I'd guess that you should try moving the needle clip down to the 4th position from the top.

 

Thanks for the info I have had a hard time getting people to reply. I think I will change the clip setting it seems to be the only thing different plus it's easy to change.

 

"I'd guess that you should try moving the needle clip down to the 4th position from the top."

Why would you want to do this? Moving the clip down to the 4th position will richen the mixture further.

FUJIMO - Yes, your main jet is too big. Putting the airbox lid back on will cause a richer fuel/air mixture.

This is how you should approach the problem. First, put your airbox lid on. We already know that your running too rich with a 170 main. Go to the local dealer and pick up some main jets (168, 165, 162, 160). Then loosen and remove your main jet access plug (bottom of float bowl), remember to turn off gas. Remove 170, put in 168. Go for a ride. If it still stumbles, put in the 165. Repeat this until the stumble is gone. Once the stumble is gone, you can drop one more main jet size down from that if you want, but no more.

You might need to get main jets that are less than 160 (i.e., 158, 155...), but probably not. You should be able to return any jets that do not work.

As for your needle, leave it in the same position (middle clip). After you have the correct main jet in, you can do a plug chop. Ride the quad for 1/4 - 1/2 mile at half throttle. Pull in the clutch and hit the kill switch at the same time. Remove your spark plug. It should have a tan color (like a roasted marshmallow). If it's more white looking, then you can move your clip down to the 4th position (from the top).

ogre