Hey guys....
The solution to the 400ex smoking problem is simple. It's not that your not taking care of your machines or that your useing the wrong oil.It's right in front of your nose.Because of the EPA, the 400ex has a crank case breather that blows right into the air intake boot aft of the air filter.Under a heavy load, the crank case vents alot! Through blowby from the piston and simply the piston moving up and down real fast.So do like I did, plug the hole and get a K&N breather(about $20)and put it on the end of the breather tube. You just might get a little better performace by not burning oil enriched air!!

 

Man, that would be the prob..I had the same problem with a friends XR250L street verson becasuse of the stupid breather..I just cut his and ran it strait down like the other XR's did

 

Where do you get this K&N Breather?, Which hole do you plug, the one in the airboot? One more question, will this cause problems if the breather gets wet? Thanks! Norm

 

Yo,Yo,Yo Norm....
Don't worry about the breather, when the engines running it's blowing out. Now, if you shut it off while your in a lake it will act like a U-boat.

To answer the other question,yes- plug the hole in the intake boot. Any other questions just let me know.... almost fogot, the breather can be purchased from any auto parts store ....Good luck

 

I had actually thought of that as a possibility, but then convinced myself otherwise.

I thought about a car, they route essentially the same way and don't smoke. I also thought about the amount of oil it takes to actually cause visible smoke.

I don't remember seeing any oily residue in my breather passages either... Surely if it is venting that much oil, there would be some in the plastic catch reservoir before the airbox.

You're right though, the old bikes just routed straight into the atmosphere, and the hoses did usually have some oily dirt stuck to them.

I'll look into my buddy's '00 XR400 and see how it's routed, just for comparison purposes.

It's something worth checking into though, I had originally dismissed the idea when mine quit smoking after changing oil viscosities...

 

I'm confused on which hole you plugged. Is the crankcase breather the ~3/4" hose that runs from the crankcase to the air box? Would you put a K&N breather filter and attach it where the hose enters the air box? Let me know if I'm on the right track.

 

I think you would pull off the plastic resevoir by the airboot, plug the hole in the airboot, cut down or completely remove the breather hose, then install the K&N Breather on the crankcase, am I right?

 

The little white bottle on mine has some black stuff in it that has appeared over the last year. I took the white bottle off stuck the hose back in the frame and taped off the hole in the air boot with black tape. Make sure no air is getting in IT WILL BE UNFILTERED!

 

Installing a breather filter directly on the crankcase seems risky to me. What if a stick or rock hits it? With what would you block off the plastic resivior in the air box?

 

mike the crankcase filter is not just hanging in the open when you put the filter on it is out of the way. packerdave found a plastic plug that fits right in the intake boot hole use the stock clamp that holds the white bottle on it. not risky its a pretty smart ideal