The last 2 times I have cleaned my air fiter I have fouled the spark plug. It will usualy run for a few minuts just warming it up then die and I won't be able to get it started. All intake and exhaust is stock except for a 42 piolet jet, which seemed to help the bottom end. I put a new plug in and rode around my yard (at varing RPMs) a few times and took out the plug again and it waas coal black and dry.

 

How much filter oil are you using and what kind? I recommend PJ1 spray-on filter oil on either stock foam or aftermarket foam filters. If you over-oil the filter (Or use the wrong kind), you can reduce airflow. Thus if that problem only happens just after the filter is freshly installed, then that may be your problem.

Hope this helps some,

Sickman

 

Since you mentioned cleaning your filter, that seems to be a clue. I wonder what kind of oil you're using..filter oil or motor oil? Are you squeezing the filter gently after oiling to distribute it and remove excess oil? If the filter is completely saturated & dripping with oil I suppose it could be sucked into the intake.
I would hope you use a good spray-on filter oil rather that motor oil, not for the above problem, but for other reasons.

 

I use about 3 table spoons of Bel-Ray foam filter oil. the stuff gets really thick. I don't dip it in oil then squeez it out because that seems like it would really get a ton of oil on it.

 

Why not run a K&N filter. I find them much easier to take care of than foam and they do last longer. Foam holds more dirt and dust in them when you clean them. A K&N with outerware will stay much cleaner out in the sand dunes where I ride. Anyway just my two duckets

 

sounds to me if you rid it and had a black plug afterwards you need to put a smaller main jet into it and check that your fuel is flowing well through your carb. DO A TEST. first- ride your atv in 4th gear wide open then pull the clutch in and kill the engine, coast to a stop. take the plug out and look at it. it should be a light tan,if it is a gray color you need a larger main jet. a black color you need a smaller main jet

 

It's pretty normal to foul a plug after cleaning and oiling a filter with pour-on filter oil, especially with a cold-blooded girl like the 400EX.

The PJ1 spray-on foam filter oil really is easy to use, and usually solves the problem of getting too much oil on the filter.