I am changing my stock 400ex air filter to a unifilter and was wonder if I should rejet the carb.I am not planing on changing the pipe at all but maybe I can gain a little just by upsizing the the jets.Any sugjestions on what size to go with?

 

I have found that the filter itself doesn't change jetting much if at all.

Now if you modify the airbox at all, that is a different story.

Testing shows that my bike is just as happy with a 165 main whether I run a K&N or the stock filter, all else being the same.

 

Well, as I have said a few times regarding jetting, The only one who can really tell you if you need to rejet is your own engine..no one else's or no one else's experience with his engine. Having said that, I'd recommend that you do a test ride/plug read with your new filter installed. Use a new plug for accurate reading. If I understand you correctly, you run the stock pipe so I wouldn't think you'd need to use more than one size fatter main jet with a Uni filter and your jet needle in the 3rd position, but again, a test ride at 3/4 to full throttle & a plug read is the final answer, coupled with the performance at that speed, of course.

 

Yep, Cal's right. I meant to put the part in my original post about doing a plug read, just to make sure. So far on 3 different 400EX's jetting changes were not required for the difference between a K&N and a stock foam. I would presume that a UNI would be even closer to stock.... FWIW...