The K&N fits fine. The airbox opening is just a little small. The stock filter is tight to get in too.

 

i've used both the uni foam and k&n....the k&n is much louder than the uni...you can hear and feel the air suction in the k&n is greater...i'm running a dj132 jet which ran fine with the uni and no snorkel....i've been a fan of k&n on all my cars...the k&n looks like it won't fit, but if you angle it right, it slides in perfectly...don't crush the cone trying to force it in....i ran the k&n with no snorkel, but the vibrations on my crotch was annoying and the engine didn't feel right...added the kdx snorkel (thanks to a member here)...adjusted the a/f a bit and i love the k&n and kdx snorkel combo a lot better than the uni and no snorkel combo...my crotch is happy and just like the sound better...

 

Nope just stick with the stock one,it filters better than any of the aftermarket ones.

 

To Brieninsac: If you are running a stock, pre-oiled K&N you are using a filter that is probably OVER-OILED. New, out of the box K&N filters, in cars, are depositing their excess filter oil on the air-mass sensor down stream and screwing them up. I think there is even some note about it in a new K&N filter or on its package.
If you want to know if any dirt is getting THROUGH your air filter you can put a little motor oil on a paper towel and wipe down any black surfaces on the carb side of the filter. It will catch stuff that gets through the filter. "Screwing it up". Say's it all, doesn't it?

 

They do appear to come well oiled. I actually hit mine with the air compressor and blew some of the oil off before putting it on.

 

Well that's a lot of votes for aftermarket and one for stock. I guess I'll try it idk where all of you are finding them for $20 the cheapest I could find was $25 including shipping and handling. I went with a uni on amazon part # NU-2364ST I looked it up on UNI website and that's what they had for klx250s/f

 

To be fair, you can't go wrong using the stock (genuine) filter. It IS what the manufacturer recommends, after all. I would consider stock as the minimum you should use, however. Stock filters aren't built to be the BEST filter, they're made such that they will warrant the machine in that configuration and no more. If you want your ride to last longer than stock then you need to filter the air, fuel and oil better than stock (and preferably don't flog the bike it's whole life) ... but that's up to you.
Do realize that a poor quality after-market filter can do more harm than good. If you're not prepared to spend some cash up front, stick with the stock filer.

 

Can anyone please tell me how you can tell if an aftermarket filter is "not good"? Or how much air it flows?

 

As a rule of thumb, if it's not more costly than an original disposable unit, it's probably crap. If it can't be washed out, re-oiled and re-used hundreds of times, it's crap.
As for air flow, there's often a rating on the packaging or on the manufacturer's web page. It pays to do your research on these things.

 

Reread the instructions for the k&n, your not supposed to use compressed air on them...