2 stroke air box

i just installed a yz250 2 stroke airbox on my yz426 every thing fit nice(much lighter than stock box)i also bought the yz250 seat any tips on making the seat fit a little better,it's really not bad just a little tight at the back of the tank and a little bit of a gap at the bottom between the airbox and the bottom of the seat.non of this is really a problem it looks great just want it a little cleaner looking is all.



can you take some pics? I have only seen 1 426 with a 250 airbox and it looked great!

motoman' i'll try as soon as i get some taken.

PICTURES!!! I've been looking for specs for a while...You can't use the YZ seat?

you can use the 426 seat but it doesn't line up well with the yz250 side panels and has a huge gap between the air box and bottom of the seat

(i tried)the 250 seat lines up better but i had to elongate the hole at the front of seat where the tank fastens on and looks much better.as far as specs,jam,what are you looking for maybe i can help??

hey motoman ive seen you out at splendora a few times and i have a two stroke air box on my 426. if i see you again ill show you. i really like the box

hey motoman ive seen you out at splendora a few times and i have a two stroke air box on my 426. if i see you again ill show you. i really like the box

Please, somebody post pix and a parts list! I'm getting the shakes!!! :)

Yea same here you really need to get those pics. Im curiuos what it looks like.

Sorry. Double post.

[ March 18, 2002: Message edited by: Shane ]

hey guys i'm workin' on the pics. i did get to ride her with the new box only about an hour so the jury is still out on any HP addition.if any one is going to try this here is some little things to be aware of:

everything bolts up pretty easy as i said before the yz250 2t seat makes it look much neater however the bottom front of the seat pushes on the bottom rear of the tank pretty hard so i had to pull the cover back and trim some of the seat base (no big deal)also elongate the hole in the nose of the seat where it fastens to the tank,this set up will not work with the ty davis tank as the back of the tank hits the front of the air box on the left side,it does work with the stock tank and acerbis tank.the 2t side panels work well with the FMF Q silencer and the stock silencer (others may also work but these two are what i have)make sure you put some type of heat shield(like moto-tile)around the area where the pipe runs below the bottom of the air box other wise it will melt the air box(don't ask me how i know)all in all i like the looks of this mod and it has to breath much better,the only draw back that i have seen so far is that the filter seems to get a little dirtier quicker ahh but what the hey.also according to my totaly innacurate bathroom scale the stock box was like 2lbs and the 2t box is less than a pound.what i bought is:

1) 99 yz250 air box $50.00

1) set of ufo 96-01 yz125/250 side

covers $39.00

1) moose 96-01 yz125/250 tall seat(optional)

$ 89.95

give it a try you'll like it and i'll get my pics asap

good luck


Do you need a new intake boot to match the new 2 stroke air box? or are they the same? How about a subframe the stock one works if I remember correctly?

motoman,yes you use the stock 426 air-boot and sub-frame

If you guys could only see my smile...I'll be ordering those parts soon! They'll look good with my new SuperMoto wheels!

I've done the airbox mod on my 250f. I'm unable to put pic's on here (I don't have a web page to download from).

The weight saving was 1lb for the airbox and no savings for the side panels. And yes, the stock seat works but the gap looks bad. By the yz seat and do some trimming.

If you PM me your email address I'll send pic's before/after of mine.


I may have the cleanest YZ250/YZ426 airbox conversion done, ask motoman 393, he's seen the pictures. My recomnedations, use 00-01 yz250 2-stroke white airbox, it fits better with the square subframe rails of the 426. The stock 426 airboot fits perfectly the the 2-stroke airbox. The exhaust pipe needs to be dented slightly to clear the airbox and I added some heat shielding to the airbox, as it is a close fit. I also recommend that you remove the side panel bumpers and brackets from the stock muffler, not required with an aftermarket muffler. The seat I used was a One Industries/SDG 96-01 YZ250 2-stroke seat, I had to trim it where it meets the tank a lot and I added a foam extension to the front of the seat to fill it in around the nose, the cover stretched enough to add about 1 1/2". It takes some time and a lot of patience to make the seat look right but once it is done the bike feels taller because the 2-stroke seat is wider across the top than the 4-stroke. Maybe motoman 393 can post the pictures on his website?

motoman393@hotmail.com email me the pics and I will post them! Later,


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