08 XT250 Airbox Uncork

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08 XT250 Airbox Uncork

Post by 4Strokes » Mon May 23, 2016 7:24 pm

Take a look at this air box (the red line is air flow). It has two chambers, three if you count the rubber tube to carb mount with its own separate chamber which is where the red arrow stops.
The first chamber is where the snorkel is part of the casting. A small section inside the box is a second piece. However it's glued together to form the snorkel (your looking through the cover for the air filter). In this first section the air filter mounts as well. This fixed snorkel is the first vacuum stage (restriction).
The second and even bigger chamber serves as a junction box for the smog pump and engine blow-by gases. It's screwed to the first chamber and has a seal of course. You can see it on the left side of the bike.
Inside the second chamber is a metal grill. It's both screwed and melted in place and is only removable from inside this section. This clearly restricts air flow by half creating its own vacuum stage. You are looking through the air filter toward the second chamber.
Just to let you know the battery blocks the right side of the air box entirely.
So the only place to open up the first stage is where I have placed the quarter.
Because of the problem of getting jets, the two staged air box (three if you count the mini expansion box), the 1 to 1 1/2 horsepower increase you might get if the job was done right, and because it's faster then the CRF230 anyways. Here is my advice. The mods for performance Lil_mynx has done prior are all you should do.

Reply by Admin on 01/25/2009 12:04:02 PM
Does the air filter cover actually hold the air filter in or down? Great pics by the way! In the fourth pic, how did you get the camera to focus on the air filter screen? Does it have a dot in the lens that you can move around to affix the focal point, or was it by accident?

Reply by GlennF on 01/25/2009 12:29:49 PM
It's a shame that Yamaha made the successor bike to the XT225 impossible to improve performance. They did put on a rear disc (the XT225 rear brake is like dragging your foot) and give it 25 more cc's, but left out the 6th gear, skid plate and zerk fittings on the suspension. Also, they should have stuck to the Mikuni carb.

Reply by HorseRider650 on 01/25/2009 12:35:56 PM
Lil_mynx did these photos. She's a professional photographer. How that shot is done is you have to use a flashlight and hold it on the point of focus (that was my job). When your flash goes off you don't see the light from the flashlight.

The air filter slides in a slot and the cover holds it in place.

Yamaha did there homework on making this bike real difficult to mod. I hope Honda doesn't go this way too because of emissions.

Reply by HorseRider650 on 01/25/2009 12:40:32 PM
Ah but Glenn, if the frame is the same and the carb to engine mount will interchange. Then you can use the XT225's larger carb and air box.

Reply by GlennF on 01/25/2009 12:52:34 PM
That would be an interesting project! Some of the 225 owners convert to a Mikuni pumper carb. Somewhere I read that there may be a fuel injected XT250 in the future. Yamaha uses it on their WR250R dual purpose bike. There must be a way to relieve the air box. I will keep looking at your pictures.

Reply by jsmiley4002 on 01/25/2009 1:22:29 PM
so where you placed that quarter is where i would need to put a air vent or jus drill? and ya i saw that screen in there and was wondering how to get it out damn

Reply by GlennF on 01/25/2009 2:03:27 PM
The screen is an "anti-backfire screen". It does just that. If the carb spits back it prevents the flames from igniting the air cleaner. I wouldn't remove it.

Reply by jsmiley4002 on 01/25/2009 3:02:17 PM
i havent drilled the exhuast i simply pulled the air box cap (covering air filter) off and took the exhaust tip/arrestor out in anticipation of drilling and ran it down the street, sounds hell of a lot better and pulls harder

Reply by HorseRider650 on 01/25/2009 3:33:01 PM
GlennF, Makes since about the screen. Man, Yamie thought this one through. Check out this problem with trying to gear up or down this XT. Look at parts 9 & 11. With the new digital speedometer speed is measured by a sensor. You can't change the chip so you are going to need to change part 9 found inside the motor. So if you change the sprockets you'll have to "use the force" to know your speed.
Reply by HorseRider650 on 01/25/2009 3:37:35 PM
jsmiley4002, You should really put the air filter cover back on. It holds the filter from coming out. Do the mods Lil_mynx posted and your XT will be better then stock and run great.

Reply by jsmiley4002 on 01/25/2009 3:48:15 PM
i know but as i look at the air filter it looks pretty secure down in its little chamber and i cant really see it popping out unless i take it off road with some moderate jumps, i am going to drill that exhaust in the very near future however, after i do so, she drilled and exposed some other screw by the choke level and gave it a turn or so? to help throttle response and warm up, thats where i get lost

Reply by HorseRider650 on 01/25/2009 3:48:52 PM
jsmiley4002, It's the only place you can make a hole. But without the jets (and no one can find them as of yet) your bike will run worse so don't mod the air box. With the exhaust done as shown on this site and the idle set at 2 1/2, it will have great throttle response. Good luck

Reply by HorseRider650 on 01/25/2009 3:57:02 PM
jsmiley4002, Look on the left side of your carb and you will see a aluminum plug towards the front. It's about 10mm in size. Remove it as shown on this site by carefully drilling a hole through it and screwing a screw into the hole so you can wiggle out the plug. But don't push to hard when your drilling that you damage the brass screw on the other side. Good luck

Reply by Admin on 01/26/2009 11:03:10 AM
What if you remove the cover and fabricate a flat piece of steel, perforated steel or just hanger wire to hold the air filter down, as such:
Great photo tip! Do you know Lil' Mynx used macro as well?

Reply by jsmiley4002 on 01/26/2009 3:29:30 PM
admin that sounds like a good idea bc right now i have mine off and it doesnt bogg down or effect throttle response and by the looks of it unless i pull a double backflip like pastrana its not coming out but to be safe thats smart

Reply by HorseRider650 on 01/27/2009 09:15:04 AM
Hanger wire would work great and be reversible if necessary. Macro...so you read the meta-data. Shhhhhhh. only photographers and the CIA know about that.

Reply by ahilis on 07/12/2009 2:48:26 PM
I just pulled the cap off my air filter and noticed that if you flip it over, left to right if you're looking at it from either side of the bike, you can screw it back on upside down. This keeps pressure on the filter and leaves a nice gap all the way around the opening. It sounds the same with the cap upside down as it does without the cap at all, so seems to be pulling a lot more air in like that. Adds a nice growl to the bike.

Reply by HorseRider650 on 07/13/2009 07:00:28 AM
ahilis, Really....Outstanding...I'll let Lil_Mynx know. Sometimes the obvious is so obvious that you never see it till someone shows you.

Reply by robertbenita on 08/23/2010 09:06:26 AM
So did the flipping of the cover do anything for the bike? Old topic, but I am brand new to the 2009 Yamaha XT250. Thanks

Reply by HorseRider650 on 08/23/2010 3:03:11 PM
robertbenita,To answer your question ...yes it helped Just remember to shim up one side of the cover due to the fact the cover is made offset. One slightly longer screw with a 5mm nut,larger then the screw going through it, will do. Sorry I don't have any pics but if you play around with the cover you'll see how it's done. You should get a even gap of around 1/4" when the cover is in place. If you do Lil_Mynx's other modes and stuff the bike runs super.

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