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250x Airbox Mods

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I've spent hours going through posts about this topic, and have as of yet not gained enough info to do this mod. I've heard countless posts about honda specs, 10 times more from people just winging it, and still am not ready to carve my airbox. I've got a brand new 05 250x, have already purchased a 42pilot, 140main, 55leak. But from what I've read, adding these seems pointless without doing the airbox mod. Unfortunately I'm still vague about where/how much to open the airbox. I ride in the rockies, lots of serious hills, lots of technical terrain. I went from a 2002 xr250r (280 big bore, hotcams stage 1) to this X. I love the power once you find it, but I miss my XR's Low end. I'm hoping with correct Jetting I can find some of that low end. Don't get me wrong, I'm in the r's 90% of the time, but when you come around a 180degree switchback at a serious incline, its pretty easy to be out of the "range". That being said, I'm trying to tune my bike to my riding conditions. General Altitude I ride is 3500-7000. My bike does good, though I only have 80 miles on it, I already know it's not running to potential. What do you recommend for what I've described? Dunno if it matters, but I'm 190lbs, 31yrs old, and will ALWAYS be on your rear fender through the tight steep stuff :applause: I know this bike can blow away my XR, but as of yet, it hasn't!

Oh5x

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Lots of Info there, thanks for link. But still unclear as to how much I should chop my airbox. Is the standard where the line is? No exceptions made to altitude? Temp? As I said, I ride pretty high, and its normally pretty hot, 80-95f.

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Had a 98 XR250 with a 277 kit.

Have an '04 250X.

I don't think the X will ever generate the low-end of the XR with stock cams, etc. Just two completely different types of power.

In general, there's only so much power to be found under the curve and when you pull it up it has to come from somewhere. In the X's case, it's from the bottom.

I put a 42 pilot in mine to help with starting and low-end response. Good move. It already had a 55 leak in it stock.

The bike is completely stock other than the pilot jet, fuel screw and bark busters.

:applause:

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Cutting the top of the airbox is really pretty easy, I have done several of them. There is a small raised line that runs along the top of the airbox, just make your cut along the inside of it. There are a couple of spots where the cut line is incomplete but you will be able to easily see where to cut. It is kind of hard to understand until you have the airbox out of the bike.

When you go to do the airbox and carb mods I would recommend doing a few other things. The first is to get a trick fuel screw so you can more easily make that adjustment, this can make a huge difference in throttle response when changing altitudes, and temps. Second, would be to get the applied racing block off kit to reroute the head vent. It makes things much easier when you have to install the airboot back on the carb. Third, would be to get the twinair powerfilter, this is a billet filter cage and a trick airfilter that is backfire resitant so you can ditch stock backfire screen. It flows air night and day better than stock, and is easier to install then the stock one.

This is also a good time to do the pink wire mod.

Here are some instructions to do the airbox job well.

You will need a couple of tools to do it properly. A drill with about a 3/8" bit. A razor knife with a brand new blade, and an x-acto knife is nice but not necessary.

To do this job with the least amount of problems, you will need to pull the seat, tank, side panels, and silencer. Then, loosen the screw on the airboot clamp and disconnect the breather tube on the top of the air boot. Loosen the top subframe mounting bolt. Remove the two lower subframe mounting bolts. You should now be able to rotate the subframe upwards and pull the airboot off of the carb. Now you can tighten the top subframe mounting bolt and the subframe should stay up giving you better access to the carb. I can give you instructions on carb stuff, but for now I'll stick to the airbox.

There are three bolts holding the airbox to the subframe. One is on top and has a strap holding the coolant reservoir lines in place. The other two are at the front of the rear fender. Remove them and the airbox should slide out the front. Now drill out the two rivets holding the snorkel onto the top of the airbox and yank that thing off. You should now be able to see the cut line I mentioned earlier. With your sharp new razor knife make a fairly light pass along the cut line one section at a time, on subsequent passes the knife will want to stay in this line. Make one or two more cuts with moderate pressure. Now you should be able to plunge the blade all the way through in one section and draw it down the line, the plastic will split nicely down the line you scribed. Repeat this process as you work your way around and it should come out looking nice and professional.

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Rick's comments are based on a bike ridden in the Colorado Rockies, so his comments should be right on track for you. I cut a 4"x4" hole in the top of my air box with it still in the bike. Be real careful and put a CLEAN rag in the intake track so you won't get any shavings in there. I also had my son hold a running shop vac in the air box where I was cutting to catch particles. Mine was cut 1 1/2 years ago and so far no problems with the bike. Have fun with it. I noticed a huge difference in the power when I cut the air box and rejetted.

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is? No exceptions made to altitude? Temp? As I said, I ride pretty high, and its normally pretty hot, 80-95f.

No exceptions for altitude/temps. As is said well above, take the air box out and you'll see the like you should cut along. The resulting opening is quite large.

Also, the construction of the air box (with the small tube-like air inlets) seems to be tuned to generate high intake air velocity at low to moderate RPM; something like the Chevy TPI engines of years past. This is usually done to enhance the low RPM part of the power curve, so if you are specifically looking for immediate, off-idle bottom end, I'm not sure that cutting the air box is the way to go. At any rate, a replacement air box is only $45 from Service Honda, so go ahead and experiment and let us know what you think. If you don't like it, you can always swap in a new one.

Another option is to shift your power curve to lower bike speed ranges by changing sprockets. You can try a 13 tooth front sprocket for around $20 and the stock chain fits. The down side is that you'll lose top speed and you'll be spinning higher RPM at all bike speeds.

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Also, the construction of the air box (with the small tube-like air inlets) seems to be tuned to generate high intake air velocity at low to moderate RPM; something like the Chevy TPI engines of years past. This is usually done to enhance the low RPM part of the power curve, so if you are specifically looking for immediate, off-idle bottom end, I'm not sure that cutting the air box is the way to go.

Hey Skip, I think that "tube-like" rubber inlet has three purposes. 1. Intake noise reduction 2. Water rejection (it's as high as possible) 3. Emission control (strong intake pulses make accurate enviro jetting (read: lean) possible. I don't think it's a performance enhancer at any rpm. Just a butt plug. High velocity combustion chamber "packing" is done at a point as close to the combustion chamber as possible (see '05 head porting), not 18 inches away on the other side of the air filter. This is my opinion, I don't recite it as hard fact, just my opinion.

Scotty

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Thanks for the info, was out of town or would have responded sooner. Bike is tore down and I will be cutting the airbox. It would be done by now but my new main jet had a crack in it so had to order another one. Appreciate the wealth of info to be had here on this subject. I'm not really interested in gearing down the bike, I do ride at high speeds when able. Just looking to get a little more punch to carry me through when I make a bad choice of gears or conditions go south unexpectedly.

oh5x

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