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Seems like every week I get at least one (usually more) PM's asking me about how I did the decapitation mod to my factory airbox...While I have no problems with helping my fellow Xr's, I keep repeating the same info over and over and over and, well you get the idea....So I decided to just take the time to post up a how-to that anybody can reference...By doing this you gain two positives...First is the additional airflow over totally stock..The factory setup is pretty restrictive and chopping the top off completely opens up the top of the airbox, allowing it to flow ALOT more air...Second if it's done the way I did, you can from this point forward, remove the airbox with the gas tank still in place on the bike....Makes getting at the carb a much quicker process....
First remove the airbox cover and filter, with the gas tank and everything else still in place...Then using a white grease pencil, I traced along the bottom edge of the gas tank onto the black plastic airbox...Then remove the gas tank..Here's a how-to on gas tank removal if you need it...http://www.vtxcafe.com/showthread.php?t=33679 Then remove the airbox from the bike..There's three screws at the carb, a bolt near the bottom, and one near the top...Also there's the crankcase vent tube in the back of the cover, and the tube for the PAIR valve system if you haven't already removed that from the bike...Once you have it off the bike, the fun parts starts...I used a cutoff wheel in my die grinder, but I know that not everybody has an air compressor at their disposal..A cutoff wheel in a Dremel will also work, and as well as a hacksaw...I chose the easiest and quickest way I had....Using the grease pencil again, continue the line you already drew around the sides and back of the airbox....Now when you start cutting, cut ABOVE the line you just drew, especially on the back....If you cut too low on the backside, you'll end up allowing unfiltered air into the carb, and you don't want that happening...So you need to stay above the flat area back there...Once it is cut, it'll look just like this....
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See that flat area at the top that I left?...Don't cut into that, or you'll be letting unfiltered air into the carb...Might take2-3 times to get it right...I cut it high all the way around, then went a little at a time until I could get it back on the bike with the gas tank in place back on the frame...I left it high on the backside, and when I remove the remaining mount locations (one gets cut off), I tilt the bottom out towards me so the backside will clear the tank bottom...Here's what it looks like with the tank off the bike...
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Big difference in the open area between this and the two small openings that were formerly the intake openings...You can also see how the filter sits in there, and why it's important to leave that flat area intact...Now all that's left is to put it back together...The crankcase vent tube on the backside can be a pain to re-attach, but it'll go back on there...Here's what it looks like all re-assembled on my bike with my waterjet-cut airbox cover by Lazerdude...Can't even tell it's been cut with it all back together..
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Now for a couple more items that need to be covered...Doing this allows alot more air to enter the carb and engine, so carb work is needed to keep the air/fuel ratio correct...Lots of other guys here have done this same mod, and I'm 99% sure that everybody here has followed Scar's carb tuning sticky to dial in their setup..We're all using 2 shims on the stock needle, and set the A/F screw to 2-1/2 turns out from lightly seated...With my particular setup (opened airbox cover + decap, I have the A/F set at 2-5/8, and could maybe use another 1/8 turn which I'm gonna try this year....
Also I recommend getting a K&N filter...The stock one is a paper filter, and with the top chopped off the airbox, there's a decent chance of water getting in there if you get caught in the rain...Water will destroy the stock filter, but a K&N will be fine for the life of the bike...
Best thing about this mod is nobody can tell it's not 100% stock...But it'll flat haul ass when the carb is dialed in to the increased airflow...Stealth mode at it's best!!..LOL!!...:mosh:
First remove the airbox cover and filter, with the gas tank and everything else still in place...Then using a white grease pencil, I traced along the bottom edge of the gas tank onto the black plastic airbox...Then remove the gas tank..Here's a how-to on gas tank removal if you need it...http://www.vtxcafe.com/showthread.php?t=33679 Then remove the airbox from the bike..There's three screws at the carb, a bolt near the bottom, and one near the top...Also there's the crankcase vent tube in the back of the cover, and the tube for the PAIR valve system if you haven't already removed that from the bike...Once you have it off the bike, the fun parts starts...I used a cutoff wheel in my die grinder, but I know that not everybody has an air compressor at their disposal..A cutoff wheel in a Dremel will also work, and as well as a hacksaw...I chose the easiest and quickest way I had....Using the grease pencil again, continue the line you already drew around the sides and back of the airbox....Now when you start cutting, cut ABOVE the line you just drew, especially on the back....If you cut too low on the backside, you'll end up allowing unfiltered air into the carb, and you don't want that happening...So you need to stay above the flat area back there...Once it is cut, it'll look just like this....
See that flat area at the top that I left?...Don't cut into that, or you'll be letting unfiltered air into the carb...Might take2-3 times to get it right...I cut it high all the way around, then went a little at a time until I could get it back on the bike with the gas tank in place back on the frame...I left it high on the backside, and when I remove the remaining mount locations (one gets cut off), I tilt the bottom out towards me so the backside will clear the tank bottom...Here's what it looks like with the tank off the bike...
Big difference in the open area between this and the two small openings that were formerly the intake openings...You can also see how the filter sits in there, and why it's important to leave that flat area intact...Now all that's left is to put it back together...The crankcase vent tube on the backside can be a pain to re-attach, but it'll go back on there...Here's what it looks like all re-assembled on my bike with my waterjet-cut airbox cover by Lazerdude...Can't even tell it's been cut with it all back together..
Now for a couple more items that need to be covered...Doing this allows alot more air to enter the carb and engine, so carb work is needed to keep the air/fuel ratio correct...Lots of other guys here have done this same mod, and I'm 99% sure that everybody here has followed Scar's carb tuning sticky to dial in their setup..We're all using 2 shims on the stock needle, and set the A/F screw to 2-1/2 turns out from lightly seated...With my particular setup (opened airbox cover + decap, I have the A/F set at 2-5/8, and could maybe use another 1/8 turn which I'm gonna try this year....
Also I recommend getting a K&N filter...The stock one is a paper filter, and with the top chopped off the airbox, there's a decent chance of water getting in there if you get caught in the rain...Water will destroy the stock filter, but a K&N will be fine for the life of the bike...
Best thing about this mod is nobody can tell it's not 100% stock...But it'll flat haul ass when the carb is dialed in to the increased airflow...Stealth mode at it's best!!..LOL!!...:mosh:
) Starts fine with no choke, just a twist of the throttle before hitting the button, and I have no complaints on driveability. SO... how much tinkering is required to get it right after the mod and how much improvement are we talking about? Also - I don't see how water gets in if you don't do the "waterjet-cut airbox cover by Lazerdude" - what am I missing? And, lastly, how does it sound sucking all that air and how noticeable is the sound? Hope that's not too many questions - just don't want to do it and then wish I hadn't. Had a Trans Am a couple of years ago that I did a lot of mods to and the more I did to it the less I drove it. Don't want to make that mistake again...
