Jake's Big Airbox Modification

Captain Jake, the Mel Bay of Motorcycle Modification presents:


What follows is documentation and photographs of the airbox modification on a 2001 Kawasaki W650. The intention is to clear away an obstruction in the breathing path. This is also known as the "Firewall Mod". Note: The work is only performed on the right side airbox.

Use this information at your own risk. This modification may violate your warranty. Most of this is gathered from various sources from the WRiders Group especially Jon Haddock, cheers Jon!

This information is provided in the simplest, clearest way I can, with lots of pictures, because that's the way it works for me. If a picture isn't clear enough for you, click on it and you'll get a MUCH larger picture with better resolution. I strongly recommend looking over this entire document before you attempt this modification. There are things pictured here that I wish I knew about before I ran into them. I have one request: If you use this information to modify your airbox, please email me and let me know if this helped you.

When working on anything mechanical, I keep a big box of powdered latex gloves handy. At the end of the job you can just throw them away, no hand-washing necessary. Or in this case, I could take them off, use my camera, then put a fresh pair on. Or, as I was working I needed bare fingers, so I took the gloves off, then when I needed to take a picture, I could put on one glove and use the camera without getting it greasy. They're available at many warehouse stores.
First step, remove the seat. On the left side of the bike there is a keyhole, use your ignition key here and the seat will click. Lift from the back of the seat and slide it backwards slightly, it will lift right off. I have a custom seat, so your seat may look different but the keyhole is in the same place. Place the seat somewhere safe where you won't kick it. Don't place it on concrete. That would pinch the seat material between hard concrete and the hard edge of the seatpan.
Next you remove the sidecover. Go around to the right side of the bike. There are three screws that secure this to the bike. The first two are on the side and are indicated by the two red arrows. I recommend using a large head screwdriver to avoid damaging these screws.
The third screw is accessed in the underseat area. It is the exactly same type of dark screw you just removed and is indicated here by the red arrow. Remove it and the sidecover can be wiggled and slid out. This part is largely cosmetic and can be scratched easily in the workshop environment so put it away somewhere safe with its three screws. Keep the screws with it or just chuck the screws under the workbench right now, which is where they'll end up if you don't put them somewhere safe right away when you remove them.

I'll be using the same picture a couple of times, there are many things to do here.

Unclip the wire connector at the top, highlighted with the purple arrow, just makes it easier if you move the wire that goes to the left out of the way.

Warning, the part outlined in blue is the air filter and is the reusable type so usually contains filter oil. You are wearing appropriate clothing, right?

The green arrows indicate a circular spring that goes around the carburetor air horn. Just roll it out of its groove towards the back of the bike.


The yellow number 1 indicates a small tube underneath the airbox (the big grey thing that occupies the majority of the picture). You can't see the tube in this picture. There is a small wire clip on the tube. Squeeze the ends of the clip together and pull it down the tube. You may be able to remove the tube at this point.

The yellow number 2 indicates a larger tube. Squeeze the goldish clip and slide it down the tube. You may be able to pull the tube off at this point. If you can't, you should be able to as you move the airbox out of its place. There are detailed pictures of the tubes below.

Remove the three screws indicated by the red arrows. The top one and bottom one are identical and should have little round washers with them. The middle screw is sunken in and may require a longer screwdriver. This is the very long screw.
Next, remove the tiny silver bolt as indicated by the red arrow, it's under the seat area, just behind the gas tank. On mine it was installed mind-buggeringly tight. I wrecked a couple of tools getting it to snap loose.
Now you'll wiggle the airbox out. This takes some doing. It probably won't pull straight out, so try moving up, down, left and right. It's rides on two long posts that run across the bike from the right side of the bike to the left. Don't wiggle too far, you don't want to break the posts.
Also watch at the top where there is a post that the silver bolt screws into. It will snag the wire bundle running across the top of the area. The thumb in this photo shows where the post sticks up behind the wire bundle. You must also slide the rubber tube at the front off of the carburetor air horn.
Then there are the two tubes that attach to the front and the bottom of the airbox. Here is the front hose. You can squeeze the goldish clip with your fingers and slide it towards the front of the bike, down the hose. As you move the box towards you, you'll be able to slide the hose off the front of the box.
This one is a little easier. This little drain tube is on the bottom of the airbox and has just a tiny wireclip holding it in place. You can easily squeeze this little wireclip with two fingers and slide it down the tube. This tube should just slip down off the box with a light pull.

Having disconnected the tubes, cleared the rubber cone off of the carburetor and gently moved the top post under the wire bundle, you can just pull the box straight out. Be certain to pull it straight off, as it's travelling along two long plastic posts, visible in the next picture.


This is what the bike will look like when you've successfully removed the airbox.

The red arrows indicate the two posts you're working along, and the yellow arrows show the disconnected tubes.

On the front side of the airbox you will find 5 screws, pointed out here by the red arrows. Remove all of them and put them in a safe place. Do not remove the screws nor the big bolt that go UP into the bottom of the airbox. That's for draining the airbox. All of these screws are identical, so don't worry about mixing them up. You can also remove the air filter at this point and put it somewhere safe away from your work area, which will soon be covered in plastic shavings.

Turn the airbox over and remove all seven of the screws from the backside, most of them indicated by the red arrows. All of these are identical except for two, which are longer. They are marked in this photograph with the green arrows. Note that the airbox is upside down in this photograph.

Now you'll have to pry apart the front and back halves of the airbox. Look for someplace along the seam that is a little loose. I found one and was able to work a piece of scrap metal into the crack to widen it, then work it along the seam.

The two halves have a thin strip of rubber between them to help seal them together, indicated by the green arrow in this photo. Don't rip it. This prying apart business can be a pain, as it seems the box halves are also lightly glued together. Keep at it, be patient.


Set the back part aside, you'll be working on the front part. This is the front half. The big blue number 1 indicates the air filter. The big blue number 2 indicates the "foam-covered tube".

The yellow arrows show the air flow to the right carburetor. The air gets to the left side carb through the holes in the back half of this airbox, visible in photo 16. Note the tiny little space it has to flow through after coming out of the foam-covered tube. The space is small and requires the air to make a very sharp U-turn. This is what we're going to correct.


First, removed the foam covered tube. Place it on your desk and put pencils in it, for it will never again see the inside of your airbox! The wall that we're going to remove used to hold up the bottom of the foam covered tube, indicated by green arrows.
Here's another look from a different angle. The entire shelf that's outlined in green is what we're going to remove.

And here the operation begins. I used a Dremel Multipro at 30,000 rpm and a cutoff wheel. I made the initial cut across the back side, holding the tool very tightly to prevent it from running off into the right wall.

Next I turned it 90 degrees and cut from the outside in. The cut from the open side to the backside MUST be made carefully, as you're cutting alongside a screw receiver. If you cut into this, the screw that goes in that hole may not grab well.


The shelf has been cut away.

The green arrow indicates the area that you must be very careful cutting near. Do not cut any closer than the green line to this tube.

I cut away the majority of the ribbed back wall, and the ridge that runs from top to bottom. Using the flat of the cutoff wheel I smoothed ridges on the back down the best I could, then rounded off the screw receiver tube. A word of warning - a Dremel Tool running at 30,000 rpm is a very dangerous thing. It got slightly away from me and cut a nice semicircular gash in my thumb. Rubber gloves will not protect from this type of assault. I was going to post a picture that I took of this as a warning, but it's not pretty. BE CAREFUL.

Now the air path that was 1 inch wide is about 4.5 inches.


Now take the top half and wash it out VERY thoroughly. I used Simple Green to wash it. You MUST remove all the plastic dust that the cutting has created. If you feel that this may not be that important, consider feeding just a half teaspoon of fine plastic shavings into your carbs. That would be VERY nasty in the combustion chambers. Clean it well. Then dry it well, I recommend compressed air to ensure the water is out of the crevices and screw holes.

Now you begin to reassemble the two halves. Here is the rubber sealing strip that goes between the two parts. Mine was glued to the flat edge of the top half, while the back half has a channel that runs around the entire perimeter that the rubber strip will drop into. Place the two parts together and go all the way around the end pushing the rubber strip into the channel as the parts come together.

Make one final check to ensure the rubber seal is in place all the way around, and push the parts together. Replace all of the screws. Don't forget the two longer screws. They go in the two holes that are visible in the middle of this photograph. Tighten all the screws pretty well, but not so tight that you strip out their holes, the receivers are only plastic and seem to strip easily.

Push the airbox back into place in the bike, being very careful to get the long plastic rods into the holes in the back of the airbox. Take your time, this is important. As you wiggle the airbox back into place, you'll have to mush the rubber cone to get it around the air horn of the carburetor. I started mine with one finger.

When you're putting the airbox back, you may hit several points of resistance. That means it's hitting something solid. It will not help to push harder, locate the source of resistance and move it out of the way, or maneuver the airbox around it. One noted tough part is the post on top of the airbox.


Then placing a finger on each side and pushing back towards the airbox with your fingers, use your palm to push the airbox into the frame of the bike. This should get the rubber cone mostly onto the air horn.


Here's a problem I ran into on reassembly. I found that it hangs up on the left side, where you can't get your fingers to pull it over the horn. I found that you can insert a screwdriver where the green arrow is and push the rubber cone over so it slips over the far side of the air horn.


Once you've checked to be sure the rubber cone is properly seated around the air horn, work the circular spring back into its groove near the carb, then push the chrome ring back against the airbox.

Reattach the drain tube at the bottom of the airbox, then squeeze the little wireclip and scoot it up so it holds the tube on the bottom of the airbox.
Move to the front of the airbox and reattach this hose. This can take a little wiggling, as this one's a little firmer. I found it necessary to pull the airbox towards me a little bit to get the right angle to get the tube back on. Squeeze the goldish clip with your fingers and slide it back towards the airbox to secure the hose.
Replace these three screws indicated by the red arrows. The top and bottom ones will have flat washers, and the middle screw is a very long one.
Replace the small silver bolt on top, tightening it down firmly. This goes into a metal receiver on top of the airbox post.
Reconnect this small wire bundle. Failure to do so may result in your motorcycle exploding when it reaches 88 mph. Or maybe not, but it looks important, so snap it back together anyways.
Gently put the sidecover in place, working it over the two wire bundles at the top. Replace the chrome screw and washer on the left side, then the dark screw and washer at the bottom.
You should have one big dark screw left over. Put it here and tighten it down. This is under the seat, behind the gas tank.
Retrieve your seat from its safe storage place. Note that it has a tongue at the front part. Angling the front of the seat down, slide the tongue into this hole and gently push down on the rear of the seat. You will hear a soft click when it's back in place. Attempt to lift the back of the seat, then the front to be certain it's on securely.
Your bike should now be back together. Go for a ride. Enjoy the additional power. You may find that this modification leads to the installation of shims under the needles....