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Jake's Big Shimming Modification

Captain Jake, the Mel Bay of Motorcycle Modification presents:


What follows is documentation and photographs of the shimming of the needles on a 2001 Kawasaki W650, though this should apply to all years. The intention is to pump up the flat spot in the middle of the powerband, which may be more prevalent upon the modification of the airbox.

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

Photograph courtesy of Jon Haddock© (2002)

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 shim your needles, please email me and let me know if this helped you, or if something wasn't clear enough.

One final important note. The needles are different from the left side to the right side. Be sure not to mix them up if you decide to do both carbs at the same time. The right hand carb has the longer needle. It is marked N8GN. The left needle is marked N8GJ.

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.

This is a shim. It's a small precision-made washer. This particular one is from Suzuki, part number 13382-44030. You'll need two of them, which cost me a little over 4 dollars (US). Darned expensive washers...

Don't forget to buy TWO of these. If you only purchase one, your W650 will only ride in very tiny circles in the direction away from the carb you've modified.


These are the tools needed for this modification. A 10mm spanner (wrench), a phillips (star) headed screwdriver, and your key.

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, undo these two bolts at the back of the fuel tank. Use the 10mm spanner (wrench) and put these bolts somewhere safe.


You'll find these two tubes underneath the right side of the fuel tank, held in place by two long metal clips. They come from the bottom of the tank and thread down through the frame to drain onto the ground. You can dislodge them from the clips with your fingers. Take note of the path they follow, between the carbs and down through the frame.

Note: California Spec models may have a third tube, which would not go between the carbs, but towards the front of the bike. If you have one of these bikes, pay attention to that hose, I don't have any photographs of it or where it goes.


Having memorized their path, pull them up and out of the frame and let them hang down the right side.


The bottom tube in this picture feeds fuel from the tank to the carbs. The top tube provides vacuum. The W650 uses a vacuum system to move fuel from the tank to the carburettors. Due to this fact, we can safely remove these hoses without having the tank douse the carbs and us with fuel.

This hose uses a metal clip to secure it to the petcock nipple. On my bike the parts that you squeeze were rotated behind the petcock. Just grab them with your fingers and move them around to the front if necessary. Squeeze them towards each other and the clip will loosen its grip on the hose.
Still squeezing the clip, slide it down the hose about an inch and a half to get it beyond the end of the metal nipple.
With that one clip slid down, you can now remove both hoses. I recommend putting a wadded paper towel beneath the hoses. This is a vacuum operated fuel tank, so as long as the petcock is in the ON position, fuel won't come running out when you remove the hose. However about a teaspoon of fuel will escape from the hoses when you remove them, which will be soaked up by the paper towel. Now remove the paper towel and CAREFULLY dispose of this highly flammable item.
Lift the back of the tank slightly (red arrow), while pulling back at the green arrow. The goal is to move the tank backwards about 2 inches.
This is the view after the tank has been removed, looking down on the frame, just behind the handlebars, as if you were sitting on the bike. These two rubber nubs indicated by the green arrows locate the front of the fuel tank by sliding into.... (see next picture)
these two metal cups on the underside of the tank. This is the view underneath the fuel tank, looking towards the back of the bike. Indicated by the green arrows here are two "C" shaped metal cups that slide over the rubber nubs in the picture above. It's a tight fit and they're heavily greased, so wear your gloves.

The tank should just lift off now. Keeping it in its normal orientation, place it somewhere soft, like grass or on a towel. A word of warning: the fuel tank may be surprisingly heavy, depending on how much fuel you have in it.

Do NOT set it down on concrete, as the paint on the sharp bottom edges of the tank would be easily chipped.

Yes, I noticed that in this picture it looks like some weird animal and I am already planning a story about it. Thanks for the suggestion.


And here is your bike without its fuel tank and seat. Naked. Don't stare, that's rude.

I thought it might be fun to go for a quick little spin around the parking lot with it in this shape as a very stripped down lightweight scrambler kinda thing. But then I realized that riding it while standing up and simultaneously pouring fuel from a cup into it's tiny fuel hose would be very difficult. But I digress...

Looking directly down on the carbs from overhead, you can see the left and right carburettors. Note that they seem slightly offset to the left. This appearance is somewhat due to the bundle of wires and their associated clip on the right side of the backbone. We'll start with the right side carb first.

This is the starter motor. If you drop carb screws, they will be magically drawn under the starter motor. Then the best way to retrieve the screws will be to pick the bike up over your head and shake it. Or you can see the next photo....


Place a paper towel under the carb area, so any dropped screws will fall off to the side of the bike. Do this for both sides of the bike. Really, this takes just a second to do, and I swear, if you don't you WILL drop the screws under the starter motor. And your hair will fall out.

Moving around to the right side of the bike you will find that there are four screws that hold on the chrome top of the carb. You will also notice that the back-left screw is very difficult to get to with a regular screwdriver. You can try to use a right-angle screwdriver or follow the next procedure to make your life easier. I strongly recommend NOT trying to get that screw loose using a regular screwdriver without the next procedure. These screws take a bit of effort to get them snapped loose and unscrewing, and they have soft heads. Using a regular screwdriver is likely to damage the head of the screw.
The part outlined in green in this photograph is the culprit. It holds the two drain tubes you removed earlier and routes a big wire bundle. It can be removed very quickly using your handy 10mm spanner. Note that the forward bolt has a ground wire attached to it. Don't forget about this. You'd be really dim to forget this.

Now with the green outlined part moved out of the way, you can insert your screwdriver behind the big bundle of wires and straight down onto that screw. It will take some downward force and a pretty good amount of torque to get these screws started. Please be sure to not let the screwdriver slip and damage the screwhead. Be patient and firm with the screwdriver and they will "pop", then you can unscrew them.


When you get down to the last screw, place your finger on top of the chrome top as you are unscrewing it. There's a spring underneath it that will be pushing up against the top as you are unscrewing the last one and you don't want that spring to pop the top off and go flying across your workspace, frightening your cat.

Indicated by the green arrow here is the spring in question. Once you have the last screw removed and safely stored with its three mates, you can slowly let the top come up.

Here you can see the fixture in the carb top that keeps the spring centered. Lift the chrome top off and set it to the side. I'm not sure if the top is directional or not, so I just put it down on a clean paper towel facing the same direction that it was in on the bike.


Gently pull the spring out of the diaphragm and place it with the carb top. I put my spanner on top of the spring so it couldn't roll away.

I found the easiest way to remove the diaphragm was to stick my index finger in the hole the spring was in and slightly cock my finger and GENTLY pull upwards. The top part of the diaphragm is very thin rubber and you do NOT want to damage it.

Pull it straight up, being very careful of the slim tapered needle that projects out of the bottom. Yes, this is THE needle, and this is as far as you will be disassembling this carburettor.


Gently pushing up on the needle from below will cause the plastic retainer to rise up out of the diaphragm followed by the needle.


Here are the two original parts and your new part, the shim.


Place the shim on the needle and slide it all the way back towards its head.

Put the needle and shim into the retainer and slide it back into the diaphragm. There's a tiny hole the needle must go through in the bottom of the diaphragm. The easiest way I found of "threading the needle" was to hold the retainer in my left hand with the needle pointing up towards my eye, and lowering the diaphragm over it, spotting the tip of the needle and guiding the diaphragm's hole over it.
Be sure the retainer is fully seated within the diaphragm.
The diaphragm assembly has a tiny hole on the bottom side in front of the needle. The larger version of this photograph shows it clearer. It should go towards the front of your motorcycle. Truth is, it will only fit in one way, but I don't want you trying to put it in backwards and scraping up bits.
Look at the high definition version of this picture. There's a tiny hole down in the carb that the needle will slide into when you put the assembly back in. Keep this in mind as you lower it into place.
Slide the diaphragm into place, gently, watching the wings go into their slots in the carburettor body. The rim of the diaphragm should lay perfectly on top of the carb body.
Place the spring down into the hole, ensuring that it goes around the plastic retainer, indicated by the green arrow in this picture.

Put the chrome top back on, watching to be sure you get the top of the spring onto the fixture in the center of the chrome top.

Holding the chrome top down with your finger, replace all four screws, tightening them snugly. When you have all four back in, go back around and make certain they're all snug, but don't tear up the screw heads.

Now you can replace the wire retaining clip, being sure you capture the wire bundle beneath it. Hey, remember the ground wire that goes around the forwards bolt? It's indicated here by the green arrow, and should be placed under the forwards bolt.

Congratulations! You're halfway done!

Now move around to the left side of the bike, it's time to do the same thing with the left carb. The procedure is identical, except that there is no wire retaining clip to remove.
Remove the four screws. I recommend using the technique shown in this picture for removing the screws on the inside side of the carb, to be sure you don't drop the screw down amongst a bunch of dirty parts. This would cause you to say things that would shock the neighbors.
The inside-right screw seems like a bugger to get at initially. Simply run your screwdriver behind big tube number 1, and shove tube number two out of the way to get at the screw. Be careful not to lose this screw down into the frame.
Again be careful when removing the chrome top, it has that big spring underneath that will, as your Mother warned you, snap out of there and spear you in the eye if you're not careful.
This diaphragm is also easily removed by inserting your finger into the center and lifting gently.
Lift the assembly out with care, it has that thin needle protruding from the bottom. Verify the location of the tiny hole on the bottom of the diaphragm assembly next to the needle, for reference when you put it back in. You want to be sure it faces the proper direction, though it can't go in the other way, again I want to be sure you don't score parts up tyring to shove them in backwards.
Place your shim on the needle. The shim here is indicated by the red arrow.
Make sure it goes all the way down the needle to lie flat against the butt end of the needle.
Place the needle in the plastic retainer and lower the diaphragm over it, guiding the needle out through the hole in the bottom of it.
Once again, here's that tiny hole. It should be on the side that goes towards the front of the bike.
Gently replace the diaphragm assembly, remember that you're dropping the needle into that tiny hole deep in the carburettor's guts and you don't want to damage the needle.

Place the spring back into the hole, around the plastic retainer.


Now when you go to replace the screws, the back-right screw goes through this little doohickey to securely locate this hose, hose number 2 from step number 41.

The screw and doohickey go together like this on top of the chrome top. Again slide your screwdriver behind the big hose to get a straight shot at the screwhead.

You're done with the carbs!

Helpful hint from Dennis Guggemos: the two rubber retainers that the tank slides onto should be lightly lubricated with grease or vaseline so the tank will slip on easier. Now go retrieve your fuel tank from it's soft location and place it generally in the right place, keeping in mind that you'll be sliding it forwards to seat it. Take the two rubber drain hoses and thread them between the carbs.
Have a look at this picture to get an idea of the routing of the hoses, though you memorized their routing before you pulled them out, didn't you? You may also take this opportunity to pose for a moment with your hand on your hip, for you are almost done modifying your own carburettors!
Place the two drain hoses back into their clips under the tank
Gently but firmly push the tank forwards so the cups engage the rubber nubs on the backbone of the frame. Look at the tab at the back of the fuel tank to gauge when you have the tank back in the right place, the boltholes should line up. Replace the two 10mm bolts.
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 power.