How to Clean/Rebuild a Motorcycle Carburetor

I was actually trying to avoid making this post mainly as there are already many detailed write-ups floating about the internet about how to do this procedure, but seeing as how I've had several requests recently for carb fixing (to pass the new emissions tests in Korea), I figured I ought to put together my own how-to for the newbie shadetree mechanic.

Disclaimer: This will require both a degree of physical strength and delicacy in manipulating the parts on your motorcycle. It is likely that various rubber bits have hardened and the aluminum carburetor bodies have experienced some light corrosion causing many of the screws to be stuck in place. Simply put: you will need to be dexterous. Even then there is no guarantee that you will not damage parts during their removal, so be prepared for your bike to experience some downtime before you embark on this.

Tools Needed: A FSM (factory shop maunal) for your bike that lists the specifications for your carburetor, Screwdrivers (short and long, flathead and phillips/JIS), potentially an impact driver, a stiff brush, carburetor cleaner, lime juice, water, a pot & stove, and a bucket or two.

A cat may also be needed... for what I'm not yet sure...

Step One: Removal 

The difficulty of this part will depend largely on the bike in question. It might be easy, but if it is tricky it will be so for different reasons on different bikes. For example the carburetor on a scooter will be hard to remove because of the large amount of bodywork in the way that will need to be removed. A sportbike will have similar difficulties compounded with the fact that you are likely not removing a single carburetor but (probably) a rack of 4 carbs. Furthermore any bike with a single frame member under the gas tank will have difficulties in removal coming from the need to slide the carb rack out of the side of the bike (see above pic).

Carb rack from a CB400SF

While the specifics of each situation will vary significantly. The basics are the same. The tank and fuel line will need to be disconnected/removed. Carbs should be drained if possible via the drain screw on the underside of the bowl. Then the collars for the airbox/carb inlet will need to be removed (on the carburetor side, NOT the engine side). Following this the throttle cable(s), and choke cable (where applicable) will need to be disconnected. For slide carbs this will involve removing the entire slide, CV carbs may actually be even more difficult due to how cramped the area is where the cable attaches.  Lastly any PCV hoses or other items obstructing the removal of the carb will need to be disconnected/moved.

Again depending on your application after these steps are preformed the carb may just fall out of the bike, or you may have to brace the frame as you wring out every ounce of strength to pull the carbs out of the inlet boots and off the bike. A large amount of brute strength may very well be needed here.

Step Two: Cleaning / Disassembly / Reassembly

The next step I recommend before you begin to take it apart is to actually use some degreaser or liquid soap, and a stiff brush, and then clean the outside thoroughly. Otherwise, chances are good that you will get some of the grime on the outside to dirty up the inside of the carbs which are (hopefully) comparatively cleaner. Once the carbs are squeaky clean on the outside, it's time to see how bad they are on the inside.

Begin by taking apart the diaphragm side of the carburetor. On slide carbs this will already have been done with the slide likely still attached to the motorcycle. On CV carbs the diaphragm will be spring loaded. Be careful not to lose track of these parts. CAREFULLY, remove the slide and diaphragm (these parts are usually integral, but some carbs may have a separate diaphragm and slide). Inspect the diaphragm and its gasket for pinholes, tears, or anything else out of the ordinary. As you remove the slide the jet needle will come out as well. You can not be too careful with this part. It is machined with a high level of precision, so if it is bent, broken, or dented your carburetor will not work. It's a good idea to pretend you're removing a cooling rod from a nuclear reactor, and use an appropriate amount of care.


Unless you are doing modifications to the carburetor its a good idea to leave the jet needle alone, what you want to do here is clean the sides of the slide and the corrosponding surfaces of the carburetor body. This might actually prove to be a difficult task, because while carburetor cleaner is perfect for this, it can easily damage the diaphragm, or any other rubber gasket for that matter. I recommend applying the carb cleaner in a manner that will not let any of it touch any bits that aren't metal. You will get a good idea for how much cleaning is needed here based on how grimy the slide felt when you removed it. Pay attention for any wear that goes beyond discoloration. Any gouges or scrapes in the metal will cause the carburetor to function poorly and should be removed with a light abrasive. After all noticeable carbon deposits are removed reassemble the carb the same way it came apart.



           Repeat this process for multiple carbs, and if possible use compressed air to test the functioning of the diaphragm. The slide should move up and down quickly and come to rest with a light click. Binding, or irregular movement can be see or heard pretty easily. As an aside, I do remember when a lapse in judgement led me to apply a little WD-40 to a slide before reassembly. This caused the slide to move too fast and caused a rich misfire that required the carb be cleaned again.





 Next on the list is the bowl side of the carburetor. This requires a little more effort in cleaning, but thankfully a little less risk. Unscrew the bowl on the bottom of the carb. Underneath you should be greeted by a plastic float. This is connected to the float needle, and in held in place by an easily removable dowel pin.

Under no circumstances should you push down on the float here as this could damage the float needle. The pin on the float needle that the float sits on is spring loaded, and its operation should be checked. With the float, needle, and pin removed it should be easier to clean out any deposits in the carburetor body and gain access to the jets. Oh yeah and before I forget, be sure to clean out the bowls as well. There is bound to be deposits of rust and varnish (old congealed fuel) in here that might (or did) clog the carb, or clog the drain screws which should also be removed to aid in cleaning them. Again be careful with what you get carburetor cleaner on here.

Next come the jets. Before you start, check the FSM so you are aware of the size of each jet. They will look identical unless you check the number stamped on the side (or top) of each of them. If you get them mixed up it will cause problems somewhere down the line. Begin with the main Jet (the big one that sticks out the most). A flathead screwdriver should remove it, but try not to damage this as it is made of soft brass, if it is scarred or nicked it is not the end of the world, but it may cause other problems in the future. The first thing to do is check the aperture. To do this hold it up to a light source and look through it. You should see a perfectly round hole, if it looks oval-ish or semicircular it will need to be cleaned (did you wonder why the can of carburetor cleaner is under such high pressure? It's to blow clogs out of these holes). Use some compressed air to clean out any liquid from the jet, and re-check it. It might be a good idea to check and clean the emulsifier tube that the main jet sits in as well. You will need a socket or wrench to remove that one. The only other removable jet should be the pilot jet. This jet is longer than the main jet and should come out in the same manner. It might be a little more difficult to clean as the aperture will be smaller. If you feel that you can't get these jets cleaned with carburetor cleaner alone, try boiling them for a few minutes with a little bit of lemon or lime juice added. The effect will be a light acid etching that should clean out any stubbornly clogged jets. Clear the jet with compressed air and re-check the aperture. As before do this one carburetor at a time.

When finished you will also want to check the float levels. With everything reassembled but the carb bowls removed all of the floats should line up perfectly both when the carb is upside down and when it is right side up. If this is not the case you will want to remove the offending float(s) and gently bend the tab (if possible) where the float needle seats so that the floats line up properly. Reassemble the carb. If the gaskets have swollen, try using a toothpick or something to poke the gasket back into place as you tighten down the screws. Be careful not to pinch any gaskets.

The last step is to check the mixture screw(s). On some bikes these will be sealed off and inaccessible. It is possible to access them be drilling out the sealed surface, but that is really beyond the scope of this article. If these are accessible, they'll be centered beneath the inlet. count the number of turns on the screw until it bottoms out lightly (if you tighten this you can seriously fuck your carb). Check the FSM for the appropriate number of turns out that these screws should be set at. Remove and clean them if possible, and when resetting them use the FSM spec if you have it, or if not, set them back to the previous amount of turns that you measured. If these are set wrong you may experience an irregular idle.

This one was shamelessly taken from google. You can see the mixture screws removed. The push-pin is for a trick to remove the broken parts if you stupidly overtighten a screw.

NOTE ROK MOTORCYCLISTS: Some of us have found the new emissions test particularly harsh and difficult to pass. After properly cleaning a friends carburetor, he still could not manage to pass the test (about 1000 ppm over the allowed mesurement for hydrocarbons). If you are in such a situation, a good way to help reduce emissions for the test is to lean out the idle circuit of your carburetor by adjusting the mixture screw slightly depending on how much you need to reduce hydrocarbons, and on weather or not your mixture screw is a 'fuel' screw or an 'air' screw. Aside from this be sure that the ignition timing is set correctly, or even a little advanced of normal.

Now, stand in awe of the smooth shiny new carb you've serviced!

Step 3: Installation / Testing

Installation is essentially the reverse of removal. To make the carbs seat into the inlet boots easier, use a little WD-40. Also, bear in mind that inlet boots or collars may have spun and will need to be reset to the appropriate position. With limited room sometimes this is not easy. I recall one carb that I serviced where the throttle linkage hung up on a screw from one of the collars. It wasn't the best of times.

Anyway, when it is all put back together take it out for a test! Chances are the idle will need to be reset once the bike warms up. Pay attention to the power delivery at different RPMs and at different throttle positions. A lack of drastic improvement isn't anything to be alarmed about, but any abnormal flat spots in your torque band or misfiring should be cause for a reexamination of the carb. Also be on the lookout for an erratic idle even after it has been reset. This would indicate a vacuum leak from a torn inlet boot, or incorrect installation. One last thing to be on the lookout for are any fuel leaks that might take awhile to be obvious.

Hopefully, after this procedure the engine will feel more responsive, have greater overall power, get better fuel economy, and help you get past any pesky emissions tests you may be facing. A good procedure post carb rebuild/cleaning is to syncronize the carbs by balancing vacuum levels between the cylinders (multiple cylinder engines only need apply). This procedure can smooth out engine vibrations, stabilize idle, and improve fuel economy.