When gasoline dries out, it can leave a varnish film on whatever it was in contact with.  In some really bad instances, this residue can be a rather thick goop.  It's also possible some small particles and rust could have gotten into the small passages and orifices of the carb.  This means that parts inside a carburetor may not move like they are supposed to and small passages meant to meter air or fuel can become plugged.  This can lead to off idle stumbles, lack of power, poor gas milage, and many other maladies.  When this happens, you can try running some sort of cleaner in the gasoline, but this often won't help because the hole that the cleaner needs to get through is plugged and gas is only drawn through it when it's open.  Disassembling and cleaning the internal parts of the carburetor is the only solution.  Disassembling and cleaning carburetors can seem like a daunting task, but it is well within the means of most people if you're at all handy.  You do have to keep carefull track of parts and be carefull not to overtighten things on reassembly.  If after reading this you don't feel comfortable tearing down a carburetor, have a professional or a friend with carburetor experience do the work.  A six pack of beer can be a usefull enticer, but I recommend saving the beer until you're finished with the carbs.

A WORD OF WARNING:  Do not mix up parts between carbs.  All carbs are unique in ways that may not be immediately obvious.  I recommend only working on one carburetor at a time while keeping track of what position the carb was in on the airbox and paying close attention to how the linkages and tubes connect to the carb.  If you do more than one carb at a time, have seperate bins ready to hold parts for each carb.

The carbs can only be removed and reattached on the air box moving from one end to the other, so all the carbs will pretty much all have to be off the plenum at the same time.  It's best to mark their locations in a way that cannot be washed off by carb cleaner (which most pens can be).  I use a number punch to keep track of them.  If possible, snap a picture or two of the airbox with the carbs installed that shows how the assorted linkages, lines, and tubes are attached.

In order to completely clean the carb, it needs to be soaked in carburetor cleaner and complete disassembly is necessary to remove all rubber and plastic parts that could be harmed by the carburetor cleaner.  These parts include slider and needle, choke plunger, throttle shaft seal, float, float needle and seat, and pilot needle seal.

Conveniently, the parts that are most likely to cause problems can be cleaned without ever removing the carbs from the airbox.  Sorry, the airbox and carbs will have to be removed from the bike to gain access to everything.  Most problems stem from plugged jets or some problem with the sliders.  If you want to do a mini cleaning job, just do the sections that cover the diaphram and float bowl.

A quick test as to whether or not the sliders/diaphrams are OK is to manually push the sliders up and let them go.  If they snap to the bottom, there's a problem.  The sliders use a small air passage for actuation and this limits the speed at which the sliders should be able to return to the bottom position.  It's not a lot of delay, so don't expect them to take a second to bottom out, but there should be a noticable delay or dragging as it returns to the bottom position.

Parts needed:
 

 Spray Carb Cleaner: My favorite is B-12 Chemtool.  There's a bunch of cleaners out there, but some are pretty weak.  You want something that will disolve varnish fast.  As an example, many years ago I bought a can of carb cleaner that was good stuff and ate varnish/grease/grime very quickly.  It sat on the shelf a lot, but I finally used it up and bought another can.  The formula had changed and the new stuff was closer to water and didn't do a very good job.

Gallon can of Carb Cleaner:  This looks like a gallon paint can and has a soaking basket included.  This is used if you're cleaning some of the larger parts, especially the carb body.  I've had the same gallon for probably 10 years and have done many carbs with it.

Compressed air source:  Not strictly needed, but I wouldn't do the full disassambly cleaning without it.  You might be able to get by with a can o' air (usually available at computer or photography supply houses) but a compressor with a blowgun would be better.

O-rings and Float Bowl Gaskets:  The floatbowl gasket and O-rings around the fuel tubes are often in poor shape after being exposed to gas for many years and may need to be replaced.  It is not always necessary to replace these parts, but they should at least be inspected for cracks or problems or you might be pulling the carbs back off again shortly.  A source for the float bowl gaskets can be found in the float bowl section later in this page.

In general, carb cleaning products are toxic and can irritate skin.  Take proper precautions while dealing with it.

Note: the guinea pig carburetor used in this article for the pictures was my first powdercoating experiment.  The carb looked like it was underwater and was in generally poor shape.  The powdercoating makes it look pretty nice in spots, but others don't look too hot due to corrosion damage on the carb or problems curing the powdercoating.  That's why the carb may look a little odd in places.

All of the pictures in this article can be clicked on to view a larger version.

Disassembly

Most things come apart fairly easily, but there are some tricky parts.

Diaphram and metering needle assembly:  The slider/diaphram is under the large circular cap on the carb.  Take out the four screws that hold the cap on while putting pressure down on the cap.  There is a large spring underneath the cap that, while not that strong, could launch the cap or the small screw you're taking out of the cap.  Remove the cap and lift out the slider/diaphram assembly.  Be very carefull to not damage the diaphram as these parts are very expensive.  Inspect the diaphram for any tears or holes.  Remove the metering needle by using a screwdriver or nutdriver to push down and turn the nylon retainer at the bottom of the slider 60 degrees.  Do not set the slider assembly down on the diaphram.  If the needle is still in the slider, the diaphram can be inverted so the assembly looks like a mushroom and the assembly can be set on that end without touching the diaphram to anything (see picture).
 
 

Float Bowl:  Remove the 4 float bowl screws and take off the float bowl.   Inspect the float bowl gasket.  If it has compressed until it doesn't protrude above the channel rim, it will need to be replaced.  The cheapest place I've found so far for these gaskets is  Cycle ReCyclers  in Indianapolis (the gaskets are new KN parts).

Remove float, fuel needle, fuel needle seat, seat washer, main jet, main jet holder, and slow jet.  All other parts are pressed in and non-removable.  Apparently some of the early versions of these carbs also had a removable start jet (smooth tube parallel to the other jets).  If you have a smooth tube with no slots or a hex head, do not attempt to remove it.  Note, the main jet (part with a screwdriver slot) screws into the main jet holder (part with a hex head) and should be seperated from the jet holder for easier cleaning of both parts.
 

Choke Assembly:  The choke lever shaft is held in place by a small plastic tube.  You need to remove the nut at the top of the actuating shaft, the shaft actuating lever, and pull the tube out.  This will allow the lower part of the shaft to come out of the carb, however the choke plunger will keep it from coming all the way out.  In order to get this out you need to unscrew the choke plunger housing and lift it up while rotating the retaining fork until it's free.  Then the rod should lift out and then the choke plunger and housing should lift out.
 
 

Throttle Shaft Assembly:  The shaft comes out easily once the throttle plate is removed.  Note the position and orientation of the throttle plate before removing it.  It only goes back in correctly one way.  There is usually a symbol stamped into the outer facing surface towards the top of the carb.  The throttle plate is held in place with two small phillips screws.  The screws that hold the plate in have been staked, meaning the end was enlarged after it was installed so it couldn't back out, therefore you must use a screw bit that fits these screws very tightly so you don't strip the screws on removal or insertion.  Let me repeat that: The screw bit used to take these screws out must be a very tight fit or you'll just strip them.  Some consider taking the throttle plates out as unneccesary because of the dangers of buggering up the screws, but I wouldn't soak the carb body in cleaner without doing this step and have never had a problem as long as I was carefull in getting the screws out.  Once the screws are out, turn the shaft so the throttle plate is perpendicular to the throttle bore and carefully pull it straight out of the bore.

The throttle shaft should then pull straight out.  Some of the carbs have return springs and other linkages attached to the throttle plate shaft.  Keep track of where all springs, washers, and linkages go.  Using a small screwdriver, carefully pry out the cap that holds the throttle shaft seal in place without getting the felt seal in between the screwdriver and cap.  Be carefull to not damage the seal retainer ring or the felt ring.  As far as I know, these are not available as replacement parts.  (Note: the carb pictured was my first powdercoating experimental carb and the retainer ring has been powdercoated into place.  They normally look a bit different).
 
 

Pilot Needle Assembly:  If the carbs have never had the pilot needle adjusted, the pilot needle will have an aluminum cover over it that must be removed.  Even if it has been adjusted, the aluminum cover may have been replaced (Note: aluminum cover not shown in picture).  To get the aluminum cover out, drill a small hole through the center of the cover, but be very carefull.  The brass (read: soft) pilot screw is ~1/4" below the cover.  I recommend using some sort of stop on the drill bit so it cannot go in far enough to damage the needle (you won't like the cost of replacements).  Once the hole is drilled, screw something like a drywall screw into the hole a short ways and use it to pull the cover out.  Once the cover is out, you should see the pilot screw.  Use a screwdriver to carefully turn the screw in (clockwise) until it bottoms out while counting exactly how many turns it takes to get there to the nearest 1/8th turn.  Do not force the screw and only use light pressure or you may damage the screw threads, needle tip, or screwdriver slot.  Record the number of turns for each carb as you disassemble them.  Once you've measured the number of turns in, remove the pilot screw, spring, seal, and seal washer.

Cleaning

Carb Body:  I soak the carb body in carb cleaner with the diaphram opening up.  I use a gallon can of Gunk carburetor cleaner (available at most automotive parts stores).  There are other brands that can do the same thing.  These cans of cleaner usually come with a basket you can put the parts in so you can retrieve them after dunking without digging around the bottom of the can.  I've had the same gallon of cleaner for over 10 years and have probably done 20 motorcycle and auto carbs in it.  Yes, I can be a cheap bastard.  It's getting to look a bit dirty, but still disolves deposits fine.  If you have not completely disassembled the carburetor, do not soak the body or any parts with rubber/plastic  in the carb cleaner.  Carb cleaner can often damage plastic parts around the carb and is real likely to damage the felt seal around the throttle shaft.  I shake the carb around a bit in the cleaner to try and get the cleaner into the passages in the body.  I soak the body in the cleaner for about 20 minutes.  I then pull the body out and scrub all accessible surfaces with a toothbrush while running water over it.  Try to get water to flow through as many passages as possible.  Rinse it off very well.  I finish up by using a blow gun to blow all the water off the carb and out of the passages.

When this is done, use the spray carb cleaner on all of the orifices and try to spray through the passages as much as possible.  The one I'm most carefull to get is the metering needle orifice.  This has a tube that points down that can collect a lot of deposits in it.  I put the carb cleaner spray straw directly into the hole and spray until it comes out clear.  The cleaner will mostly come out one of the jet holes in the bowl.

Diaphram and metering needle assembly:  I spray some of the carb cleaner onto a paper towel and wipe clean the metal part of the diaphram assembly, but take care to not get any of the cleaner on the diaphram rubber.  This is easily damaged , expensive, and will probably not deal with carb cleaner very well.  Either soak or wipe off the metering needle in the same way.  If the diaphram really needs cleaning, I use Honda spray cleaner and polish.  I don't have any long term experience with this stuff as a cleaner, so I can't really say it won't damage the diaphram.  Inspect the needle.  I had one carb that had a glob of metal on the side of it that was a manufacturing defect.  I used some 600 grit sandpaper to carefully blend the glob back in with the taper of the needle.

Float Bowl Parts:  This is where things are most likely to be gummed up in a way that affects the bikes running.  Soak the main jet, main jet holder, and slow jet in carb cleaner.  These have a number of small holes that are easily plugged up with small particles or goo, causing all sorts of riding problems.  Scrub them with a toothbrush, trying to push some bristles through the holes.  Do not use a piece of wire on the holes.  The jets are brass and a piece of wire will likely change the size of the hole, meaning you just rejetted your bike in a completely unknown and probably bad  way.  The tough one is the slow jet (the one on the right in the picture).  It has a very fine hole about midway down the body that can be tough to clear if it's plugged.  The hole is so fine you're not likely to be able to find a piece of wire fine enough to push through it, so don't even try.  Try to spray carb cleaner through it in both directions with the spray straw.  If all else fails, use a fine sewing needle and alternately try pushing it lightly through the hole from each end (don't attempt to push it through) alternating between the needle and carb cleaner.   Spray carb cleaner through the rest of the holes and inspect to make sure they're all clear.  Make sure the float bowl cover is clean.

Inspect the fuel needle seat and screen.  Make sure the spring loaded pin in the end of the needle moves freely.  If the screen looks clean, I wouldn't mess with it.  If the screen is dirty, I clean it with quick blasts of spray carb cleaner, but try to not overdo it.  Make sure the seat that the needle tip sits on is clean and free of any particles.  If the seat is dirty or has deposits, you can try carb cleaner on the end of a Q-tip to clean it off.  If that doesn't work, a bit of brass cleaner on the end of a Q-tip can be used as a last resort, but the more you mess with the needle/seat, the more likely it won't seal when reassembled.  Make sure the needle tip is not deformed.

Carb Reassembly

Throttle Shaft and Throttle Plate:  Place the felt ring into the retaining piece and carefully tap it into place, insuring you don't pinch the felt ring.  Carefully insert the throttle shaft, making sure the felt ring goes around the shaft and doesn't get forced into the shaft bore, and push the shaft all the way in while slowly twisiting it so it doesn't pull the felt seal in with it.  SabMag carbs have a small nylon washer that goes between a C-clip and the carb body.  Do not forget this washer as it's very important for aligning the throttle plate.  Hook up any springs or linkages that tie to the carb body now.  Carefully insert the throttle plate in the same position it was originally in and carefully close the plate with the throttle shaft while wiggling the throttle plate and throttle shaft around until the screw holes are lined up and the plate is completely blocking off the bore with the throttle shaft as far as it will go in the closed position.  Never force it or put a lot of pressure on it.  Put the throttle plate screws in place and lightly tighten.  These will be tightened permanently later.

Shimming Needles:  You may have heard the term "shimming needles" when people talk about modifying these carburetors.  I mention it here for illustrative purposes only.  Shimming needles involves putting washers on the  metering needle before inserting it into the diaphram assembly so it is raised a little farther out of the metering orifice, causing a richer condition through the entire RPM range.  Unless you are trying to correct a lean condition or some sort of drivability problem, I don't recommend this.  This modification is likely to lower fuel milage and could cause other drivability problems if it isn't done correctly.  If you don't mind yanking your bike apart repeatedly to experiement with this, it can  cure things like flat spots in the engine RPM range.  I'll yield to others with more knowledge in this area on when these washers should be used and how thick the washers should be.

Diaphram and Cover:   Insure both the diaphram slider and diaphram bore are clean.  Place the needle and needle retainer back in the diaphram assembly.  Sometimes the diaphram assembly goes in easily, sometimes it's a real bitch.  Some diaphrams were either manufactured smaller or have shrunk over the years so the seal on the edge of the diaphram doesn't like to stay in the seal groove.  For those that are a bitch (the carb pictured here was definitely in that category), here's what I do:

First make sure the diaphram assembly slides up and down in the bore without dragging or sticking.  If it does, figure out what's causing the problem and fix it.  Sometimes stickyness can be caused by deposits in the metering orifice, so if you didn't clean it before, do it now.  Slide the diaphram assembly into the diaphram bore with the diaphram pointing down so it looks like a mushroom.  Put a small object like a small screwdriver handle in the carburetor throat so it stops the diaphram slider from going all the way down so the slider's top is slightly higher than flush with the top rim of the carburetor body.  Push the diaphram assembly into the bore, making sure the needle goes into the metering orifice, until it rests on the object in the carb throat.  The diaphram should compress with a fold like the one in the photograph.  Run your finger around the edge of the diaphram pushing it into the groove until it all remains seated.  It might take awhile and you need to get the tension around the edge as even as possible.  Make sure you line up the air bleeder hole seal.  Carefully put the spring into the diaphram assembly and place the cap over the diaphram.  Make sure the cap air bleeder indentation matches up with the bleeder hole and make sure the diaphram doesn't pop off the seal lip before the cap is squarely in place.  Insert the screws and tighten without letting the cap leave the surface of the carb body.  If this isn't done correctly, the diaphram edge could get pinched and you could damage the diaphram and/or cause some wierd drivability problems that are very tough to track down.  Remove the object you put in the carb throat and push up on the slider with your finger and let it go.  It should move smoothly up and return to the bottom position smoothly when you let go.  It should NOT snap back down to the bottom of its bore.  If it does, it's likely that the diaphram edge is not seated correctly. 

Note:  If you didn't take my advice before and didn't keep track of what part goes with what carb, the diaphram springs between front and back carbs are different.  The springs for the front carbs should be slightly longer than the rear carbs.

Choke Assembly:  Put choke actuator fork into slot of the choke rod, insert the top end of actuator rod through the solid shaft guide, and wiggle the choke plunger into place moving the whole unit downward as you go.  Screw down the choke plunger cap, insert the actuator rod sleeve, center the actuator rod spring, and screw the actuator rod end on.  The choke rod should move out smoothly and should pull itself back in until it's seated without assitance when using the actuating lever.

Pilot Screw:  Place the seal and seal washer into the bore in that order.  Look down the hole with a flashlight to insure they are both square at the bottom of the bore and the metal washer is on top. Put the spring and then the pilot screw into the bore and carefully tighten it until it bottoms out.  Turn the screw out the same amount that you originally measured during disassembly.  If you don't know its original position, look in the bike manual for the correct number of initial turns.  The caps that were originally pulled out are destroyed in removal, but I found some 1/4" Nylon caps at the local True-Value in the miscellaneous parts drawers that seals this hole up nicely.  I've been told dealers stock replacement aluminum caps.  Do not close these holes off until final running adjustments have been made.

Float Bowl Components:  Screw in pilot jet, main jet holder, main jet, and float needle seat with aluminum gasket/washer.  Don't overtighten.  Place needle clip over float tab and install float.

When you check the float level, you need to tilt the carb so that the float tab just touches the needle pin and the needle tip is fully seated.  It should not compress the pin in the end of the needle in any way.  If you check the height with the carb inverted, the weight of the floats will compress the pin and you'll get a false reading.  Check the float level by putting a precision ruler on the deck of the carb base and sighting across a line across the bottom of both floats.  If you have access to both sides of the carb, check the float level on both sides and use the average if their levels are different.  The carb in the picture's float level is a tad below 7mm.  I'm not sure what bike this carb is from (it was a V45 of some sort), but the float height is probably low.    There's two ways to correct the float height.  One is to bend the tab on the floats.  This works, but a small bend creates a large difference and if you're not carefull you can damage the plastic float.  The other way is to change the washer height under the needle seat.  I've used some Suzuki needle seat gaskets (they're very thin paper-like material) to shim the needle seat in the past.  I don't know the part number of the gaskets, but a dealer could probably come up with some if you take in the needle seat.  Another thing you need to watch is that the float tab stays relatively perpendicular to the needle pin.  If it gets cockeyed, it could cause binding in the needle.

Make sure the gasket groove in the float bowl cover and the carb body base is clean.  Any debris could cause a leak.  Screw on the cover, and carb reassembly is done.

Reassemble Carbs on Airbox:  Make sure the sealing groove at the mouth of the carb is clean.  Place the carb back on the box making sure the rubber plenum gets seated in the carb groove.  Inspect all of the O-rings for cracks or other problems as you go and replace as necessary.  Of particular importance are the O-rings on the fuel supply tubes, as these are quite likely to have deterioriated in the presence of fuel for many years.  I can't be specific about the exact reassembly steps because each bikes airbox is unique and the steps differ for different bikes.  You need to place the carbs back on the airbox moving from one end to the other hooking up fuel supply tubes, breather tubes, and linkages as you go.  Make sure you get the throttle shaft linkages in place correctly as you go.  Some of the choke linkages that go between carbs have to be on the carb before the carb is put in place.  Make sure you get the fuel inlet tube in the same position.

Centering Throttle Plates:  After all linkages and springs are back in place (do not forget the springs that go between the throttle shafts of adjacent carbs), back out the adjusters for the throttle plates so the throttle plate can be closed completely.  Loosen the throttle plate screws and wiggle the shaft so it moves to its natural full in position.  Wiggle the throttle plate until it's fully closed and centered in the bore.  The idea here is that the throttle shaft is positioned by the C-clip/nylon washer on the outside of the carb and the throttle plate is perfectly centered and not touching anything in the throttle bore during normal operation.  Tighten the throttle plate screws securely while continuously making sure the plate is centered on the throttle shaft.  Do this for each of the carbs.

Bench Synchronizing:  If you look in the bore of the carbs, you see some small holes that the throttle plate passes as it's opening.  I am not talking about the large hole midway from the carb mouth to the throttle plate.  I'm talking about the hole right at the base of the throttle plate (you'll need to look at the blowup of the picture to see it).  Adjust carb #3 with the adjuster knob until the first hole is just fully visible.  Adjust the adjustment screws on the remaining carbs until their plates are in the same position.  Open and close the throttle a few times and check the adjustment.  Back out the adjuster knob the whole way and carefully open and close the throttle plates so they bottom out.  Readjust the knob so you can just see the hole in carb 3 and check the position of the other plates.   If it looks good, move the main adjuster knob out until the plates are just open and you're done.  There should be no dragging or binding when you open and close the throttle plates.  A real syncronizing will have to be done once the bike is running, but this should get it pretty close.

Putting Airbox/Carb Assembly Back on Bike:  Put the carbs back on the bike and reassemble.  Easy, right?  Quit throwing sh%t!  Ok, my way of putting these #@$% carbs back on the bike.  I put the forward boots completely on the heads.  I put the rear boots on, but turned upwards so they're barely completely around the intake tube on the bike.  I fully seat the front carbs into the front boots and touch the rear carbs to the boots.  They should be very close to going straight into the boot opening.  Put light pressure on the boot with the carbs and use a thin screwdriver to sweep across the top between the boot and carb.  The boots should go over the carb bore and a slight pressure down should seat the rear carbs.

Finishing up:  Once the bike is reassembled, the bike needs to have the carbs balanced, the mixture set, and the idle speed set once it's running.  The bench synchronizing probably left the throttle plates closed too much to idle normally, so you'll need to goose the throttle to start the bike and keep it running.   Better this than have it jump to 4000 RPM on a dry startup.  Once it starts, adjust the initial idle setting.  The final adjustments are an iterative thing, where you'll probably have to do each of them more than once because the adjustments will affect each other.  But, that's a topic for another time.