Cleaning carbeurators on 1982 XS400 -- Day Two

Now that you have removed the carbeurator, it's time to clean it.

First, a quick tour of the outside of The Heart of Darkness.

Anterior view. The fuel line (5) simply lets gas drip into the float bowls (other picture). The carb throats (1) have butterfly valves (1) to control the output of gar-air mix into the engine. The throttle cable comes through a bracket (4a) and into the throttle lever (2). The choke cable goes through a bracket also, connecting to a barrel clip (4b) to pull on the choke lever (3a, 3b).

Posterior view. The fuel line (4) connects to the gas tank, and drips gas into the float bowls (2). The diaphragms (underneath 1) use air pressure to gasoline out of the float bowls, via a piston and needle (3).

The basic idea of a carb is this:

• Fuel drips down the hose and into the fuel line, into the float bowls.
• The float bowls fill with gasoline.
• Air pressure from the engine sucks downward on the diaphragms, and the diaphragms push upward on springs. These two forces are carefully balanced, so the piston-and-needle assembly allows gasoline to be sucked through a valve.
• The valve has little holes called "jets"
• The springs and needles are all carefully matched to generate the correct mix of gasoline and air, and can be thrown off entirely by any single thing being sticky, cruddy, too loose, or too tight.

So, let's get cleaning.

Step 1. Start by removing the diaphragm caps, to expose the diaphragms and the springs which hold them in place. Remove these and clean them thoroughly with carb cleaner. A few blasts should do it, but a soft toothbrush helps too.

The bracket that holds the throttle and choke cables. This is attached to the diaphragm caps. It's not necessary to clean it, but I like to.

The caps, the diaphragm springs, and the diaphragms.

Dorsal view of the sockets where the diaphragms used to be. Visible at the bottom of the socket is the "Venturi" valve, which you will see in more detail later.

Be sure to clean thoroughly. The whole system can be fouled up by sediments and sticky films that aren't necessarily visible. A soft toothbrush helps, as does a high-pressure blast of carb cleaner.

Tip: These screws tend to get stripped, and we did spend a half hour with the Dremel thanks to a stripped screw. My local hardware store had replacement bolts with hex caps, for 35c apiece. In the future I can use my socket wrench.

Step 2. Remove the float bowls. Clean them thoroughly. There will likely be some sediment or even rust, particularly at the bottom of the bowl. The bowls have a gasket to help seal them to the carb body, and if the gasket is old it will probably not survive the process. Fortunately, they're inexpensive.

The float bowls, and what's left of the gasket. They are not supposed to be brown.

Float bowls before and after.

When cleaning the bowls, also pull off the drain screw and blast it clean.

As with the rest of the carb, the general rule is Blast Everything Spotless Clean. Not a lot to say about the bowls, except that that's where the crud will be most visible.


Step 3. Inside the float bowl is a simple (but tedious!) mechanism consisting of floats and a valve, which works similarly to how a toilet works. It consists of:

• The floats. These act like toilet floats, shutting off the flow of gasoline when the bowl is full.
• A needle which is pushed upward by the floats, shutting off the flow of gasoline from the fuel line.
• A main jet, which sucks up the gasoline through the Venturi valve.
• A smaller jet called the pilot jet.

Start by removing the float pivot pin and then the float, to get a better view of the needle and jet. Blast these clean, inside and out.

Ventral view, and upside down. The floats (1) dangle into the bowl, and gasoline makes them float upward. These push into the needle (2, hidden) to prevent more gas from coming in. The pilot jet (4) should not be modified but should be cleaned. The main jet (3) will be removed and cleaned.

The float and pin have been removed, as have the needle.The needle has a screw and bracket, but they don't hold it in. I don't know why.

Step 4. Unscrew the main jet screw (the big brass one), and you can push the main jet (aka the Venturi) upward into the carb body. Again, keep blasting with carb cleaner, getting every trace of sediment and scum off of everything.

Be sure to clean the jets (the little holes) everyplace: from the bottom, from the top, all of the little ones in the Venturi. Some folks will stick a wire into the Venturi jets to scrub them, but that can damage or enlarge the jets, so I just rely on the blasting power of the can.

Posterior view. Having pushed the Venturi up out of its little "turret" from the float bowl, I see that it's covered in scum.

Step 5. Don't neglect the exterior of the carb! The interior is sensitive to crud, but the outside has springs and levers that can also get gummed up with dirt, oil, mouse hair, etc. You don't need to get every inch of the exterior spotless, but do spend some time cleaning all those moving parts.

Step 6. Go over it again. Yeah, I'm serious. Blast it, brush it, dry it, and keep at it. Even an invisibly thin film of crud can make a difference.

Step 7. Let it dry overnight.

Step 8. Put it back together. This really is, as they say, "the opposite of removal"

• Slide the Venturi back downward. Be sure to line up the notch and the peg, and to push it down all the way. Then replace the brass cap.
• Put the needle back under the float tang, reinstall the float, reinsert the float pivot pin.
• Put the float bowls back on.
• Put the needle and piston and diaphragm back together, and put them back into place along with the spring, then put the covers back on. The diaphragm will fit into a notch indicating the correct positioning, and it is sticky enough that you can push it down and have a few seconds to get the cover on too.

Step 9. Reassemble it. If you forgot exactly how everything went, refer to the previous blog post.

• Put the carb back onto the bike the same way you took it off: airbox and fender, four throats and clamps, choke and throttle cable, and tighten things down.
• Put your fuel petcock to PRI for a few seconds, and listen for the gas dribbling into the float bowls. Then turn it back to ON.
• Now try to start the bike.

It may not work. Invent some new curse words that would make George Carlin and Eddie Murphy flinch, and see our next exciting blog post... float bowl adjustment!