KEIHIN CARBURATORS FOR 4-CYLINDER HONDA MOTORCYCLES

Set of 4 Keihin carburetors marked 089A and used on 1976 CB550K

GENERAL NOTES:

All carburetors perform the same function: mixing air and fuel for supply to the combustion chamber (cylinder). The ideal carburetor would do this in the perfect ratio of about 15:1 all the time. I say “air and fuel” putting air before fuel for emphasis. Air is crucial to the combustion process. The fuel (gasoline) cannot burn without oxygen or will burn only partially if there isn’t sufficient amount of oxygen. In turn, your engine will run inefficiently. How well the carburetor takes in air is important.

Carburetors utilize the exact same principle of operation as the one employed in a spray bottle. When you press the lever on your Windex bottle, air from outside is forced in. This pressurizes the air inside. The pressurized air applies a force on the liquid inside, which then travels up the tube and out the spray nozzle.

It is the pressure differential that does the trick. Higher pressure inside the reservoir, lower pressure outside.

The reservoir in a carburetor sits at the bottom and is normally called a “bowl”. The fuel inside is at atmospheric pressure (since the bowl is open to the atmosphere via an overflow pipe). In order for the fuel to come out of the bowl and go into the combustion chamber, the pressure above the bowl must be made lower than the pressure inside.

Creating such a pressure differential is the chief principle in carburetor design.

When the piston in the cylinder starts moving from its highest to its lowest position it creates suction (like a syringe). With the intake valve in the cylinder head open, this suction pulls air through the carburetor. A wind, in essence, is generated moving from the outside through the carburetor and into the engine.

This wind moves through what is termed a Venturi (an open pipe more or less) and passes right over the bowl containing the fuel. Now, according to what is known as Bernoulli’s principle, the faster the wind the lower its pressure. So, the air in this “wind” is at a lower pressure than the pressure inside the bowl and there we have it: our desired pressure differential.

The fuel in the bowl now moves up a tube (called a jet), mixes with the air in the wind and into the cylinder the mixture goes.

A spark, a powerful explosion, a gorgeous rumble in the exhaust pipe and you are under way.

WHAT’S WHAT IN A CARBURETOR AND WHAT IT DOES:

Considering the simple job a carburetor needs to do, it’s shocking to discover how intricately complex it is.

But watches are like that too. To perform a simple job (transfer the tension of a spring to a mechanical movement of two hands telling the time), a watch must employ some 100 parts. A typical ROLEX watch has about 150 parts; all of them precision-machined.

Well, the bank of four Keihin’s on your HONDA CB bike has over 300 parts (some 70 or so per carburetor and about 40 on the rack/stay) all of which must be put together with precision and tuned perfectly in order to perform as intended.

I like to know how things work. I wish all service manuals took the time to explain how things work and why you are doing the work they describe. It is frustrating to simply follow a procedure (loosen nut A, undo bolt B, remove part C, etc.) without knowing the principles behind it.

And so, what are these 300 parts and what do they do?

Let’s take a look.

The fuel bowl is also called “float bowl “. HONDA manuals refer to it as “float chamber body”.

It operates on the same principle as a toilet bowl:

Fuel flows into it by gravity. As the fuel level rises, it lifts a float. The float then pushes a “float valve”, which eventually shuts off the fuel supply from the tank when the fuel is at a certain level. When fuel is consumed by the engine, the fuel level in the bowl drops, the float drops too, the float valves opens, more fuel flows in from the tank and the cycle repeats.

Inside the fuel bowl chamber, there are some very important components, so let’s remove it and take a look:

Carburetor body.

Top cap. Covers and protects the throttle valve.

Fuel bowl. It houses fuel and is directly connected to the fuel tank via a fuel hose.

Air Screw. It regulates the air that mixes with the fuel from the “slow jet”.

Choke lever. It opens and closes a “choke valve” in order to restrict or allow air to come into the carburetor.

Drain plug. Used for draining the fuel out prior to long term storage.

A quick word about the hot topic of jets, jetting and re-jetting. The main jet (a small brass piece with a hole in the middle) provides an outlet for the fuel to leave the bowl when the pressure above it drops. The diameter of the hole determines how much fuel can go through. Hence, the jet numbering/nomenclature. Main jet #100 means the diameter of the hole is 1.00 mm, #98 means a diameter of 0.98 mm, etc. The main jet (and its associated important components) does an excellent job of providing fuel to the engine at almost all RPMs except

Float.

Slow Jet

Leaf Spring. It keeps the main jet in position and it also prevents the screws securing the bowl to the carb body from loosening.

Main Jet

Float Pin.

Float Valve. A “tang” on the float pushes the float valve upwards to shut off fuel. The float valve drops down by gravity as the level in the bowl drops.

very low and idle. To solve this problem, designers added another jet and called it “slow jet”. The slow jet’s job is to provide a little bit of additional air/fuel mix so when combined with that from the main jet at fully closed throttle, the engine can idle smoothly. It is also a brass piece with a hole in the middle. But in this case the hole is much smaller and its cross-sectional area is about 7 times smaller than that of the main jet meaning that the amount of fuel going through the slow jet is at least 7 times smaller than the corresponding amount going through the main jet.

Both the main and slow jets are constantly submerged in fuel and the air/fuel mix they provide is routed through different passages inside the carb, but their individual mixes, of course, end up the same place. Both jets provide air/fuel mix to the engine at all times. It is a mistake to think that the slow jet (also called idle jet) stops functioning as soon as the motorcycle starts moving.

To re-jet or not to re-jet. Much has been written about this and much fuss has been made about it too. My advice is: don’t do it. Unless you know exactly what you are doing and why, I would advise that you leave the jets alone. If you must re-jet, always use genuine Keihin jets. Many people say they can’t think of anything worse than replacing a perfectly good original jet with a smaller or larger cheap aftermarket jet. Genuine Keihin products always carry the Keihin logo on them. Here is an example.

Shown above are the air screw and its spring (whose function is to keep the air screw in position).

We’ve done the bottom and middle of the carburetor so let’s take a look at what we find at the top. Once you’ve removed the top cap, here is what you’ll see:

Air Pipe. Supplies air to mix with fuel from the main jet when the choke valve is closed.

Choke Valve. When closed, no air can enter the carburetor except air through the air pipe.

Air hole (passage) for the air screw. The only air that can be regulated by the air screw must pass through this hole.

Tanged Washer

And here is the Venturi opening extending from the air intake side of the carb all the way to the other end.

Pry open the tanged washer and remove the hex screw (bolt). Now the throttle valve and the link arm can be removed. They look like this:

THROTTLE VALVE

LINK ARM: The many components of the link arm.

Before explaining what is what on the throttle valve and how it works, let’s take a look at its components:

And here is a close-up of the needle itself:

Notice the taper in the needle.

Throttle Valve

Throttle Valve Needle.

We need a look at two more things before putting the picture together. Looking down into the carburetor’s body where the throttle valve came out of, here is what we see:

Notice the hole in the middle with the brass material around it. This brass piece (at the bottom of which is the main jet) is called an “emulsion tube” and looks like this:

Okay. Too many components, indeed, but a very easy to understand function. As the throttle valve moves up and down in the carburetor’s body (it’s a snug fit), the needle moves in and out of the “emulsion tube” (whose job is to help turn the fuel into tiny droplets, vaporize it so to speak or as they call it sometimes, atomize it. It’s much easier to ignite vaporized fuel). Because the needle is tapered, it allows more fuel to come out of the bowl when the needle moves up and less fuel to come out when the needle moves down. The needle’s movement up and down is directly linked to the throttle grip via the throttle cable, which itself operates the link arm. As you twist the grip, the needle moves up, more fuel comes in, the engine revs up and the bike moves faster. Release the grip and the needle moves down restricting the amount of fuel coming in and consequently slowing the bike. The needle regulates the amount of fuel.

But what regulates the amount of the all-important air? The throttle valve does that. As it moves up and down in the Venturi opening (in sync with the needle because they are connected together in one assembly), the throttle valve changes the size of the Venturi opening and that allows more or less air to be sucked in. Again, twist the grip and the throttle valve goes up. More air comes in and more fuel comes in. Exactly what we want.

I hope this gives a good argument against plunging ahead and re-jetting your carbs because you put in a new exhaust system or changed the air intake by installing “pods” or velocity stacks. The main jet, emulsion tube, needle length and taper, the throttle valve shape, degree of taper and height (as well as other factors such as the level of the fuel in the bowl) are all designed carefully by experts in order to ensure the most practical compromise. Changing only a single component (the main jet in the majority of the cases) will certainly impact the performance of all other components and will cause the engine to run less efficiently.

Keep in mind that the ideal air to fuel ratio is around 15:1. Carburetors are normally not very good at putting in those 15 parts of air to 1 part of fuel. That’s why superchargers, turbo charges, compressors, etc. were developed.

To summarize, here is a very crude description of what a carburetor does: