keihin carburetors explained

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.
Top cap. Covers
and protects the
throttle valve.
Carburetor body.
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.
Fuel bowl. It
houses fuel and is
directly connected
to the fuel tank via a
fuel hose.
Drain plug. Used for
draining the fuel out prior
to long term storage.
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:
Main Jet
Slow Jet
Float Pin.
Float.
Leaf Spring. It keeps
the main jet in
position and it also
prevents the screws
securing the bowl to
the carb body from
loosening.
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.
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
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).
And here is the Venturi
opening extending from the
air intake side of the carb all
the way to the other end.
Choke Valve. When
closed, no air can enter
the carburetor except air
through the air pipe.
Air Pipe. Supplies air to
mix with fuel from the
main jet when the choke
valve is closed.
Air hole (passage) for the
air screw. The only air that
can be regulated by the air
screw must pass through
this hole.
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:
Tanged Washer
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:
Throttle Valve Needle.
Throttle Valve
And here is a close-up of the needle itself:
Notice the taper in the 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: