How Carburetors Work
The modern internal combustion engine is
merely an air pump; it pumps air in and air out. One of the first things
performance enthusiasts learn is to get more in you have to get more out.
Performance exhaust is useless if you dont increase the air/fuel mixture
coming into the engine.
Ok you know that, so you also know bigger has
got to be better, a bigger carburetor will allow more air to enter the engine
along with a free breathing air box or air cleaner. This is true but only when
other factors are taken into account like, jetting, and the type of cams used.
Venturi Effect The carburetor, which is basically a large
tube or Venturi. Vacuum created by the downward stroke of the
piston causes the higher pressure in the float chamber to force the fuel/air
mixture through the fuel/air outlet passage (or ports) in the carburetor
venturi and vaporized, by the fast moving air stream. A needle sits in the hole
to regulate the fuel flow. The throttle cable or engine vacuum operates the
needle up and down to regulate the fuel in the air stream.
carburetors are as simple as the one above, some have more than one hole fuel
to be drawn from. Most all-modern carburetors have an idle speed adjustment
screw it is actually a valve; the end of the adjustment screw is shaped like a
needle and it regulates fuel flow when the engine is running at low speed.
For higher speed operation a needle valve is attached to the slide,
which is operated by vacuum or cable. In cases where the high-speed needle is
attached to the slide, the slide is raised and lowered by the throttle cable.
When the throttle is open the needle is raised up out of the emulsion tube
allowing more fuel to mix with in increased air allowed by the rising slide. As
the throttle is closed the slide and needle come down reducing air and fuel.
carburetors used on modern motorcycles also have a moving flap in the venturi
to regulate the airflow, a butterfly throttle valve (or throttle plate). On
most carburetors this is where the throttle cable is connected. As the throttle
plate is opened it allows more air into the engine. Fuel is fed through a fixed
orifice or jet controlling the amount of fuel in addition to the idle port.
Some of the more expensive aftermarket carburetors and most of the
newer orignal equipment carburetors have additional circuits or passageways and
ports for fuel to enter the air stream. In addition to the idle circuit, a
transfer, midrange or air circuits may be used to fine-tune the fuel mixture.
Remember the idle port will provide fuel during the entire operation of the
carburetor. As the air speed increases the needle or main jet will add fuel in
addition to the idle circuit.
The most modern carburetors are Vacuum
controlled or CV (Constant Vacuum) type. A vacuum operated diaphragm controls
the opening and closing of the slide (or piston). The CV Carburetor also has a
butterfly throttle valve (or throttle plate).
idle, with the throttle plate closed and the air stream cut off, idle speed is
maintained by fuel metered through the slow jet. Air from the slow air jet
mixes with the fuel and is delivered to the idle port at the vacuum side of the
throttle plate. At low speed as the throttle plate is cracked open the transfer
ports are exposed to the vacuum side of the throttle plate and additional fuel
is directed to the barrel of the carburetor. With throttle plate cracked open a
quantity of fuel also enters the air stream from the needle jet. The idle and
transfer ports supply additional fuel to the carburetor barrel to assist during
the transition period from idle to mid range.
The venturi opening is
reduced by the low position of the vacuum piston (Slide). This enables initial
air stream velocities to be higher than normally attainable with fixed venturi
carburetors. The higher air stream velocities provide greater quantities of
fuel necessary for good acceleration.
As the throttle plate is opened,
airflow increases through the carburetor and the pressure drop in the venturi
near the needle jet increases.
The low pressure in the venturi
travels through the vacuum port in the vacuum piston to the chamber above the
diaphragm. The chamber beneath the diaphragm is vented to atmospheric pressure
by a passage from the chamber to the carburetor inlet. The higher pressure at
the underside of the diaphragm overcomes spring pressure and moves the vacuum
piston upward in proportion to the pressure difference between chambers.
As the throttle plate is opened further, the pressure difference
between the chambers above and below the diaphragm increases and the vacuum
piston moves further upward. The venturi opening increases and the needle is
lifted further out of the needle jet. The quantity of fuel and the volume of
air are simultaneously increased and metered to the proportions of engine
demand by the variable venturi and needle lift.
The accelerator pump is
used to supply fuel during sudden or rapid throttle openings. It quickly
injects fuel into the venturi; the extra fuel provides for smooth acceleration
and reducing throttle hesitation.
For cold starting carburetors are
equipped with a choke or an enrichener circuit. The choke is a flap type valve
similar to the butterfly or throttle plate. It is located on the inlet side of
the carburetor and reduces airflow making the fuel mixture richer. As the
engine warms up the choke is opened up and the carburetor returns to normal
operation. An enrichener is a separate fuel circuit, manually controlled by a
cable when it is engaged it provides additional fuel making the idle mixture
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