The _carburetor_ or _mixing chamber_, as it is sometimes called, is a
device used for obtaining an _explosive mixture_ of gasoline and air.
It consists, as shown by the accompanying drawing, of two principal
parts, an air pipe and gasoline pipe, the latter running through the
wall and discharging into the center of the former.
[Illustration: Fig. 21--Simple drawing of a Carburetor.]
In order to make sure that the amount of gasoline flowing out of
the gasoline jet shall be just the right amount at all times it is
necessary to provide a little gasoline tank, which forms a part of
the carburetor casting itself, which is known as a _float chamber_,
so that the amount of gasoline in the main tank will not affect the
amount discharged at the nozzle. You can see why this is necessary if
you think of a water tank or a dam. If the water was almost up to the
top of the dam and you should bore a hole through the wall somewhere
near the bottom, the water would flow out faster than if the water was
low. By putting this little gasoline tank in the carburetor itself and
keeping a certain height of gasoline in this smaller reservoir, which
always automatically shuts off the supply at the right time, you can
make the pressure, and therefore the flow of the liquid, always the
same. The illustration will show this plainly. For instance, when the
gasoline gets low the little float will gradually drop down until the
ball on the end of the float stem will open the valve in the gasoline
pipe. The gasoline will then flow in from the tank until the proper
amount has filled the float chamber and caused the float to bob up to
its former position, carrying the ball, which closes the gasoline off,
up with it. By this means the requisite amount of gasoline is always
kept in the float chamber.
The amount of air entering the mixing chamber is controlled by changing
the size of the hole through which the air enters and the quantity
of gasoline admitted is regulated by means of a needle valve in the
gasoline pipe.
Although many carburetors, in fact most of them, do not look like
this drawing, yet their action is the same, and by careful study you
will find that the same principles enter into their construction.
Fig. 22 shows an actual sectional drawing of a carburetor used on a
four-cylinder motor. In this particular carburetor, however, the float
chamber and float surround the mixing chamber, and the float valve,
instead of being directly under the float, is at the right hand side
and is operated by means of a lever. The needle valve, which is the
little round rod having a “T” handle, running up through the center of
the mixing chamber, controls the amount of gasoline flowing from the
gasoline chamber to the nozzle. The air comes up through the bottom
and around the gasoline jet. At the left you will notice a small valve
which opens downward, which you do not find on the other carburetor.
It is known as an auxiliary air valve and allows a certain amount of
air to be added to the mixture, a small quantity of which is sometimes
needed to keep the mixture just right. The throttle valve, which looks
like a damper in a stovepipe and which controls the amount of gasoline
vapor going in to the engine, will be seen in the upper pipe.
[Illustration: Fig. 22--A Typical Four-cylinder Carburetor.]