Brief Elementary Fundamentals
Before taking up the subject of the principles of the different
makes of carburetors, a few elementary fundamentals as to the
types of carburetors will be discussed here.
Carburetors in general can be divided into five general types,
as shown by A, B, C, D and E.
In this group, A shows in the simplest manner possible the plain-tube
type of carburetor of which Stromberg, some models of the Schebler,
Hayfield, and others are examples.
The plain-tube type derives its name from the fact that there
is but one constant air passage intake in which is placed the
fuel nozzle (2).
At B is shown a metering-pin type of carburetor, examples being
the Hayfield, Schebler model T, the Stewart (metering valve)
and Ball (S Ball (metering screw).
In this type of carburetor the size of the fuel jet (2) is increased
automatically with the opening of the throttle or butterfly valve
Sometimes the metering pin (N) is controlled by an auxiliary
air valve or simply by air suction, as in the Stewart carburetor.
The auxiliary air-valve carburetor is shown in C.
Here the air flow is increased by the suction of the engine which
opens a valve (V) usually retained by a spring.
Normally this valve is closed and air is taken
in for idling through a con
C start air intake of small size.
As the engine increases in
L f peed, the auxiliary valve is
~~2 d1a a from its seat and furnishes additional air to give the fuel mixture
correct proportions. Auxiliary air valves are found on many makes of carburetors.
The compensating-jet type carburetor is shown by D, examples being the Zenith
and Stromberg carburetors.
Here one, or sometimes two jets are placed in the throat of the carburetor
near the butterfly valve when the latter is seated.
The engine gets its mixture for idling from the small jet (3) just above the
butterfly throttle valve (T), and for normal speeds, from the jet just below.
When the throttle (T) is opened, the main jet (2) in the center furnishes the
The multiple-jet or expanding type carburetor, such as the Miller and Master,
is shown by E.
In this carburetor a barrel-type throttle valve (T) is used in place of a butterfly
valve, and as this barrel revolves when the throttle is opened it uncovers
a series of fuel jets (M).
The air intake surrounds these jets and an idling jet (3) is placed above the
barrel, as the other jets do not come into action when the throttle (T) is
In all the above diagrams the air flow is indicated by arrows and the fuel
by the shaded portion.
The Necessary Agents Essential for Good
Modern carburetors make use of three natural agents in vaporizing and gassifying
any volatile liquid. They are heat, air velocity, and vacuum.
Heat supplies the energy to change the fuel front a liquid to a vapor state.
Air velocity is made use of and controlled through restricted air venturi tubes
to break up the fuel and carry it quickly to the combustion chambers before
combustion can set in.
Vacuum is made use of to separate the air molecules, rendering space thus gained
between the air molecules more susceptible to collection of combustible vapor
With these fundamentals in mind, the reader should now be able to analyze and
understand the principles and operation of all makes of carburetors.
The shape of the cylinder head and inlet manifold can largely influence the
rate of combustion. A type of cylinder head as shown on page 1314 and manifold
(page 115) is a type which promotes turbulence which remakes the flame spread
The following pages will be devoted to the study of some of the modern types
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