THE STEWART CARBURETOR
Fig. 39 shows the Stewart carburetor used on Dodge Brothers cars,
which is of the float feed type in which a fine spray of gasoline
is drawn from an aspirating tube by a current of air induced by the
engine pistons. The supply of gasoline being regulated by a float
which actuates a needle valve controlling the outlet of the feed pipe.
This tube is also called the spray nozzle. This type of carburetor is
commonly used on automobile engines.
It consists of a float chamber containing a float, functions of which
are described below, a mixing chamber in which the gasoline spray is
reduced to vapor and mixed with air (i. e., “carbureted” in proper
proportion).
The float and valve maintain a constant or even supply of gasoline for
the carburetor.
The gasoline flows from the filter Z into the float chamber C through
the inlet valve G, which is directly actuated by the float F, so that
it closes or opens as the float rises or falls. As the float rises the
valve is closed until the float reaches a certain predetermined level,
at which the valve is entirely closed. If the float falls below this
level because of a diminishing supply of gasoline in the float chamber,
the valve is automatically opened and sufficient fresh gasoline is
admitted to bring the level up to the proper point. From the foregoing
it will be seen that the float chamber in reality serves as a reservoir
of constant supply, in which any pressure to which the gasoline has
been subjected in order to force it from the tank is eliminated. When
the engine is running gasoline is, of course, being constantly drawn
off from the float chamber through the aspirating tube L, as will
be described later, to meet the requirements of the motor, but in
practice the resulting movement of the inlet valve is very slight and
hence the flow of gasoline into the float chamber is nearly constant.
The gasoline inlet valve is also called the “needle valve.”
[Illustration: Fig. 39. Stewart Carburetor]
Between the float chamber C and the engine connection of the carburetor
is an enclosed space called the mixing chamber O. This compartment is
provided with a valve for the ingress of free air.
Extending into the mixing chamber from a point below the surface of the
gasoline in the float chamber is a passage, L for gasoline, ending with
a nozzle, so constructed that gasoline drawn through it will come forth
in a very fine spray. This is called the aspirating tube, atomizer, or
more commonly, the spray nozzle.
The air inlet AA to the mixing chamber on the carburetor used on the
Dodge is in the shape of a large tube extending from the carburetor to
a box on the exhaust manifold. Air supplied from this source is heated
in order that vaporization of gasoline may be more readily accomplished.
A cold air regulator is interposed between this tube and the carburetor
proper so that in hot weather cool air may be admitted. This should
always be closed when the temperature of the atmosphere is below 60 F.
The action of the carburetor is as follows: The suction created by
the downward stroke of the pistons draws air into the mixing chamber
through the air ducts (drilled holes HH). The same suction draws a
fine spray of gasoline through the aspirating tube L (spray nozzle)
into the same compartment, and the air, becoming impregnated with the
gasoline vapor thus produced, becomes a highly explosive gas. In order
that the proportion of air and gasoline vapor may be correct for all
motor speeds, provision is made by means of a valve A for the automatic
admission of larger quantities of both at high motor speeds. The
ducts are open at all times, but the valve is held to its seat by its
weight until the suction, increasing as the motor speed increases, is
sufficient to lift it and admit a greater volume of air. The valve A
is joined to the tube L, hence the latter is raised when the valve is
lifted and the ingress of proportionately larger quantities of gasoline
is made possible. This is accomplished by means of a metering pin P
normally stationary, projecting upward into the tube L. The higher the
tube rises the smaller is the section of the metering pin even with its
opening, and hence the greater is the quantity of gasoline which may be
taken into the tube. The carburetor thus automatically produces the
correct mixture and quantity for all motor speeds.
The metering pin is subject to control from the dash, as will be
explained later, by means of a rack N, and pinion M. To change the
fixed running position of the pin, turn the stop screw to the right
or left. Turning this screw to the right lowers the position of the
metering pin and turning it to the left raises it. As the pin is
lowered more gasoline is admitted to the aspirating tube at a given
motor speed, thus enriching the mixture.
A wider range of adjustment of the position of the metering pin may be
had by releasing the clamp of the pinion shaft lever and changing its
position with relation to the shaft. This should never be attempted by
any save experts in this class of work.
The carburetor used on the Dodge Brothers car is so nearly automatic in
its action that it is not effected by climatic conditions, or changes
in altitude or temperature. It automatically adjusts itself to all
variations of atmosphere. It is, therefore, wise to see if the causes
of any troubles which may develop are not due to derangements elsewhere
than at the carburetor before attempting any changes of its adjustment.
Make all adjustments with dash adjustment all the way in.
The metering pin should not be tampered with unless absolutely
necessary.
If replacement of this pin should become necessary, it may be
accomplished as follows: First, remove the cap nut at the bottom of the
rack and pinion housing. Next, turn pinion shaft slowly from right to
left (facing toward the carburetor) until the bottom of the metering
pin appears at the bottom of the pinion shaft housing. Continue to
turn the shaft slowly in the same direction, releasing the connection
to the dash control if necessary, until the rack to which the pin is
fastened drops out. The palm of the hand should be held to receive this
as the parts are very loosely assembled. The pinion shaft should be
retained at the exact position at which the rack is released. Install a
new metering pin, the way to do this will be obvious, and return the
rack to its proper mesh with the pinion. Replace dash attachment (if
detached), replace cap, adjust per instructions given on previous page.
The loose assembling of the metering pin in the rack is for the purpose
of providing for freedom of movement of the metering pin and in order
that binding in the aspirating tube may be avoided.
The gasoline filter is installed on the carburetor at a point where the
fuel pipe is connected.
The pressure within the gasoline tank forces the fuel through the pipe,
through the filter screen (ZO in the filter) and thence out through the
opening to the carburetor.
The filter cap CC may be removed by turning the flanged nut on the
bottom of carburetor to the left, thus releasing the inlet fitting.
The filter screen or strainer should occasionally be cleaned. This may
be readily accomplished by removing the filter cap to which the screen
is attached.
The filter should be screwed up tight when replaced.