Stewart Carburetors
lifting of the head of the metering valve (M) away from its seat
(Q).
The metering valve measures the fuel at its lower end and the air
at its upper end, the two being mixed in the chamber (R).
In other words, the metering valve under all conditions of operation
automatically measures and delivers to the mixing chamber the most
efficient proportions of fuel and air. The high velocity of primary
or initial air passing through the holes (P) and around the nozzle
insures thorough breaking up or atomization of the fuel at all
speeds.
Stewart Carburetor Adjustments
The Stewart carburetor is exceedingly simple, there being only
one point where any change in adjustment is possible. This adjustment
is properly made at the factory when the carburetor is in-stalled
on the car and no change should be made unless it is positively
known that the adjustment is incorrect.
Very often symptoms of carburetor trouble, such as misfiring, backfiring,
lack of power, overheating, etc., are produced by causes foreign
to the carburetor.
Therefore, before attempting any change in the carburetor adjustment,
make sure that the compression is good and equal in all cylinders,
that there are no air leaks between the carburetor and engine,
that the ignition is timed correctly and delivering a hot spark,
and also make sure that the spark plugs are clean and otherwise
in good condition with gaps correctly set,. It should be known
that the fuel supply to the carburetor is sufficient and unrestricted.
The only change of adjustment possible in the Stewart carburetor
is that of the relative height of the tapered metering pin (W)
to the opening iii the center of the valve at (S).
The metering pin is carried by a circular rack (T) meshing with
a pinion shaft (Y) to which is attached the dash adjustment lever
(K).
By turning the adjustment screw (V), the fixed or running position
of the metering pin is changed.
Turning the screw to the right (clockwise) lowers the metering
pin, thereby increasing the fuel opening, and thus makes the mixture
richer.
Turning the screw to the left (counter-clockwise) raises the metering
pin and makes the mixture leaner.
To obtain the best adjustment, first run the engine a sufficient
length of time to get the jacket water up to normal running temperature.
In this connection, the engine should not be too hot as the adjustment
thus obtained will likely be too lean for road conditions.
With normal jacket water temperature, make the adjustment with
the engine running idle, having the spark fully retarded and the
low throttle stop screw (Z) (located at the top of the carburetor)
so adjusted as to give an idling speed of approximately 250 r.
p. m.
Gradually turn the adjustment screw (V) to the left, thereby making
the mixture leaner, until a point is reached where the engine runs
unsteadily or stalls, when the adjustment should be reversed; that
is, turned to the right, making the mixture richer a notch, at
a time until a point is reached where the engine will fire evenly.
It is most important that the adjustment be made at idling speed
with fully retarded spark. This one adjustment, when properly made,
automatically insures correct carburetion throughout the entire
range of car operating conditions.
Cold Air Shutter
The cold air shutter regulates the temperature of the air supply
to the carburetor. It does not change the mixture proportions.
It should be closed under all normal conditions and only opened
for extreme high temperatures, such as exist in very warm climates.
Dash Control
For starting and warming up, the Stewart carburetor enriches the
mixture by means of the dash control, which pulls down the tapered
metering pin, thereby increasing the size of the fuel orifice.
The air supply is in no way restricted. This means that the mixture
can be made sufficiently rich for prompt, efficient starting and
warming up in cold weather without flooding the engine with excessive
quantities of raw fuel as with the use of an air strangle or choke
valve.
When starting the engine cold the dash control should be pulled
all the way out but should be immediately returned part way as
soon as the engine begins to fire.
The amount which the control will have to be left out for warming
up depends upon the weather conditions. During sununer weather
a slight amount is sufficient, whereas (luring extremely cold weather
it may be necessary to leave the control out as much as half way
for part of the warming-up period.
A general rule for the amount which the mixture should be enriched
by means of the dash control for warming up is to have the control
out as little as possible and still obtain satisfactory acceleration.
As soon as the engine comes to normal running temperature, the
control should be pushed all the way in. If this is not done, considerable
fuel will be wasted.
Carburetion During Cold Weather
Good starting can be obtained under extreme cold weather conditions
as follows: Advance the spark nearly to the running position and
set the throttle lever on the steering column so that it opens
only slightly. In other words, the throttle should be opened just
a little beyond the idling position. The (lash control should be
pulled all the way out while the engine is being cranked.
When the engine commences to fire, the dash control should be left
way out and the throttle lever should also be left as for starting,
permitting the engine to run for eight or ten seconds. This will
allow the combustion chamber walls to become somewhat heated, when
the control can be pushed part way iu and the engine accelerated
as may be required.
The above applies only when starting in extremely cold weather.
t-nder ordinary weather conditions the dash control should he returned
part way immediately or as soon as the engine fires.
Locating Adjustment Arm on
Pinion Shaft
As explained before, rotation of the pinion shaft moves the metering
pin up or down, thus making the mixture leaner or richer. Motion
of the dash control is transferred to the pinion shaft through
the adjustment arm K.
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