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10-54
GROUP
10-FUEL
SYSTEM
SECONDARY THROTTLE SHAFT AND LEVER
Fig. 16 - Choke Unloader Operation
Fig. 17- Secondary Throttle Lockout
before the engine starts so as to block
air flow into the
venturis.
When the
engine is cranked, engine vacuum
then causes fuel to flow from the
main metering and idle systems to
start the
engine.
Choke Plate Pulldown
When the engine starts, some air is
needed to prevent flooding.
The
choke plate is pulled open slightly by
the effect of engine vacuum and by
the operation of tht choke piston.
The piston, also located in the choke
housing, is actuated by vacuum as
soon as the engine
starts.
Piston
travel is limited by vacuum bypass
slots in the choke piston cylinder wall
and a slot in the
piston.
When the
slots meet, vacuum starts to bleed off,
preventing
further
piston
travel.
Thus,
the choke plate is opened just
enough to prevent flooding, but not
enough to lean out the cold mixture.
If the engine is accelerated during
warmup, vacuum drops and the choke
closes for an instant ... the mixture
is thus made richer to prevent a
"stumble" or
stalling.
Exhaust-Heated Air Control
As the engine warms up, inlet manifold
supplied heat warms the thermostatic
choke spring. The choke closing force is
gradually relaxed and the choke plate
opens as the engine becomes
warm.
Fast Idle Cam
During the engine warm-up
period,
it is necessary to have a faster idle
speed to prevent stalling. The fast
idle cam
(Fig.
15) increases the idle
speed by cracking the throttles to
admit
more air.
The cam is rotated into position
under the fast idle adjustment screw
bv the automatic choke lever. The
adjustment screw is mounted on the
throttle
lever
and
actuates
the
throttles when the cam contacts it.
The cam has several steps, and moves
to succeedingly lower steps under the
fast idle adjustment screw as the en-
EJ
IN-CAR ADJUSTMENTS AND REPAIRS
CARBURETTOR ADJUSTMENTS
The
adjustments
described
and
illustrated
in
this
part
should
be
performed
as required to retain the desired
engine
performance
characteristics. Refer
to the Specifications when evercarburettor
adjustments are made.
IDLE SPEED AND
FUEL
MIXTURE
All carburettors are equipped with idle
fuel
mixture
adjusting
limiters. The
limiters control the maximum idle richness
and help prevent unauthorized persons
from making
overly
rich idle adjustment.
The plastic idle limiter cap is installed
on the head of the idle fuel mixture
adjusting
screw(s). (Fig. 18).
Any adjust-
ment made on carburettors having this
type of limiter must be within the range
of the idle adjusting limiter. Under no
circumstances
are the idle adjusting limiters
or the
limiter
stops on the carburettor to
be
mutilated or deformed to render the limiters
inoperative.
A
satisfactory
idle
should
be
obtainable within the range of the idle
gine warms up. Thus, there are pro-
gressively !ewer fast-idle speeds.
Choke Unloader (Dechoker)
If the engine becomes flooded while
the driver is trying to start it, the
choke unloader
(Fig.
16) permits the
choke, by using the throttle lever, to
be opened. Pushing the accelerator
pedal to the floor mechanically moves
a projection on the throttle lever
against the unloader face on the fast
idle cam. The cam partially opens
the choke plate.
Secondary Throttle Lockout
The secondary throttle lockout
(Fig. 17) also is actuated by the choke
system to prevent the secondary
ven-
turis from delivering fuel when the
engine is cold. The throttle lockout
lever engages with a lever on the
secondary throttle shaft, and holds
the throttle plates tightly closed dur-
ing choking. When the choke opens,
the lockout lever is disengaged so that
the secondaries can operatE>.
adjusting limiters, if all other engine
systems
are
operating
within
specifications.
At pre-delivery, follow the Normal
Idle Fuel Settings for both Engine Off and
Engine On and in Step 1 of Additional
Idle Speed and Fuel Mixture Procedures.
Other fuel system adjustments should not
be
required at pre-delivery
service.
Followins are the normal procedures
necessary to properly adj\lSt the engine
idle speed and fuel mixture. The specific
operations should
be
followed in the
sequence given whenever the idle speed Qr
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