RUNNING
ADJUSTMENTS
ZENITH-STRCN\fBERG
CARBTTRETTORS
(SERIES
175.CD)
Starting from
Cold
The
mixture is enriched
for
cold
startins when
the choke
control
is
pulled, This
operates
a
le.er
t6t
which
rotares
the
starter
bar
(20)
to lift
rhe air
valve
(18)
and needle
(29),
thus
increasing
the
area of the
annulus
between needle
and
iet
orifice.
Simulraneously,
a cam
on the le\er
(6)
opens rhe throtile
belond
irs normal
idle
posirion lo
provide
increased
idling speed.
according to
the setting
of the
screw
(4).
When the motor
fires
the increased
depression
will lift the
air valve
(18)
ro weaken
the inilial
starting
mixture and
Drevenr
the
engine stalling through
over richness.
While the choke remains
in action
the car may
be driven
away but the
control knob
should
be released
or
pushed in
gradually as the engine
attains
normal
working temperature.
This will
progressively decrease
the extent
of enrichment
and
the degree
of throttle
opening
for fast-idle
to the
point
where
rhe screw
(4)
is
out of contact with
rhe
cam
on the choke lever
and
the throttle is permitted
to
return to
the normal
idle position
as determined
by the setting
of the
throttle
stop screw
(3).
NOTE : The
accelerator
pedal
should not
be depressed when
startins from cold.
Normal
Running
With the
opening of the
butterfly throttle,
manifold
de-
pression is transferred,
via a drilling
(25)
io the
air valve, to
the chamber
(24)
which
is sealed from the main
bodv
bv the
diaphragm
(
I6).
The
pressure difference
between chanrber
(24)
and
that
existing in
the bore
(26)
causes the air valve to lift,
thus
any
increase
in engine
speed or load will enlarge
the effective
choke
area
since the air
valre
lifr
is proporrional
to the $eighr
of
air
passing lhe throlle
(27).
Bt rhis means
air
velocirl
and
pressure drop
across the
jet
orifice
remain approximately
constant
at all speeds.
As the air valve
(18)
rises it withdraws
a tapered
meterins
needle
(29),
held in the
base of the air valve
by
the
screw
(101
from the
jet
odfice
(19)
so
that fuel
flow is increased
relative
to
the greater air flow.
Acceleration
At
any point in the throttle
range a temporarily
richer
mixture is needed
at the noment
of
further
throttle
opening.
To
pro\ ide this. a
da\hpol or hldraulic danrper
is airange-d
inside
the hollow
guide
rod
(17)
ol the air valve.
The rod is filled with
S.A.E.20 oil to within a
+" of the end
of the rod
in which the damper
(14)
operates. When the throttle
is opened, the
immediate upward motion
of the air valve
is
resisted
by this
plunger during which time the
suction or
depression
at the
jet
orifice is increased to euich the
mixture.
The downward movement
of the air valve
(18)
is assisted
by the
coil spring-(15).
1?
,l
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