Triumph TR4A - Running Adjustments; Carburettor Operation and Cold Starting

RUNNING
ADJUSTMENTS
I
I
ZENITH.STROMBERG
CARBURETTORS
(SERIES
175.CD)
Starting
from Cold
(Fig.
5l)
The
mixture
is enriched
for cold starting
when the
choke
control
is pulled.
This oPerates a
lever
(6)
which
lotates the
start€I
bar
(20)
to
lift
the
air valve
(18)
and needle
(29),
thus
increasing
the
area of the annulus between
needle
and
jet
odfice.
Simultaneously,
a
cam on the lever
(6)
oPens the
throttle beyond
its
normal
idle position
to provide increased
idling speed,
according
to the setting
of the sqew
(4).
when
the motor
fues the increased depression
will
lift thc
air valve
(18)
to weaken the initial
starting mixture
and
Prevent
the
engine
stalling
tbrough 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 throtde
opening for
fast-idle to the
Point
where
the screw
(4)
is out of contact
with the cam on
the choke
lever
and
the
thotde
is permitted to return
to the nomal
idle
Position
as
determined
by thc setting
of the throttle
stoP sqew
(3).
NOTE:
The
accelerator pedal
should
not
be
dePressed when
starting
from cold.
Normal
Ruming
With
the opening
of lhe butterfly
throttle, manifold
de-
pression
is transferre-d,
via a d llingi25)
in
rhe air
valve.
to
ihe chamber
(24)
which
is sealed
from the
maio body by
the
diaphagm
(16).
The
pressure
diference
between
chamber
(24)
and
that
existiog
in the bore
(26)
causes
the air
valve to
lift, thus any
increasi
in engine
sDeed or
load
will enlarge
the effective chokc
area since
thj air
valve lift is
proportional
to the
weight of
air
passinq
rhe
throttle
(27).
By
tfus means air
velocity and
presiue arop
across
the
jet
orifice
remain
approximately
constant
at
all speeds.
As the air
valve
(18)
rises
it withdraws
a taPered
metering
needle
(29),
held
in the base
of the air
valve by
the screw
(10),
from
the
iet
orifice
(19)
so
that fuel
flow is
increased relative
to
the gr6ater
air flow.
Acceleratiotr
At any point
in the throttle
range
a temporadly
richer
mixture iC
needed at the
moment of
further
tfuottle
opening.
To
provide
this, a dashpot
or hydraulic
damPer
is aEanged
inside
the hollow
suide
rod
(17)
of the air
valve.
The
rod is filli
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
resiited by
fiis
plunger diring
which time
the suction
or
depression
at the
let
orifice
is increased
to enrich
the mixture.
The downward
movement of
the air
valve
(18)
is assisted
by the
coil spring
(15).