J.A.P 6 - Page 14

ALf
TRNATIV€
VARIABII
MAIN
JET
SECTION CARBURETTER
TYPE
2{7.2,
GENDRAL
DESCRIPTION
'fhe
21'l-2
Carburetter
showu
above
in
cross
section is
a vertical
instrument
in
general
use on
ight
Commercial
Vehicles,
Marine
Engines
Stationary
Industrial
Plaqt and various
types
of
Mobilo
Agricultural
equipnent.
'fhc
float
chamber
is ofi-set
ia order
to keep
it ar closc
ar
possible
to
tho main
discharge
tube, thus
ensuriog
high
anglo
operation
in any
direction
without
flooding,
or
stalling.
The lustrument
caq be arranged
to
tako all
sir through
the
uain intako
which lr
invariably
protccted
by an
air
cleaner,
Thtl
feature
ls called
Ior
when working
under
dusty
condltion!
close
to
the
ground,
if
long
lile
and
reliablo
servlce are
to be
obtalned.
The carburetter
consists of
two maia
castingr,
thc upper
or barrel
portlon
being
secured
to the
lowor
bowl
portion
by 6ve
acrews,
ono of
which
is
shown (8).
Itr order
to keep thl3
carburettcr
as siEple
as
possiblo
we
hÅve
uscd
ono
princlplo
jet
and
a slow running
jet.
Suitable alr
bleeding
is arranged
to atomiso the
luel and
to tnaintalo
a correct
mixturc strength
under
all condltionr
of
engine
operation. The
drawlng
rhowr the
princl-
ple,
or mail
jet
(2)
covered by
thc largo
hexagon
plug
(l).
The slow
running
jet
lr
sko shorvu
(15).
Thc
air
bleeding to
the maln
jet
$yttem
is controlled
by
the air
jet
(14),
and thb
air
3upply
is
teke! Irom
tho main air
lntaks.
It
will
be
uotlced
that thls air lssues
lrom
thc holer
({),
(6)
aad
(6),
at high engine
speeds
whcn tho
fuel iu
thc
maln
dischargo
tube
(7)
Ialls
to ltt
lowest level.
Fuel
metered
by thc slow
runnitrg
jet
(lS)
ir
rtomlsed
by an alr supply
tsken through
thc
malu
sir lntakc
aDd controlled
by the
serew
(12).
Tht.
mixturo
i$suer through
tho ldle
dlrcharge
channcl
(10)
and
tho
progresrion
orlåce
(11).
Ttc
doat
chaEber
coDtains a normal
typo float (10)
ead
thc
PAes.
24
usual
combined
needle
seating
valve (lg),
The
correct
Iuel
level is
autouatically
provided.
by
the
size
of
the
float
chaDrbcr
and the
float,
together
with
the
position
of
the ncedle
seating,
the
latter
part
is
usually
fitted
with
one
washer,
but
two
washer
catr
bc used
iI it is
desired
for
anv reason
to reduce
the fuel
lcvel
in
the iloat
chambcr,
MAIN
ADJUSTMENT
The
comblnation
of choke
tube,
main
jet
antl
alr
jet
will be
Iound
corrcct
for
the
engine
to
which
the itrstrument
is
fitted and
it
should
not
be
uecessary
to
alter
these
parts
when
deallng
with
ortliuary
running
trouble.
Cleanliness
is the
keynote
for
good
results.
Takc speclal
cåre to
use a
suitablo
screw_
driver
when rcmoving
the
main
Jet
in
order
to
prevent
damage
to
the thread
in
the
carburetter
casting.
The mair
jet
adjuster
(22).
is a
tapered
Deedlo
pa$lng
into
the main
jet
orificc
and
controlling
thc
flow of fuel lnto
the
jet.
lVhen
turoed
ln a
clockrvise
direction as
far as
possible,
all fuel
flow
to
the
naln
jct
will be
coDpletely
cut-o{I.
On
the other
haqd,
if
the
adjuster (22)
is
opened
up ln
an anti-clockwise
dlrection
more
than about
two
coruplete
turn8,
the full luel
flow of
the main
jet
rvill
be
provided.
Do
not screw the needle
into
the
jet
with
ercessive
lorce,
as this
will
damage
the taper
of
tho
needle
&ud tho correct settiug
specified
will no longor
apply.
The
packing
nut
of the adjuster
should
be
tighteDed
slightly if
the luel drips
from
the adjuster,
A
gasket
must
alwayE be used
between
the two
halver
oI
the
carbur€tter.
SLOW RUNNING
ADJUSTMENT
Tbis should
be carried
out
vhen the
engine
is
hot, tho rniuirnum
ruuning
speed ls
usually
set
sround 550/600 r.p.m.
A apring-loaded
adjustlDs
rrow lr
provided
close
to
the throttlo
lever
by
means
of which the
exact throttle
opening
cau be
adjustqd
for idling.
The
head of ilris
scre\y should
be turned
clockwise
to increase
the idle specd and
uicc o./sø.
The slow running
mixture
screw
(12)
rvill
provide
a richer
idle n:ixture
if turned itr a clockwise
direction
by reduciog
the
supply oI
slow
running
air,
On the
other
hand if there
is evldenco of rich
running,
i.e.,
black
smoke
lrom the
exhaust when
idling,
this
screw
should be
given
a
quarter,
or
one-half
turn
in
an anti,clockwise
direction.
The
usual
setting
is
about
one corlpletc
turn open from
the fully
home
position,
but
of course
this varies
slightly
frorn
orre
cngine to
atroflrer.
COLD
STARTING
(see
also
page
3)
The rich
mixture
necessary
to nteet
this conditlon
is
provided
by closing
the
choke
or air strangler
(3)
and
at the
sarnc time
set
the throtile
to about
one
third
of its
full movement
opcn.
(See
SIARTING,
page
3). As soon
as
the
engine is mnuing
the
strangler
flap
will open
autoulatically
to
adrnit air; however,
as
the
engioe
warms
up
the control
operatiDg
the
strangler
must
be rnoved
to the futly
open
position,
HOT
§TARTING
(seo
also
page
3)
When
the
etrgine is
hot
or warm, the
choke
ls
not
lcquired,
and
the throtde position
is
not
irnportant.
II
thc
engine
does not
Inrncrliately
respond
check
the
usual details,
such as pctrol
tap ln
"
ou
"
Ilosition,
etc.
GENERAL
The carburetter
is
an accurate
netering
lnstru-
ment.
Consequently,
absolute
intcrnal
cleanliness
is
essential
if
satisfactory
resuits
are
to
be obtained,
\Vhen
the
cornplete
instrument
is rerrroved
and
disDratrtled
for cleanitrg
purposes,
it is
a
good
plan
to
clean
the outside
of
the carburetter
tholoughly
belore any
dismautling
takes
place.
The
parts
should
bo carefully
set out
on a clean
sheet
of
papor
and
the main
castings
blown
out with
compressed
air if
this
available.
The strangler
flap complete
with
its
spindle
should move quite
freely
against
the light
anchoring
spring
when the
strangler
lever is
held in
the closed
positioa
and
the flap is
pushed
with the
finger.
It rvill be
noted that
the light
spring referred
to is secured
in
one of
the notches
of
the spling
anchor
plate,
and the
spring loading
can
be readily
adjusted.
The standard
position
is
the top or
"
l2 o'clock
"
notch.
IR commoD
rvith all
air
intake
choke valves
the
straogler flap
nust be
cornpletely
closerl
against
the air intake
bore
when the extemal
lever is rnoved
to
the fuUy closed
position.
When
replacing
the carburetter
take care to
use a thin
flange
Basket
as a thick
gasket
may
tend to
squeeze
out cirusing
the flange on
the oarburetter
to bend and
this
would allow
æ air leak
to occur.
When dealing
with a flange
in this way
it can
be trued up in the
ttsual
nlanner
with a file.
Before
atternpting
to
start
the
engine,
subsequent to
refitting the
carbur-
etter,
it is a
good
plan
to
check the
throttle cotrtrol
atrd also
the choke cootrol
to make
quite
sure these
parts
work corrcctly,
giving
the full
amomt of
travel,
etc.
The
l{aker's
adjustment
oI the
carburetter and
setting lor
tlre main
jet
adjuster
gives
correct
mlxturo
strcngth
for all
conditions
up to about
3,000 feet
altitucle
above
sea level.
Wherr operating
8t higher
altitudes
it may
sometimes
bo necessary
to deal wiUr
8yDptoms
of rich running
and
possibly
loss
of
power.
In these
cases, the rnain
jet
adjustcr
will
bo found Dost
useful in
obtaining
a
clean
ruDning
exhaust.
Turning
the
adjuster in
a
clock-
wise direction
one-quarter
of a
turn at a
tinre will
bo found
eflectlve in
weakening
the rnixture
delivered
by the carburetter,
In
any casc
of difficulty
our orvn
Servico Dcpartment
or
the nearest
Zenith Carburetter
Servico
Station should
be consulted.
Prcu 26