J.A.P 6 Reference Book, Diagrams, Spare Parts Lists 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