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Changing
the size of the cut-away
(
designated
by
cut-away
No.)
.
the
pressure actuating
the
needle valve
can be altered to
change the amount
of fuel
flow and
causes
a
change
to
the
fuel
mixture.
The va lve
with
a larger cut-away
number
will
produce
a
leaner
fuel
mixture.
However, the
range
of its
effectiveness is
mainly
at
low
speed.
from
idling
speed
to
approxi
-
mately
1/4
throt tle
opening
and has no
effect
above
1/2
throttle opening.
A
throttle
stop
screw
sets
the
thrott
le
va
lve
in
the idle position.
Screwing in
on the stop
screw
will cause the throttle
valv
e
to rise.
and
backing
off
will lower
the
throttle
valve.
6.
Slow Jet
1 he slow
jet
regulates the
fuel
flow during idling
and small
thrott le opening, and
perm its
t
he air
to
en
ter
throu
gh
the
air
bleed to m ix with the fuel
for
atomization.
The
slow jet is similar
to
the
main
jet
in that the
lar
ger
t he
jet
size number, the
great
er
will be the luel
flow and
consequent
ly a
richer
fuel
air
m
ixtur
e. (
Fi
g.
3-20)
7.
Air
Scre
w
The
air
screw regulates
the
amount
of
air m1x1ng
with the
fuel in
the slow
speed system by controlling
the amount of
pilot
air
bleeding with
the fuel enter-
ing
throu
gh
the
slow jet.
In this way,
the proper
fue
l
air mixture is
maintained.
Screwing in
the air
screw w
ill
produce
a rich fuel-a
ir
mixture by
restrict·
ing the
air
bleed
hole
and
backing off
on the
screw
will
resu
l
t
in
a
lean
mix
ture.
COMPRESSION
(Compress the fuel air
mixture in the
cylinder)
Piston
The
piston
plays an important role by performing
the
intake,
compression, power
and
exhaust
funct ions.
It is alternately
cooled by the
intake
fue l-air
mixture
or exposed
to
the
ho
t
gases
resulting
f rom
the
com-
bus
tion.
If the piston
is
close
ly fi
tted
agains
t
the
cyl
in-
der wa
ll
w
i
thou
t
clea
ranc
e
as shown
in
the
ri
g. 3 2
1,
it
would
not
operate
smoothly
and
may
result
in
seizure.
On the
other
hand,
excessive
c learance
between
the
pis
ton
and cylinder
wall will result in
insu fficient
intake
of
ruel
-air mi
xture,
causing low compression, oil
pum-
ping (oil
enters
the combustion
c11amber
)
, etc ..
and
conseQuence
poor engine performance.
Therefore,
a
good
seal must be maintained between
the
pis ton and
cylinder
wall.
For
this purpose, piston rings are instal-
led
to
provide
the necessary
seal.
The
piston is
made
of aluminum
die
cast ing
equiva-
len
t to
SAE 8630.
This ma
teria
l
is light and
has good
heat conducting
proper
t
y
so th
at tile heat
f
rom
the
combust
ion
can
be
dissipate
rapidl
y.
Fur
the
rmor
e,
t
his
mater
ial
has
a
small
coeffic ient
of
expansion,
thus
minimiling
the
expansion
o
f
the piston
at elevated
temperature
and
permits a
small
piston
to cylinder
clearnace design.
19
Fig.
3-20
Slow jet
'!)
0
ring
2
Genuine parts
mark
'3
Slow
jet
®
Sl
ow
jet
NO.
l
Fig.
3 -21
Piston
1
1
Cylinder
'2)
Piston
(3
Piston
rings
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