•
•
•
7-2
CARBURETION
AND
FUEL
SYSTEM
Tl1e
S L338
D,
S L433B, E L4338,
SW433C
and
EW433C
are
equipped
wi
th a
KE
IHIN
diapl1ragm
typ
e carburetor
CD42-38 (v
en
t
L1
ri bore 38 mm),
the
GP338
is
with
tw
o
KEIHIN diaphragm type
ca
rbur
etor C 038-30 (ventury
bor
e
Th
ro
t tle
shutter
J-
-----~
I
dle
disc
harge po
rt
S
low
a
djusting
sc
rew
I
~....,....,.~.....,.....,.....,.....,~
~
Fuel
pump
diap
h
ragm
Fue
l
pump
c
ha
mber
R
eg
ulat
or
cha
mb
er
M ain d i
aphr
a
gm
Dia
phra
gm
sp
r ing
ENGINE
MAINT:ENANCE
-Carburetion and Fuel System
30
mm),
and the GP433B
is
two
KEIHIN
diaphragm
type
c.arburetor CD38-32
(ventury
bore
32
mm).
(These carbure-
to
rs
are
know
as
floatless
type
carburetors.)
Secondary
ventury
Primary
ventury
•
Main
discharge
port
Choke
shutter
Main
nozzle
Main
adjusting
screw
11111
Inlet
control
lever
In
let
tens
io
n spring
~----
----Ou
tlet
check valve
Spring
sea
t
Scr
ew
L..--IL-J-
-----
~
-
---
L
oc
k
nut
Adjustment
screw
Fig. 7-8
The diaphragm carburetor draws the fu
el
at
a proper rate
fr
om
th
e fuel tank where the fuel
is
then metered to the
engine according
to
fuel consumption in the engine.
The diaphragm type carburetor, adopted
by
Yamaha,
has
the
following
three main mechani
sms:
1.
Mixing
bo
dy
supplying
an
air-fL1el
mixture
to the
•
en
1nge.
2. Fuel pump drawing the
fL1el
from
the fuel tank
to
the
carburetor.
3.
Regu
later maintaining the
pump
fuel pressure cons
tant
by
reducing
or
increasing
it
in
order
to
deliver a
proper
air-fuel
mixture
according
to
engine speed.
The regulator
of
the diaphragm
type
c.arburetor plays a role
similar
to
the
float
of
the
float
type
carburetor. The fuel
level in the
float
chamber
is
likened
to
the regulator
•
internal pressure.
Accordingly,
the
regu
later internal pressure must be
kept
at
a constant level
just
as
the fuel
_level
of
the
float
chamber
type carburetor, because a
proper
air-fuel
mixture
must
be
supplied
to
the engine according
to
its
requirements . .
To
this
end, the regulator internal pressure (difference between
the
regulator set fuel level and the main and idle discharge
•
ports) must be
optimum
for
feeding a proper air-fuel
mixture
.
For
this
reason,
the
above mentioned three main com-
ponents
are
arranged
as
follows:
The
mixing
body
is
provided
with
the fuel
pump,
and
the
regulator is posi-
tioned apart
from
the
mixing
body
so
that
the
regulator
may maintain positive pressure. In
addition,
the
regulator
pressure setting is designed
to
be varied freely.
The
diaphragm
for
the fuel
pump
is
made
of
thin
stainless steel
so
that
it
quickly
respondes
to
variations in engine speed.
As a result,
it
may
be
said
that
the fuel
pump
is superior
in
discharge performance
as
well
as
in
durability.
As the engine runs, the pulses
which
are generated
in
the
crankcase are carried
from
the
impulse
port
to
the
fuel
-77
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