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7-2 CARBURETION AND FUEL SYSTEM
Tl1e
S L338D,
S L433B,
E L4338,
SW433C
and
EW433C
are
equipped wi
t
h
a
KE
IHIN
diapl1ragm
type
carburetor
CD42
-38
(ven
t
L1 ri bo re
38
mm)
,
the
GP338
is with
two
KEIHIN
diaphragm
type
carburetor
C
038-30
(ventury
bore
Thro
t
t
le shutter
J------~
Idle discharge
p
o
rt
Slow
adjusting
screw
I
~....,....,.~.....,.....,.....,.....,~~
Fuel pump
diaph
ragm
Fuel
pump
chamber
R
eg
u
la
t
or
c
ha
mber
M
ai
n d iaphragm
D iaphragm
spr
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-
t ors are know
as
floatless type
carburetors.)
Secondary ventury
Primary ventury
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Main
discharge port
Choke
shutter
Main
nozzle
Main
adjusting
screw
11111
Inlet
control
lever
I nlet tension spring
~--------Outlet
check
valve
Spr
ing
seat
Screw
L..--IL-J------~----
Lock nut
Adjustment
screw
Fig
.
7-8
The
d
iaphragm
carburetor draws the f uel
at
a
proper rate
from
the
fuel tank
where
the
fuel is then metered
t
o
the
engine according
to fuel consumption
in the engine.
The diaphragm
type carbu
retor
, adopted
by
Yamaha,
has
the
following three
main
mechani
sms:
1.
Mixing body 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 constant
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
t
y
pe
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
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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
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