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3. 6 CARBURETOR
1. Air System
Air passed through the air cleaner is inhaled
by engine from the air intake 121 through ( 1) and
(151.
In the venturi portion
(1
I, ( 161
is
projected
and lowered
:by
its own
weight
to the
direction
to minimize the venturi
area.
(Fi~. 3. 124)
In case
when
the
amount
of air
inhaled by
engine
is small, the vacuum piston
is
generally
lowered
and forms primary
venturi ;
thus, maintains the
flow
speed
in the venturi and
improves
fuel gasifying.
Accordingly with
the
increase
of the
air
inhaling
volume, the
flow speed
in
the
narrow
venturi por-
tion ( 1 I
becomes
faster than that
of
the air
intake
(21 ;
thus, the pressure of the
venturi
portion ( 1 I
becomes lower than that of
the air intake (2)
(becomes
negative pressure), the difference in the
press
ure is transmitted as
the
pressure
difference
in between the upper and lower surfaces
of
the
vacuum piston at portion (a) and portion (bl. and
the force to pull up the vacuum piston is actuated.
The faster the flow speed of air intake and venturi
portion ( 1 I an
(2),
(to be
more
specific, more the
difference of flow speeds
of
air intake and venturi
portion) becomes, the negative pressure
becomes
larger ;
thus
the vacuum piston (16) is pulled up in
a value suitable to the flow speed, and the venturi
area is increased.
Further, the engine speed is raised at
high speed,
the vacuum piston ( 16) rises up to the top at least,
and
forms
the secondary venturi sectional surface
which is sufficient to maintain the maximum
output.
(Fig. 3. 123j
2. Fuel System
a. Flow system
Flow value of fuel
which
passed through the
main jet is controlled at slow jet through pilot jet,
and simultaneously, the fuel is mixed with air from
the slow air jei at the air bleed of
slow
jet.
The
mixture jets from the pilot outlet ( 121 and bypass
( 131 near the periphery of the throttle valve. (Fig.
3.
125)
On
the
pilot
outlet
( 121, the pilot screw
which
regulates the mixture is provided.
(Fig.
3.
129)
b. Main system
A part of the fuel
which
passed through the
main jet is flowed into the slow
system
as explained
above ; however, the other main portion of the
fuel
is mixed with air from the air jet within the
air
bleed of the needle jet holder, and jets out
Fig. 3. 124
Vacuum
Piston
CD
Vacuum piston
®
Jet
needle
@
Fig. 3. 125
Pilot
jet and
slow
jet
CD
Pilot jet
®
Slow
jet
59
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