Toyota 3R-B 1969, Repair Manual

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5-6 F U E L S Y S T E M - Car buret or
T h e ca rb u re to r is a three group construction consisting of the air horn group,
main body group and the flange g r o u p .
T h e prim ary b ore provided with a double venturi consists of the slow system,
high speed system, accelerating system, automatic choke system and the ther­
mostatic valve system enabling the air-fuel mixture to be supplied to the engine
as required f o r normal driving conditions.
T h e secondary bore provided with a triple venturi consists only of the high speed
system to meet the requirement for high speed, and fo r sudden acceleration.
1. Float chamber & air vent system
T h e float chamber s e r v e s as a
constant level fuel r e s e r v o i r . It is
n ec e ss a ry to maintain the fuel level
at a constant height re g a rd le s s of
whether small o r la rge amount of
fuel is being withdrawn.
T h e level gauge glass is installed
as a c o v e r fo r visual inspection
of the fuel le v e l. T o maintain the
fuel level at a constant height, the
float chamber is provided with a
float pivotted on an a rm , a needle
valve and a seat.
If the fuel enters into the float chamber faster than it is withdrawn into the
engine, the float (1 ) r is e s , lifting the needle valve ( 2 ) , causing to seat firmly
against the needle valve seat (3 ) . If the fuel withdrawn is quite fast, opposite
operation will take place to compensate to balance the fuel level height. T h e s e
items a r e v e r y important to obtain p r o p e r perform an ces of the ca rb u retor.
A i r vent tubes (5 ) a re provided to maintain the same air p r e s s u re in the
air horn, and the float cham ber. Th is type compensates the clogging effect
of the a ir c lea n er.
2. Idling & low speed system
When the throttle valve is fully
closed, o r slightly opened, the
main n ozzle will not discharge any
fuel.
A s the a ir passing through the
venturi being v e r y slow and little
t h e r e fo r e , no vacuum is generated
in the venturi. F o r this reas on ,
the c a rb u r e to r is incorporated with
another circuit (idle and low speed
system ) to supply air-fuel mixture
during this type of operation.
T h e p r e s s u r e difference between
the float cham ber and the idle port (6 ) is due to the high manifold vacuum
which f o r c e s the fuel through the idle and low speed system.
F ig . 5-4 Idle & L o w
S p eed System
G1735
5
F i g . 5-3 Float Chamber Y6713
& A i r Vent