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Fig.2-84. Part throttle and full throttle enrichment systems:
1 - secondary throttle valve; 2 - main fuel jet, secondary barrel; 3 - fuel jet and tube, full throttle enrichment; 4 - main fuel jet, primary barrel; 5 - primary throttle valve; 6 -
vacuum port; 7 - part throttle diaphragm; 8 - ball valve; 9 - fuel jet, part throttle enrichment; 10 - fuel passage; 11 - choke; 12 - main air jets; 13 - injection tube, full throttle
enrichment
Main metering system. Fuel through gauze filter 4 (Fig. 2-
82) and needle valve 6 is fed to the float chamber. From the float
chamber fuel flows through main fuel jets 9 to the emulsion wells
to be mixed with air, escaping from emulsion tubes 1 which are
built as one piece with the main air jets. Through atomizers 2 the
air-fuel mixture flows to the primary and secondary venturi.
Throttle valves 8 and 10 are connected so that the secondary
barrel starts opening when the primary one is 2/3 open.
Idling system runs on fuel from the emulsion well behind
fuel jet 7 (Fig.2-83). Fuel is fed to fuel jet 2 which is fitted with fuel
cut-off solenoid 1. At the jet outlet the fuel is mixed with the air
coming from the port and from a flared part of the venturi (to
ensure the carburettor smooth operation at idling). Emulsion goes
under the throttle plate through the correcting orifice adjusted by
idle mixture adjustment screw 9.
Air correction systems. Following the throttle opening but
prior to activation of the main metering system, the air-fuel mix-
ture enters:
- the primary barrel through idling fuel jet 2 and air correction
system passage 8, located at the level of the closed throttle plate
edge;
- the secondary barrel through outlet orifice 6 being slightly
over the edge of the closed throttle plate. Fuel flows from jet 4
through the tube to be further mixed with air from air jet 5 flowing
through the idling air passage.
Part throttle enrichment (economizer) operates when cer-
tain vacuum level is maintained on throttle body 5 (Fig. 2-84).
Fuel is fed from the float chamber through ball valve 8. Valve 8
remains closed as long as the diaphragm is held retracted in the
intake pipe by means of the vacuum. With wide open throttle, the
vacuum decreases allowing diaphragm 7 spring to open the
valve. Petrol which flows through part throttle enrichment jet 9 is
added to fuel passing through main fuel jet 4, providing a richer
mixture.
Full throttle enrichment (econostat) operates at full engine
load, at high, close-to-maximum speeds and wide open throttles.
Petrol from the float barrel through jet 3 (Fig.2-84) is delivered to
the fuel pipe and further through injection tube 13 into the sec-
ondary barrel resulting in a richer mixture.
Accelerator pump is of diaphragm type, mechanically oper-
ated by cam 6 (Fig.2-85) on the primary throttle spindle. With the
throttle valve closed, the spring retracts diaphragm 3 thus filling
the pump with petrol through ball valve 8. As the throttle opens,
the cam operates lever 5, whilst diaphragm 3 forces fuel through
ball valve 2 and atomizer 1 into the barrels enriching the air-fuel
mixture.
The pump capacity is invariable and is determined by the
cam profile only.
Choke control unit. Choke lever 4 (Fig.2-86) has three pro-
files. Its outer edge 4.3 operates throttle lever 11 through adjuster
screw 10 to start the engine when cold and to further rise the
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