In old days,Fiat cars are most popular brand in India due to its very easy start in cold conditions.This easy start is obtained by Solex Craburettor which is a down draft type carburettor and has special provisions to supply,richer mixture at starting of engine and weaken it gradually till the engine reaches its normal running speed.Solex carburettor are available in many design and all have following work circuits,
01) Starting Circuit:-
The throttle valve remains in closed position during starting.The petrol is supplied to the starter petrol jet through first passage from the float chamber and the air through the starter air jet for starting operation.
Starting Valve which have different sizes hole,is made from flat disc.The position of various holes can be adjusted in front of starter petrol jet by starter lever and then air is mixed coming from starter Air jet .This air-fuel mixture,passes through another holes of starter valve,in a passage of the carburettor,below the throttle valve.The suction stroke of the engine sucks this mixture into the cylinder.This mixture is rich enough to start the engine.
After the engine starts and speed increases,a weak mixture is required,therefore,a small hole of the starter valve is brought in front of the starter petrol jet by means of starter lever,there by reducing the quantity of petrol,which weaken the air-fuel mixture.Similarly next smaller hole of the starter valve is brought in front of starter petrol jet till the engine attains its normal speed then the starter valve is closed by bringing the starter lever to its off position.
02) Normal Running Circuit:-
At normal running speed,starting circuit is closed and throttle valve is opened.The normal running circuit consist of main jet which receives the petrol through second passage,from the float chamber as the throttle valve is opened sufficiently,the air is drawn through the venturi where the petrol mixes up with it forming a suitable mixture for the normal running of the engine.In this case,only throttle valve,governed the air-fuel ratio.
03) Accelerating Circuit:-
The engine requires an extra rich mixture,during acceleration period.To obtained extra rich mixture,the fuel is pumped under pressure into the main air passage or in the venturi through an injector.
Diaphragm pump is used to create pressure,which is actuated by a lever connected to the accelerator.The pump sucks the petrol from the float chamber through the pump valve and forces it through third passage into the main passage through an injector above the venturi of the carburettor.
04) Idling And Slow Running Circuit:-
During the idling operation,the throttle valve is kept closed and the engine receives the mixture through a port opening below the throttle valve,whose area can be varied by an idle adjusting screw according to the need of the engine.The petrol is supplied to a pilot petrol jet from the main jet fuel circuit through fourth passage and the air from a pilot air jet .The petrol and air thus supplied mix up in the idle passage and go to a port situated below the throttle valve from where the mixture is sucked by engine.
During the slow running,the engine draws the mixture from the idle passage through a hole situated above the throttle valve when the throttle valve is partially opened.
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