speed or normal road load conditions, Is great enough to hold the power valve diaphragm down, overcoming the tension of the spring on the valve stem and holding the valve closed. When high power operation places a greater load on the engine and manifold vacuum drops below a predetermined value, the spring opens the power valve. Fuel from the fuel bowl flows through the power valve and into passages leading to the main fuel well. Here the fuel is added to the fuel from the main fuel system, enriching the mixture.
As engine power demands are reduced, manifold vacuum increases. The increased vacuum overcomes the tension of the valve stem spring and closes the power valve.
STARTING THE ENGINE
When cold
Switch on the ignition noting that the generator warning lamp lights up. The electric fuel pump will also be heard operating. This is normal.
Press the accelerator pedal down to the floor once. THIS MUST BE DONE to allow the automatic choke mechanism to open the throttle to its correct position for cold starting.
Operate the starter WITHOUT MOVING THE AC-CELERATOR PEDAL. The engine should start im-mediately and continue to run at a fast idle speed. As the engine warms up, its Idling speed will automatically return to normal provided the accelerator pedal has been moved.
From extreme cold (below 100F; —120C.)
When starting under extreme cold conditions FULLY DEPRESS THE ACCELERATOR PEDAL THREE TIMES before operating the starter. This pedal movement allows the accelerator pump to Inject a small quantity of fuel into the inlet manifold which helps starting under extremely cold conditions.
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When warm
When the engine is warm, switch on the Ignition and operate the starter without moving the accelerator pedal. Should the engine hesitate to start, depress the accelerator pedal slightly, releasing it when the engine starts.
The accelerator pedal must not be pumped or agitated as this will cause the accelerator pump to inject an excessive quantity of fuel into the hot Inlet manifold. This fuel will quickly vapourise and produce an excessively rich mixture that will prevent the engine from starting.
Starting a very hot engine
If difficulty is experienced in starting a hot engine the accelerator pedal should be slowly and fully depressed and the starter operated. Release the accelerator pedal immediately the engine starts.
DIAGNOSIS OF FAULTS
Condition of ignition system and engine
The correct functioning of the carburettor is dependent upon the proper functioning of the Ignition system, satisfactory compressions in all engine cylinders, freedom from air leaks in the induction system, and correct operation of the crankcase ventilating system regulator valve. In consequence troubles are sometimes difficult to trace and the carburettor is often suspected when it is not at fault.
This fact is seen clearly If reference is made to the causes given for the various faults shown In the Diagnosis Guide In Section B.
A considerable amount of useful Information is also given in Section B under the heading “Ignition System”.
Carburettor diagnosis guide
The following diagnosis guide covers faults that can only occur in the actual carburettor.
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