Crankcase Ventilation System

On later models a special crankcase-breather system is fitted. The engine fumes are drawn from the crankcase, through aflame-trap and a ventilation valve, into the inlet manifold. The valve regulates the flow of air and fumes into the manifold, thus preventing excessive weakening of the mixture when the engine is idling. Clean air is drawn into the engine to replace the fumes, through a filter which is incorporated in the oil filler cap. This cap should be renewed as a complete assembly after 12,000 miles in service, or earlier if it becomes clogged.
To test the system, take off the oil filler cap when the engine is running at idling speed, at normal operating temperature. If the valve is functioning correctly the engine will speed-up by approximately 200 r.p.m. (which represents an audible change in engine speed) when the cap is removed.

If there is no change in the idling speed, remove the spring clip from the ventilation valve and dismantle the valve as shown in. Clean the metal parts with petrol. Hard deposits on these parts can be softened by dropping the parts into boiling Vater for a few minutes.

The diaphragm must be in good condition. If in doubt, replace it. Do not stretch or distort the spring. When reassembling the valve, make sure that the metering needle is correctly centred and that the diaphragm is seating properly before refitting the cover and spring clip.