Assuming a cold engine the main valve (10) will be open to a degree determined by the temperature of the main bi-metal (22) the fuel needle (15) will be raised by the diaphragm spring (31) until it is restrained by the second bi-metal (3) and the fuel well (16) will be filled to the level of the fuel in the float chamber.
On cranking the engine air is drawn past the lightly spring-loaded air valve (26) through the main valve seating (13) and into the inlet manifold. Fuel is drawn into the engine through the fuel orifice (18) temporarily enlarged by the lifted needle (15) also through the well orifice (17) and up the jet tube (14) to the main valve (10) producing a very rich mixture to wet the inlet manifold rapidly and so shorten the cranking time. When the engine runs, manifold depression acting via passage (29) draws down the needle diaphragm (30) and allows the spring (31) to lower the fuel needle to its normal idling position. The increased manifold depression also draws the main valve (10) slightly towards its seating due to small out of balance forces acting on the valve and its diaphragm. The engine will then run at the required speed as set by the adjusting screw (1) the mixture strength being temporarily increased by the discharge from the fuel well (16). When the well is emptied, the well orifice (17) acts as an air bleed to the jet tube, air being drawn into the fuel well via passage (12) from the bi-metal chamber (19).
As the engine temperature rises heated air is drawn through the device, a proportion of this passes through the bi-metal chamber via passage (27) and orifices (9) and (33) raising the bi-metal temperature and progressively closing the valve until the full working temperature is reached. At this point the valve will be fully closed but subsequent running will induce sufficient heated air to be drawn through the bi-metal chamber to maintain the bi-metal temperature and keep the main valve closed.
Before full running temperature is reached extra enrichment is needed for acceleration. This is provided in response to falling manifold depression, small carburetter throttle openings result in the main valve (10) opening slightly due to a reduction of the pneumatic load which was tending to close it, whilst further opening of the carburetter throttles reduces the depression sufficiently to allow the needle diaphragm spring (31) to push the needle (15) upward thus opening the fuel orifice (18) and increasing the fuel supply until such a.time as either the increase in the engine speed raises the manifold depression or the throttle is closed.
On stopping the engine the heat stored in the heavy section heat shroud (20) and the thermal insulating properties of the moulded valve body (11) and the bimetal cover (2) ensure that the cooling rate of the bimetal matches that of the engine so that the device will only come into operation at the required temperature.
Carburetter tuning
On models fitted with the Automatic enrichment device, it is most important when tuningthe carburetters to ensure that the A.E.D. is functioning correctly as follows:
(a) The engine must be thoroughly warm and at its normal running temperature.
(b) The A.E.D. must be inoperative, that is, completely cut off and is not enriching the mixture. This can best be done by first noting the engine idling speed. Stop the engine, remove the delivery hose of the inlet manifold. Blank off aperture in inlet manifold. Start engine, idling speed should be slightly lower than that previously noted, as there is an air bleed from the A.E.D to the inlet manifold. If the engine speed drops more than approximately 50 rpm this indicates a leak in the A.E.D. unit which must be rectified. Finally replace the delivery hose.
Then
tune the carburetters as detailed in the appropriate Workshop Manual.
