Assignment

The carburetor should provide the engine with a rich mixture when cold starting, because the fuel condenses on the cold inlet manifold and cylinder walls making it non-combustible.

Function

There used to be carburetors, where the choke-flap (above picture) had to be hand-pulled by the driver (choke). In the above picture an automatic choke is shown, in which, when switching on the ignition, a bimetal is, in this case, only electrically heated, whereby, the spring tension for the closing of the choke-flap diminishes depending on the temperature.

Typical for the cold-start are the completely closed choke-flap, and the slightly opened throttle-flap (see picture below). To produce such a rich mixture of 3 kg of air to 1 kg of fuel, the closed choke-flap reduces the air intake drastically, and at the same time, through the increased vacuum, more fuel is drawn in through the main jet system. Through the switched on ignition the electro-magnetic idling cut-off (see figure below left) opens the idling channel. This reliably prevents the engine, when the ignition is switched off, from running (post-dieseling) when hot spots in the combustion chamber (e.g., glowing oil-carbon) could ignite the in- flowing fuel-air mixture.

During cold starting, at higher RPMs, the choke-flap must be opened by the pull-down can (see figure above right), otherwise the engine is given a mixture which is too rich, causing the spark plugs to become wet, and the engine is 'flooded'.

Important

A cold start is more of a strain on the petrol engine than a longer motorway trip, because the condensing fuel washes the oil from the cold cylinder walls and dilutes it. Therefore, - particularly if the oil-level increases - the oil must be changed more often. 11/10