Why is it considered dangerous to pump the throttle lever when starting an engine?

Pumping the throttle lever when starting an engine is considered dangerous primarily because it can increase the risk of fire in the carburettor air intake. When the throttle is pumped, it allows additional fuel to enter the carburettor, which can create a rich mixture within the air-fuel mixture that is being drawn into the engine. If the mixture becomes too rich, it can lead to fuel pooling in the carburettor or even in the intake manifold. If this excess fuel comes into contact with any ignition source, whether it be a spark from the engine’s components or heat generated during the starting process, it could ignite, leading to a fire hazard.

In addition, while there are other potential effects of pumping the throttle that could involve starting difficulties or mixtures becoming too rich, these do not pose the immediate and significant danger that the risk of fire does. Therefore, understanding the implications of how excessive fuel can affect ignition systems is crucial for safe engine operation, highlighting why this action is discouraged during the starting process.