When is the mixture supplied by the carburettor considered weak?

The mixture supplied by the carburettor is considered weak when the proportion of air in the mixture is greater than required. A weak mixture typically indicates that there is an excess of air compared to fuel, which results in a higher air-to-fuel ratio. This condition can lead to incomplete combustion, as there is not enough fuel to utilize all the air for effective operation.

In the context of engine performance, having a weak mixture can affect engine efficiency and power output, often resulting in hard starting or poor acceleration. It’s essential for engines, especially reciprocating engines, to maintain the correct fuel-to-air mixture to ensure optimal performance. If the mixture is weak, the combustion process can become inefficient, leading to various performance issues.

Other options relate to different characteristics of the mixture or engine behavior but do not specifically address the condition of having an excess of air. For example, insufficient proportion of air would indicate a rich mixture, while using a lower grade of fuel or insufficient power for take-off are indicative of issues unrelated to the air-fuel mixture's strength.