What would be the likely effect of prolonged running with a weak mixture?

Prolonged running with a weak mixture typically leads to overheating in a reciprocating engine. A weak mixture means there is an insufficient amount of fuel compared to the air being fed into the engine. This imbalance can cause the engine to run hotter than normal for several reasons.

Firstly, a weak mixture results in a higher combustion temperature because the available fuel burns less efficiently and creates more heat. Additionally, with less fuel, there is insufficient thermal mass to absorb the heat generated during combustion, leading to excessive heat build-up in the engine components.

Furthermore, a lean mixture can affect the cooling characteristics of the engine, as the combustion process can create hotspots on cylinder walls, leading to potential engine damage due to thermal stress. Ultimately, this can also cause components like valves and piston rings to wear more quickly.

The other options represent scenarios that do not typically occur as a direct result of running with a weak mixture for an extended period, such as consistently failing to reach the correct operating temperature, which is generally associated with a rich mixture or insufficient cooling system performance. Carburettor icing usually occurs in certain environmental conditions that are unrelated to mixture strength. High oil pressure might stem from factors like oil viscosity or pump issues rather than mixture ratios.