If over-priming causes a fire to start in the engine's carburettor during starting, what is the best immediate action?

In the scenario where over-priming has caused a fire to start in the engine's carburettor during starting, the best immediate action is to keep the engine turning on the starter motor and select "idle cut-off." This approach is effective because maintaining the rotation of the engine helps to draw fresh air into the carburettor, which can help to extinguish the fire.

By selecting "idle cut-off," you are effectively stopping the fuel flow to the engine, which reduces the amount of fuel available for combustion and minimizes the risk of a larger fire. This method leverages the engine's operation to address the emergency situation directly without shutting it down completely, which could allow unburned fuel and fire to exacerbate the situation.

In contrast, other actions may not effectively mitigate the situation. Evacuating the aircraft and calling for help, while important, does not address the immediate fire threat. Shutting down the engine might leave residual fuel and heat that can allow the fire to continue. Selecting a weak mixture and rapidly increasing RPM could potentially introduce more fuel into the carburettor, which can worsen the fire situation. Thus, maintaining engine turnover while cutting off fuel flow is the most effective method to handle this hazardous situation.