Which instrument provides information on the fuel primer system used for starting an engine?

The instrument that provides information on the fuel primer system used for starting an engine is the carburetor air temperature gauge. This gauge measures the temperature of the air entering the carburetor, which is directly related to fuel vaporization and engine starting performance.

During engine starts, particularly in cold conditions, the fuel primer system enhances starting by ensuring that the fuel is properly vaporized as it mixes with air. If the air temperature is too low, it can lead to poor fuel vaporization, resulting in difficult starts. The reading from the carburetor air temperature gauge helps assess whether the air temperature is conducive to effective fuel mixing and engine starting.

While the other instruments listed provide essential information about engine operation, they do not specifically indicate the condition of the fuel primer system or the effectiveness of fuel vaporization in the starting process. The oil temperature gauge measures the temperature of the engine oil, the pressure gauge typically refers to fuel or oil pressure, and the RPM gauge indicates the engine speed, none of which are directly linked to the function of the fuel primer system.