Which of the following will decrease volumetric efficiency in a reciprocating engine?

Volumetric efficiency in a reciprocating engine refers to the engine's ability to fill its cylinders with air during the intake stroke. This efficiency is influenced by several factors, including the temperature of the air entering the engine, the physical design of the intake system, and how the engine's valves are timed.

Improper valve timing can significantly reduce volumetric efficiency because if the valves open or close at incorrect times, it can lead to incomplete cylinder filling or the mixing of exhaust and intake gases, which results in lost performance. Sharp bends in the induction system also play a crucial role; when the air intake path has sharp bends, it creates turbulence and increases resistance, making it harder for the engine to draw in the necessary air efficiently.

High carburetor air temperatures would warm the intake charge. Warmer air is less dense than cooler air, and since less dense air leads to a reduction in the mass flow rate of the air being drawn into the engine, it too negatively impacts volumetric efficiency. Therefore, all these factors contribute to decreased volumetric efficiency by either impeding air flow or reducing air density, making option C the correct answer.