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

Volumetric efficiency in a reciprocating engine is a measure of the engine's ability to draw in air-fuel mixture compared to the amount that the cylinders could theoretically hold. Sharp bends in the induction system can significantly impede airflow, leading to turbulence and losses in pressure. This disrupted airflow reduces the amount of air-fuel mixture that can be inducted into the cylinders during the intake stroke, ultimately decreasing the engine's volumetric efficiency.

Factors like full throttle operation typically enhance volumetric efficiency because they maximize the intake of the air-fuel mixture. Low cylinder head temperatures might affect combustion but are not directly related to the intake airflow characteristics. Similarly, while improper valve timing can also lead to inefficiencies, it primarily affects the timing of intake and exhaust events rather than the flow characteristics induced by a design feature like sharp bends in the induction system. Thus, sharp bends themselves are a major contributor to reduced volumetric efficiency.