Reciprocating Engines Which of the following will decrease volumetric efficiency in a reciprocating engine? 1. Full throttle operation. 2. Low cylinder head temperatures. 3. Improper valve timing. 4. Sharp bends in the induction system. 5. High carburetor air temperatures. a. 2, 4, and 5. b. 1, 2, 3, and 4. c. 3, 4, and 5.