What effect does improper valve timing have on engine performance?

Improper valve timing can significantly reduce the engine's horsepower. When valve timing is not correctly synchronized with the movement of the pistons, it can lead to several detrimental effects on the combustion process. For example, if the intake valves close too early or too late, it may restrict the amount of air-fuel mixture entering the cylinder, which subsequently affects the engine's ability to generate power during the combustion cycle.

Furthermore, if the exhaust valves do not open or close at the appropriate times, it can prevent the efficient expulsion of combustion gases, leading to back pressure and reduced performance. Overall, the efficiency of the engine becomes compromised, leading to a decrease in horsepower. The other options, such as increasing fuel efficiency, enhancing cold starting capability, or improving throttle response, are not directly associated with improper valve timing, which predominantly leads to reduced engine output and efficiency.