What effect does improper valve timing have on engine performance?

Improper valve timing can significantly affect engine performance, primarily leading to a decrease in horsepower. Valve timing refers to the precise moments when the intake and exhaust valves open and close during the engine's operation cycle. If the timing is incorrect, it can result in several adverse effects on the combustion process.

When valves do not open and close at the optimal times, the engine can suffer from poor air-fuel mixture intake and exhaust gas expulsion. This leads to inefficient combustion, causing the engine to produce less power than it is capable of. Additionally, improper valve timing can create issues such as back-pressure during exhaust, hindering the engine's ability to move exhaust gases efficiently and affecting the overall performance.

In contrast, the other options presented imply positive outcomes that contradict the impact of improper valve timing. They do not accurately describe the consequences associated with poorly timed valves. For instance, improved fuel economy, better exhaust emissions, and increased idle stability are all results of proper valve timing, not improper timing. Hence, recognizing that improper valve timing is linked with a decrease in horsepower reflects a fundamental understanding of engine mechanics.