What is one common consequence of incorrect valve timing in a four-stroke engine?

Incorrect valve timing in a four-stroke engine can lead to backfiring into the induction system. This happens because improper timing may prevent the exhaust valves from closing at the right moment or cause the intake valves to open before the exhaust gases are fully expelled. As a result, when the pressure in the combustion chamber is altered, it's possible for the unburned fuel and air mixture to ignite in the intake manifold, causing a backfire.

The other outcomes mentioned, like increased horsepower, improved fuel efficiency, and lower emissions, are generally positive side effects that proper valve timing encourages, rather than consequences of incorrect timing. If the timing is off, these benefits are usually diminished, highlighting the critical role of precise valve operation in engine performance and efficiency.