What is a common result of improper valve adjustment?

An improper valve adjustment can lead to an engine misfire due to the incorrect timing and operation of the engine's intake and exhaust valves. When the valve clearance is not set correctly, it can result in valves opening too late or closing too late. This can disrupt the air-fuel mixture entering the combustion chamber, causing incomplete combustion. Consequently, this incomplete combustion can lead to misfiring, where the engine fails to fire in a smooth and continuous manner, resulting in reduced power output and uneven engine operation.

In contrast, the other options suggest positive outcomes (increased horsepower, better fuel efficiency, reduced emissions), which are not associated with improper valve adjustments. Instead, a well-adjusted valve system ensures optimal engine performance, contributing to smooth operation and effective combustion.