A fairly common ignition system problem experienced with engines powering rotary lawn mowers is __________.

A sheared flywheel key is a common ignition system problem in engines that power rotary lawn mowers due to its critical role in aligning the flywheel with the crankshaft. The flywheel is essential for the timing of the ignition system, as it stores rotational energy and helps keep the engine running smoothly. When the flywheel key shears, the connection between the crankshaft and the flywheel is lost, causing the flywheel to rotate independently of the crankshaft. This can lead to improper timing of the spark, resulting in difficulty starting the engine, misfires, or a complete failure to run.

In lawn mowers, which operate at relatively high speeds and under varying loads, the risk of the flywheel key shearing can increase due to vibration and sudden impacts from hitting debris. Regular maintenance and inspection can help mitigate this risk.

The other options pertain to different issues that may arise in small engines but are not as directly related to the ignition system as a sheared flywheel key is. For example, a cracked cylinder block could cause significant compression loss but isn't tied to the ignition timing. A stuck valve leads to performance issues related to the engine's breathing but does not affect the electrical ignition timing. Water in the oil indicates a different