When is removal of the flywheel necessary?

The removal of the flywheel is primarily necessary to inspect the flywheel key and keyways. The flywheel key is a critical component that connects the flywheel to the crankshaft, ensuring that they rotate in unison. If the flywheel key becomes damaged or sheared, it can lead to timing issues, affecting engine performance and operation. Inspecting this part allows for the identification of any issues that could lead to more significant mechanical problems.

The other options involve maintenance tasks that can typically be performed without removing the flywheel. For instance, spark plug maintenance and carburetor cleaning focus on engine components that are generally accessible without the need to disturb the flywheel. Valve lash adjustment usually addresses the valve train components, which again don't necessitate flywheel removal. Thus, while these tasks are essential for engine performance, they are not directly related to the condition and functionality of the flywheel itself.