What effect does a sheared or partially sheared flywheel key have?

A sheared or partially sheared flywheel key primarily affects the ignition timing of an engine. The flywheel key is designed to securely attach the flywheel to the crankshaft, ensuring that the timing of the ignition events coincides accurately with the engine's piston movements. When the flywheel key is compromised, it may allow the flywheel to rotate out of its intended position.

This misalignment results in ignition timing being shifted either too early or too late relative to when the piston reaches the top of its compression stroke. Correct ignition timing is crucial for engine performance, as it ensures that the fuel-air mixture ignites at the optimal moment for maximum power output and engine efficiency.

While a faulty flywheel key may lead to engine stalling or impact how loudly the engine runs, these are secondary issues stemming from the primary concern of incorrect ignition timing. In some cases, it could also affect fuel efficiency, but the most direct and immediate consequence of a sheared flywheel key is the ignition timing being off, making this the most accurate choice.