When there is no spark detected with the ignition tester, what is a likely issue?

When there is no spark detected with the ignition tester, a likely issue is that the flywheel key has sheared. The flywheel key plays a critical role in the operation of small engines; it helps maintain the alignment between the flywheel and the crankshaft. If the flywheel key is sheared, it causes a misalignment which can prevent the ignition coils from functioning properly. As a result, the engine may not generate the necessary spark for combustion.

Conversely, while a clogged air filter, weak battery, and bad fuel could contribute to engine problems, they would not directly cause the ignition system to fail in producing a spark. A clogged air filter would restrict airflow, affecting combustion rather than the ignition spark itself. A weak battery could lead to difficulty in starting the engine but would not necessary prevent spark generation, especially if other components are functioning correctly. Bad fuel can cause poor engine performance and misfiring, but again, it does not correlate with the absence of spark detection in the ignition system. Thus, the failure of the flywheel key is the most direct cause of no spark being detected.