No RPM drop during a mag check could indicate a broken __________________?

A lack of RPM drop during a magneto check often indicates an issue with the P-lead, which is the wire that connects the ignition switch to the magneto. When the ignition switch is turned off or placed in the "off" position during the mag check, the P-lead should short the magneto and cause the RPM to decrease because one of the magnetos is no longer firing. If there is no RPM drop, it suggests that the P-lead may be broken or malfunctioning, preventing the electrical connection from properly shutting off the magneto.

In contrast, problems with other components such as the fuel line, ignition coil, or starter motor usually result in different types of performance issues, not specifically related to the RPM drop during a magneto check. Therefore, while issues with those components could lead to other operational problems, they would not directly explain the phenomenon of no RPM drop during the magneto check.