The Dog and Lemon Guide

A spark occurs in the ignition switch at the moment of switching the main power on, because the current at the contact point is excessive. With a repetition of starting operations, the contact point gets worn out excessively, causing its contact pressure to decrease, and carbide accumulates on the contact point. As a result, the contact point may move due to vibration during travel, disturbing electric continuity. At worst, the engine stops suddenly. Incidentally, the engine can be restarted, as the carbide pieces are nudged off when the contact point is moved by engine starting operation.