What is the purpose and operation of an impulse coupling?

• A. A fixed timing device.
• B. A battery-powered timer.
• C. A device that cools the magnet.
• D. An impulse coupling is a spring-loaded device that locks the magnet during starting, winds up, and releases to spin the magnet for a hot and retarded spark near top center.

Answer: (D)

The impulse coupling is a spring-powered starting aid for a magneto. Its job is to give the engine a hot, reliable spark at the right moment during cranking. While you’re cranking, the mechanism locks the magneto rotor and winds the spring. When the starter releases, the spring suddenly drives the rotor, delivering a rapid spin that creates a strong spark near top dead center and with timing that’s retarded compared to normal running. Once the engine starts, the impulse mechanism disengages and the magneto returns to its ordinary timing.

This isn’t a fixed timing device, a battery-powered timer, or a device to cool the magnet. It’s specifically about storing energy mechanically to produce a powerful, properly timed spark for starting.