Why Two Mags?

[onetrack]

Capot - Magnetos with an impulse starter (often referred to, as an impulse coupling) have a setup consisting of small spring-loaded flyweights located on a spring-loaded, rotatable, driven plate within the magneto housing.
The aim of an impulse coupler is to retard the spark by about 15° at cranking RPM, as well as deliver a hotter spark, than would otherwise be available at cranking speed.

The magneto is flexibly connected through the impulse coupling by means of a spring on the driven plate, so that at low speed the driven plate is temporarily held by stop pins that engage with the flyweights, while the magneto shaft is rotated until the piston to be fired reaches approximately TDC position.

At this point the driven plate is released by other pins or lugs on the magneto shaft, running up ramps on the flyweights, which releases the flyweights from the stop pins... and the spring on the driven plate, kicks the driven plate back to its original position, resulting in a quick twist of the rotating magnet.
This, being equivalent to high-speed magneto rotation, produces a hot spark.

After the engine fires enough to sustain running (around 150-400 RPM), the flyweights in the coupling fly outward due to centrifugal force, and lock the two coupling members together. This makes it a solid unit, returning the magneto to a normal timing position relative to the engine.

The presence of an impulse coupling is identified by a sharp clicking noise as the crankshaft is turned at cranking speed past TDC on each cylinder.
Often the flyweights become magnetized and do not engage the stop pins, resulting in a malfunctioning impulse coupling. Additionally, congealed oil on the flyweights during cold weather may produce the same results.

Single cylinder engines or twin engines that fire 360° apart, have single flyweights for a single impulse every revolution of the magneto shaft.
In engines where a cylinder firing is required every 180° of the magneto shaft rotation, there are twin flyweights, which produce an impulse every 180° of rotation of the magneto shaft.