The gear ratio for the Main drive Gear and Magneto gears is two and a quarter. The contact breakers give 2 sparks per revolution and
run at two and a quarter times the engine speed, each giving nine sparks every two revolutions of the engine.
The firing order therefore is 1, 3, 5, 7, 9, 2, 4, 6, 8.
See Magnetos simply explained for additional details.