NOT sure how you are driving the Fairbanks mag, but I"m GUESSING it"s a 4 cylinder version and only the spark for one cylinder is getting to the spark plug. Or, if a single-cylinder mag, the gear ratio between the mag and the camshaft is wrong.
Spark SHOULD get to the plug EVERY time the crank turns ABOUT 180° from the time the exhaust valve closes. The exhaust valve will close every 720° crankshaft degrees ASSUMING the engine is NOT above governed speed, so the governor is latching up the valve pushrod. As it is now, sounds like you have spark at the plug, then engine turns though 3 compression cycles (2160°), then the spark plug "fires" again.
Tell us more about the mag you are using and how it is driven.