Two cylinders will fire at the same time. 1 and 4, then 2 and 3.During each rotation of the crankshaft, 360 degrees, each set of points open at 180 deg. of each other, causing a spark to occur in two of the cylinders. The other set of points fires 180 deg. later, so that in every 360 deg. of crankshaft rotation each plug is fired once. One extra spark occurs during the time when there is no combustable material in the chamber. Visit kzrider.com for lots of great information.