[Gompers]

I was under the impression that wasted spark refers to the spark plug firing on the exhaust stroke as well as the compression stroke.

The two coils fire at different times, (90 degrees apart from one another) but they both fire every time the magnet goes by, so once every 360 degrees of flywheel rotation. The cylinder only has a compression stroke once every 720 degrees of flywheel rotation (4 stroke engine). So one of those fires is "wasted".

On a non-wasted spark engine, like a K series, the engine only fires once every 720 degrees of rotation, since the cam travels at half the speed of the crank, and ignition is timed off the cam, not off the flywheel.

I believe (though I could be wrong...haven't ever really messed with one) that the SAM on a CH22/25 engine just advances the timing based on engine RPM, but doesn't actually work like a fully electronic ignition would in a non-wasted spark engine, only firing the cylinder once every 720 degrees.

It's also why when you do static timing on a KT series, which has two "ignition" lobes on the cam, you should set it, rotate the engine 360 degrees and check it/set it again. It can be off quite a bit depending on the condition of the camshaft lobes. The KT manual says to set it between the two.

So, on a 4 cycle wasted spark engine, any spark plug firing will give you a 360 degree rotation of the engine. On a non-wasted spark engine, it would give you 720 degrees of rotation. That's why you would theoretically need to divide by 2 to get the engine RPM.