[cooperino]

Originally Posted by Gompers

Wasted spark was used back in the day to simplify the ignition system and is used today to reduce emissions. The plugs would only fire in pairs if they they have cylinders with strokes that are 360 degrees out from one another. Otherwise, you don't have a reduction in ignition parts. This is how M18 and KT17s work. One magneto (for a M) and one coil (for a KT) with two leads that fire at once, since the cylinders are 180 degrees opposed.

Also, pretty much all single cylinder magneto ignition small engines are wasted spark since the magneto fires every time the flywheel magnet comes around.

From wikipedia: "Most single cylinder [four-stroke] engines use the wasted spark system in order to capitalise on the simplicity and reliability of the flywheel magneto. These engines need a flywheel to run smoothly, and the heavy current-generating magnets help provide the momentum while delivering a zero-maintenance drive to the ignition system. Bolted to the end of the crankshaft, this flywheel rotates twice for each compression stroke."

Obviously there's no second plug to fire in this case.

This won't work on V twins with shared crank journals since the firing interval is not equal. So one piston is 90 degrees away from TDC when the other one is at TDC, regardless of which one is on compression and which is on exhaust. Each cylinder has it's own ignition system and it fires on every revolution, hence "wasted spark". It's basically two individual ignition systems each running like a single cylinder engine would.

As RAC says above, there's no cam position sensor to know which cylinder is on which stroke so it fires every time. I suspect this was done to keep things simple(r).

Very good read and well written explanation.. thank you!