This might be a good time to share my story again about wasted spark ignitions and why I don't like them.
Back in the early 80's, the industrial engine manufacturers were trying to figure out how to get rid of the dual 8 cylinder magnetos for 16 cylinder engines, and they hit on the idea of using a single 8 cylinder unit and double firing it, and waste spark was born for big industrial engines. I was offshore with one of the first experimental units, and we were trying it out on a 9390 cubic inch, twin turbo V16 engine driving a gas compressor. We tried to start the monster several times but didn't have enough air pressure to spin it fast enough for the mag to generate a good spark. Finally we ripped the regulator out of the air supply and gave it plenty of air, and when it did light off, it let out a mighty backfire that shook the platform. Not only that, but it blew the exhaust manifolds, and turbochargers off the motor, and launched the 36" silencer and about 20 feet of 18" exhaust pipe out into the Gulf of Mexico.
Needless to say, experimental waste spark ignition systems were not a big hit that day.
As it turned out, there was an accumulation of unburnt fuel gas in both the intake and exhaust manifolds, and when the plugs fired on the exhaust stroke, with an open path to the exhaust, well, that's all it took.
So now some 20 years later, I'm still not a fan of waste spark ignition systems, because backfires like David's can still occur when there's unburnt gas in the exhaust system.