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http://www.nrhsperformance.com/tech_singlefire.shtml

HEY CAMY 18

I HAVE A 1ST YEAR EVO SPORTY {1986} & A 1999 FXSTC {BIG TWIN EVO}. AS I UNDERSTAND IT, MOST H-D, FROM 1984 & UP, FIRE THE PLUGS, ON BOTH THE COMPRESSION & EXHAUST STROKE, MY THEORY ON THE EXHAUST STROKE; PLUGS FIRE ON EXHAUST STROKE TO BURN OFF ANY EXCESS FUEL THAT DIDN'T COMBUST ON COMPRESSSION STROKE, KEEPING THE EXCESS FUEL FROM RUNNING DOWN THE CYLINDERS, PAST THE RINGS, & DILUTING THE OIL. WEAK COIL, BAD PLUGS, RICH CARB ARE ALL SYMPTOMS OF FUEL NOT TOTALLY IGNIGHTING. I'M USING SCREAMIN EAGLE IGNITIONS & COILS ON BOTH H-D'S, ON THE BIG TWIN EVO, I'M ALSO USING BOSCH 4 PRONG PLATINUM TIP PLUGS, #B4478, USING THEM IS LIKE PUTTING IN A SET OF CAMS, THEY BURN ALL THE FUEL, THEY'RE AMAZING. ROBERT BOSCH INVENTED THE SPARK PLUG CIRCA 1876, I BELIEVE THEY FIRED UP KARL BENZ FIRST WOODEN TRIKE.

The difference between single and dual fire ignitions sometimes gets lost in theory, the claim that firing on the exhaust stroke to consume residual gasses and engines running smother as a result are incorrect, if there was residual fuel in the exhaust cycle of an engine and enough residual oxygen to ignite it, it means that cylinder did not fire on its power stroke in the first place and the explosion from the exhaust would I think get your attention, that is a misfire.
One benefit of single fire is to be able to individually time each cylinder at set up and also split the timing when on the dyno for best results.
As for wearing out plugs, technically a plug does not spark it ionizes in other words it super heats the gas charge between the electrode and ground strap , a good electronic ignition will actually be heard more than seen or you will hear a click and a tiny lightening bolt at the plug when grounded against the engine and the ignition activated ,this is the hallmark of a strong ignition, the louder the click the stronger the spark energy , if you see blue flashes etc that is the metals being burnt or eroded so plug life is extended well over what would be achieved with points ignition and some early electronic systems.
If the performance of your engine improves noticeably with a plug change and the plugs are not actually worn, it is an indication of a fault elsewhere in the engine its just that the plug is the first thing to fall over in the chain of events, ash deposits ,fuel loading ,poor ignition components etc show up as bad plugs .
When Harley Davidson went to the Hall Effect generator in the early 80’s it was one of the best things they did, it opened the door to a lot of good product.
For years the old Dyna S dual fire ignition was used on fuel V twins ,if ever an engine would bite your ass for mistakes, oversights ,incorrect setup it was THAT engine ,there are a few divots in workshop floors, pit areas, and strips to bear testament to the explosive nature of this fuel to those not vigilant. A point of interest is that every time a fuel engine has run or rotated with the fuel system on, the plug leads have to be removed from the plugs and isolated before the ignition system can be turned off or rotating the engine again because of a potential for an untimed spark ,the engine is then blown down with plugs removed to clear any residual fuel from the cylinders before attempting to start the engine again, these things can lift a cylinder with spectacular results!