Google is your friend my friend

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.