B & S 18hp ovh intek not starting

Hi I have a Craftsman mower 12 years old. It has a 18hp ohv B & S motor. I started it up for year and let it run for 5 mins. I changed the oil, filter, plug. And had good gas in it. I went to start it and all it did was spin over and over and then made a big bang. I checked the spark and it was bright and steady. I cleaned the coil and fly wheel. Still did the same thing spun over and bang. Am I missing something here. Gas is going to the carb. 120lbs pressure.

Thanks

B & S 18hp ovh intek not starting