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

Time to remove the valve cover and inspect the valve clearances. I know you said 120 lb. of pressure, but that can be misleading. When you post back, include the engine model, type and code numbers so we can find the valve clearances for you.

Hi model-31H777, Type- 0605 E2, Code 050621ZE. I adjusted them between .004 and .005. I did fine the aluminum push rod with a little bend in it. And put a new one in. And adjusted the valve again. I changed the carb with a new one. In of stead of rebuilding it. I went to the top with the piston and went down about a 1/4 inch
and both rockers were loose and adjusted the gap. I saw that on you-tube site. The thing that is weird all I did was change the oil, filter, gas, and battery and things
went to hell after that.

Thanks