Outdoor Power Equipment and Small Engines - Engine gun fire Noise and Smoke

I have a Craftman Lawn Tractor Model # 917.256510 with Briggs & Stratton engine Model # 28N707 I/C Gold 15.5HP OHV. It was running fine but after i left the tractor outside for one week with a few rains, It now has alot of black smoke and sometime MAKE NOISE LIKE GUN FIRE OUT OF THE MUFFLER after i start the engine and engage the blade. Some one told me that there may be some water in my fuel and the carburator so i need emty the fuel tank and let it run out of gas. After I change the spark plug, fuel filter and let my engine run until it out of gas but there're still alot of smoke and the gun fire noise still happen. I'm a handy man, If any one know what happen to the engine please let me know. I already check the oil, air filter and it's still in a good condition

Does the oil smell like gas? Did the air filter get wet?

the oil doesn't smell like gas and the air filter is clean. I already change the spark plug, fuel filter.