Outdoor Power Equipment and Small Engines - Briggs and Stratton 21hp Vanguard engine

I have a Husqvarna YTH2148 lawn tractor with a Briggs and Stratton Vanguard 21 HP engine that my cousin gave me. It was running rough even though it had only 50 hours on it but it had been sitting for 4 months. I have changed the oil (SAE 30), the air filter and pre-cleaner, the fuel filter (extended version), and the spark plugs (E3.18). The old plugs were very fouled and not the correct ones indicated from the B&S site. I could not remove the oil filter yet. With all the changes, it ran great for approximately one hour (One cutting session). The next time I used it, it started right up and then ran great for about 30 minutes and then started running rough again with little power and a very strong gas odor. I turned it off and pulled the plugs. The plugs were blackened and appeared to have gas/oil residue. I cleaned and rechecked the gaps on both (both were at .30). It seems as if the mixture is too strong and the engine is flooding. I have printed the Husqvarna owners manual, the B&S Operators manual and parts manual which offer little help. I am going to order the repair manual #272144 but it does not help me cut my grass this weekend. Any ideas?

I'd say you have fuel migrating into the oil by way of the carburetor's inlet needle and seat that is not shutting off the fuel supply properly within the carb. Have a smell of your oil, does it smell like gasoline? Have you noticed that the oil level has increased? These are two clear indicators that you have a carburetor in need of removal, cleaning and reconditioning with the correct Briggs carb kit and reinstallation.