Riding Lawn Mower Won't Start?

I have a Murray riding lawn mower with a 12hp Tecumseh engine. The engine cranks just fine, I have spark after replacing the coil and plug, I also have fuel after cleaning the carb and the tank and cleaning the fuel filter. My only thought after that is compression, but there is a good amount of air coming out of the muffler when the exhaust stroke comes around. But it just wont start, is there any way to time those engines or anything or what am I missing?