My lawn mower won't run...?

recently i have started having problems with my lawn mower. it's a troy built tb260 self propelled walk behind mower, with a briggs and stratton 190cc engine. i start it, and it will run for about 20 seconds then it sounds like it's sloting down, and starts idling really rough until it dies completely. when i try to start it again, it just idles and dies. after a few tries it won't even do that. i changed the air filter, and the spark plug. it has plenty of gas, and the oil is good. i really am stuck on this.