What makes my lawnmower start and then quit running in 6 seconds?

I have a Yard Machine, 6 h.p., 22-inch cut, self-propelled, big wheel lawnmower. It has worked great for the last two years. Now, after priming, it runs for 6 seconds and quits. I have installed a new spark plug, cleaned the gas tank and gas line, and cleaned the air filter. I have even left the gas cap off to see if the engine was not receiving gasoline. The engine does not have an adusting screw at the carburator, or least I have not located it. This is a mystery to me. I generally can repair anything mechanical, except this lawnmower. Question: what makes my lawnmower start and then stop, after 6 seconds?