I have a 14 hp murray riding mower with a 42" deck. While cutting the lawn today, it started to sputter as if it was out of gas. I know it wasn't because I had just filled it. The mower died and won't start. It gets spark, and plug is clean. If I pull off the air cleaner and drip some gas into the carb it will run but won't stay running for more than a minute. Any ideas?