I ran the battery down on a 16 1/2 horsepower briggs and straten engine on a sabre riding mower. When I jump started it, it made a loud backfire through the air intake. It ran OK a week ago until I ran out of gas, then it ran real rough. Now it won't start. What is my problem?

Hi,
It has a sheared flywheel key. Always throttle down the engine before shutdown to prevent it from happening.
Peace,
Clarke