how to fix Craftsman II 14hp riding mower?

Trying to figure out what is wrong with my riding mower. It is a Craftsman II 14hp 5 speed 38" cut. It is rather old, but this year it has decided that it doesn't want to stay running. I have been able to get it to turn over using some starting fluid, but it will run for only 5-6 seconds and then quits again. I have removed the old gas and put new fuel in hoping is was just some bad gas, but this hasn't solved the problem. The Battery is in good shape. I have taken the plugs out and cleaned them. I have done most of the basic things that I though would help, but no luck yet.