I have a Craftsman 19.5hp riding lawn mower. It starts and runs okay, as long as i keep my foot on the clutch. When i put it into gear or just let off the clutch or engage the blade, it turns off.
Most likely a problem with the seat interlock switch. The engine is supposed to stop if you get off the mower while it is either in gear and/or the blades are engaged. If the switch is bad, it may 'think' you are not on board, and therefore refuse to work.