Sears lawn tractor model 917-271815 Engine shuts down when blade is engaged or clutch is released, otherwise continues running as long as clutch is depressed and blade is not engaged. An interlock or other switch has apparently broken. Which part do you think it is? The schematic lists a bunch of switches. Glenn Troester

Sounds like the switch that tells the mower if you have it in reverse (shutting down the engine)

If it were the Seat switch I don't think it would run even with the clutch in and blades disengaged.

Google run my craftsman lawn tractor in reverse and you'll probably find where to find and possible even disable (or fix) this switch

Thanks JP. I agree: a faulty seat switch would disable the ignition system entirely.
It won't let me run in any gear forward or reverse. The only way I can keep the engine running is to sit dead still with the blade disengaged and the clutch fully depressed. Very weird. I'll have to bite the bullet and do some trian and error tracing and testing.