I have a 1986 48 inch Sensation lawntractor with a Briggs Twin 18 Opposed engine. It hadn't run for 18 months. I installed a new starter motor, carburetor and ignition switch. When I turned it on it ran perfectly. I drove it a short distance, turned it on and off 8-10 times with no problem. The next time I tried it, I got plenty of crank but no spark. What could it be: sheared flywheel key? Ignition coil? Safety switch.? Where do I look first? Thanks for the help