Small Engine Troubleshooting Problems? Ask a Mechanic Now.
If your mower is not turning off when the key is turned to the off position, replace the ignition switch and/or check the kill wire that goes to the coil. These are the 2 factors when killing ignition to mower.
Am I not understanding the question here? I am guessing you are familiar with the ignition coil set up and wiring of the ignition system.
Jon is right. The key switch can kill engine in two ways, one is the shorting out the kill wire to ground, to kill spark. It also should kill the voltage to the fuel solenoid, which will also kill engine. So it seems the key switch is doing neither. Replace the switch.
I see question is still open, did you find problem.