I have a 16 hp Briggs Vanguard on a Karcher hds 1200 hot water pressure washer. It's electric start but it won't shut off when key is turned off. I haven't had this thing that long so it's still a work in progress. I did take all of the shrouds off and powder coat them I don't see that causing any issues unless something isn't making ground somewhere. It looks like the grounds are all by wires anyways. I did replace a bad diode on the coil ground wires but that didn't do anything.
Check the cut out wire to the ignition coils.
It should be ground with the ignition off and open with the ignition on
I have never seen a mower ignition switch which grounds out through its body, most have a ground terminal ( marked G or 0 ) and either a short ground strap to the body or are wires into a common ground through the loom.
If the starter does not turn off check the wires on the solenoid.
Should only see 12 V with the key in the S position.
The wire from the coils runs to a connector that's insulated from the shroud. From there a wire runs to oil sensor and another wire to the key switch. I checked continuity to ground from there with key off and nothing. There is a yellow wire on the key switch that runs to starter and it's always grounded. Makes me wonder if the yellow on back of switch is suppose to make contact with the wire that runs to that connector that has coil and oil sensor wires.
Thanks for the follow up
It is good to know that some karchers use switches that ground out through the body of the switch.
I am here to learn every bit as much as every one else.