SSsssteamer, good diagnostics. I never thought about the added current of advanced spark causing the switch contacts to stick.
So, do you really have a Stanley Steamer?
There's usually a couple that run the New London to New Brighton run each year. There used to be an 1899 Locomobile steamer at local shows, but I haven't seen it in over 20 years. That was one of the most beautifully restored cars I'd ever seen.