Re: 1965 mcculloch 45 hp electric start
That is called runaway. It is initiated by revving it up too high in neutral. And "too high" isn't very high. Scott/McCulloch motors were especially bad about doing that. There should be a vacuum switch (switches?) to prevent that. At least, to the best of my knowledge, they were on that one.
This is a typical diagram. May not be correct for yours.