Re: Flathead "spark knock" ???????
Maybe it was to improve the idle and initial throttle response with a big cam. At low RPM a long overlap cam will allow a lot of exhaust gasses to remain in the cylinder, which dilutes the incoming charge. This slows down the burn rate so more initial advance is needed to get a better idle. With added initial timing, you obviously need to reduce the overall amount of advance so that total timing is not excessive.
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