Some may have at the end of the 70s the reason for it is to allow the timing to be advanced when under high vacuum conditions. (near no load and at many speeds) At idle the vacuum can be high but the centrifugal advance is retarded to near the initial static timing. (IIRC all distributor systems from the early 50s on up have centrifugal advance under the breaker plate) Then when the engine is increased in speed (but under little load) the centrifugal advance and the centrifugal both advance the spark. this allows better fuel consumption and still avoids spark knock at high vacuum low load conditions. When the governor opens the vacuum advance pulls the timing back to load condition to, again, avoid knock. The centrifugal advance is only RPM controlled, so the advance does change with engine speed under loaded, or partially loaded conditions. Early tractors were designed without consideration to pollution, or smooth running. Magnetos used in that era had no advance or other timing mechanism other than the impulse coupling delay to get them running without breaking the cranking persons arm. After starting they ran poorly at idle because the timing was set at full throttle full load timing. Jim
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Today's Featured Article - Chores - by Frank Young. The ceaseless passing of time! It is at once our friend and our enemy. It measures our progress and it makes us old. Like most features of our life, few things are all good or all bad, and most such judgments depend on our own perspective or viewpoint. In our particular hobby, we enjoy the nostalgic return to the days of our youth as we recreate many of the scenes that took place on the family farm that served as the stage for the first few acts of the play that is our live
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