With respect, I disagree with one of your statements. I believe the percentage of time the points are open or closed is the same at all speeds (until point bounce/flutter happens at some 5000 rpm (unlikely with a Farmall for very long) The dwell is constant. The time the points are closed is longer at low speeds, but so is the time they are open. Thus the duty cycle is identical at starting and operating speeds, and is the ratio of open to closed.
The ignition needs to work harder (produce higher firing voltage) at idle than at high speed and load conditions. A partial vacuum 20"of HG is harder to fire a spark through than compressed air and fuel at 140psi. Looking at an ignition scope under variable throttle/load/speed conditions will illustrate this with clarity.
Cold resistors do pass more current than hot resistors do, but the load from operation is basically the same on the primary due to the slow speeds of these engines allowing the coil to have optimum saturation, and continuous "duty cycle". On 1/3 off 2/3 only gets into overheating if the on is 10 minutes, and off is 20 minutes. Even then it would probably cool enough in 20 minutes to avoid damage. (exaggerated for emphasis) Jim
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