the copper wire in the primary coil windings warms up as current flows through. the resistance then rises. AS the new 12 volt coils primary winding has many coils of very thin wire a temperature change of 20 or thirty degrees will change the resistance of the wire in the primary circuit. This will correct for the changes in primary circuit voltage.
quote from how things work
Copper is the most widely used electrical conductor because it is inexpensive and second only to silver in electrical conduction. Although low, copper does provide some resistance to current that is passed through it. The baseline resistance for a copper wire is based on the cross-section of the wire and the length of the wire. The resistance to the current generates heat. Resistance is temperature dependent, so as the temperature of the copper conductor increases, so does the resistance.
please give one of your new coils an amp reading cold and the hot. The primary circuit in the coil you describe is the variable resistance needed for the change in primary voltage. No old ignition systems will ever work correctly without control on output coil voltage.
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