Re: what's the function of resistor wire on coil ?
A little more information for you. As Don mentioned, the resistor rerduces the voltage to the coil...sort of.
The reason the resistor is there is to prevent the coil from overheating when the ignition is on and the engine is not, and at low rpms. It does this by limiting the current flow through the coil.
The coil has very little resistance, but quite a bit of primary inductance. Without the resistance, when the points close, the current in the inductor would ramp up linearly to a very high value, causing the coil to overheat. With the resistor, as the current ramps up the resistor drops some of the voltage...the result is an logarithmic curve, which has a maximum current value no matter how long the points are closed.
Why bother explaining this? It may change the type of coil you decide to use. The lower inductance, the quicker the coil current comes up, which favours high rpm spark energy (flamethrower etc), at the expense of low rpm energy.
A standard coil has comparably larger inductance which favours lower rpm use. Given we use lower rpms mostly (Under 5k) I feel the stock coil is better for the application.
The electronic ignitions tend to have internal current limiting, and therefore don't need an external resistor. This is ideal as the increase in current through the coil primary is as fast as possible, but the current is also limited to prevent coil overheating.
C