Some ignition coils require an external resistor while others do not. The
same physical coil can be made for either configuration so it's important
to forget the book and check to see what ignition coil you physically
have. If it has ever been changed in the past, it's possible that it was
changed out for the wrong type.
Spitfires use a ballast resistor. Some other makes use a resistance wire.
Others use the special coil. Your mileage may vary.
What is it used for, you might wonder? The coil runs much better for long
periods of time at a lower voltage than the standard 12-14 volts that the
system is running at. If you run a higher voltage, then you heat up the
oil in the coil and it will start degrading. There are two circuits for
the ignition- one for starting and one for running. During the start
cycle, the ballast resistor is bypassed and a pure 12 volts is sent to the
coil for that hot spark. Once it starts and you release the key, the
circuit is switched over to the ballast resistor which limits things down
to around 8 volts at the coil.
If you want to eliminate this setup, you can just run 12 volts to the coil
through any key-on power feed to an ignition coil that has an internal
resistor and will be labeled for such use. It's a matter of preference
what you want to do.
Now how is it I can work through Spitfire electronics without any trouble
but I can't get a single cylinder Yamaha motorcycle to run? AAARGH!
-Vegaman Dan
-68 Spitfire Mk III "Giles"
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