An ignition coil is a combination transformer / inductor that turns your 12 volt switched current into a high voltage 'spark' current.
What makes a coil 'work' is the speed at which the low voltage is changed from 12 to 0 to 12, and the number of turns of the primary winding in relation to the secondary winding.
You cannot do very much about the speed at which the 12v switches. Electronic ignition replaces the mechanical 'points' with an electrical switch which is many times faster. This gives a bigger spark and is why modern kart systems are moving to electronic ignitions.
Nor can you do anything about the number of turns of the windings.
However there are a number of faults that can occur with the coil that affect its performance, the insulation between strands may break down or the wires may break and very rarely they may be affected by corrosion.
However you can measure the resistance of the windings, which should tell you whether these faults have occurred.
Using an ohmmeter, or multimeter switched to Ohms, measure the resistance between the two 12v contacts on the coil.
I suspect that you will get a read between 0.3 and 1.7 ohms. This is the resistance of your primary winding (my Formula Blue coil, about 0.8 ohms)
Then set one connector in the spark plug cap and the other on the earth terminal of the coil (or the metal 'bridge' to which the earth is connected). This will give you the resistance of your secondary coil and should be between 8000 ohms and 18000 ohms. (FB coil, 9470 ohms )
If you find that you cannot get a reading between the spark cap and earth, take the spark cap off and test between the HT cable core and the earth. If you get a reading then, you need to investigate your spark cap... (Been there, done that).
If your readings are out side the tolerances given for the Rotax coil ( and I cannot find them quickly, so I have used generic 'kart coil' ranges) then you have a problem. The coil may be damaged.
Keeping a record of the readings will tell you whether the coil is changing. (Which might be insulation failing for example, since the resistance is proportional to the length, a change in resistance could be effectively a change in the length, which would equate to a reduction in the number of turns in the winding and hence the efficiency of the coil to deliver a big fat spark!)
I don't think ( and cannot find a reference anywhere to the idea) that you can say that one working coil of resistance X is better or worse than a coil with resistance Y purely on the basis of the resistances, at least not where X and Y differ only by small amounts.
You can find lots of references to the idea that your primary winding should have at least 3 ohms resistance. This seems to be true for points driven systems, (as on older motorcycles, Harleys, Triumphs etc) our electronic systems use a much lower resistance in the primary winding.
There is one other value you can check, the voltage that arrives at the coil, it should be 12 volts, and anything less may be the result of problems in your loom or electrical connectors) Been there and done that too.
Does that help?
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coil testing? by 'moondog78' (09 Jan 2015 @ 18:10)
