Should have looked before I leaped. The following is from the British Sprite
Site and answers the coil backwards question.
Addendum: The importance of connecting the coil the right way round
At this point you may be wondering what difference it makes if you don't swap
the coil connections. Whilst the car will still run ok, having the coil
connected round the wrong way can cause weird ignition problems, including
mis-fires when hot. The reasons for this are somewhat esoteric but for those
who are interested then here is why:
Whilst the coil will work when connected the wrong way round its effective
output voltage can be reduced by up to 40% so weakening the spark .
Additionally if the LT side of the coil is reversed then the HT Polarity is
also reversed so the electrical pulse and hence the spark at the plug travels
in the opposite direction to the normal (normal direction for the spark is
from the center electrode to the spark plug arm).
Electrons leave a hot surface better than a cooler one, and in service the arm
of the spark plug is more efficient at dissipating heat than the central
electrode. Therefore running the system in reverse will give a weak spark for
this reason in addition to the reduced HT voltage.
If the spark is travelling the wrong way you will also get above average
erosion on the spark plug arm as the greatest erosion happens at the electrode
from which the spark leaves. The central electrodes of spark plugs are
specially designed to help compensate for this but obviously the arm is not.
Above average increases in the spark plug gap can indicate that the system is
operating in reverse and obviously incorrect spark plug gaps will again lead
to a weaker spark.
A neat little trick appeared in a Practical Classics magazine recently for
checking if the coil is connected the right way round: Place the tip of a
pencil between the bare end of a disconnected HT lead and the top of the plug,
leaving a gap between each. If the polarity is correct then a spark appears
between the lead and the pencil and there is a flare on the plug side,
otherwise it will be the other way round.
Larry Miller
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