You can use an ordinary scope. Pick up the trigger signal from the 12 volt
side of the coil. The problem comes in dealing with the high voltage
side. You will either need to build a resistor divider, a capacitor
divider, or try capacitor coupling tokeep from blowing out the front end of the
scope. Wraping a band of aluminum foil around the spark plug wire will
act as a capacitor coupler, but if you want to compare spark signals, you will
have to make sure you get the foil the same on each wire. CAUTION: If you
wires are leaky, you can still blow the scope. In order to get all four
(six, eight) signals on the scope at the same time, you will have to also
use one of the spark wire signals as the trigger. Use seperate foil for
the trigger signal so you don't load down the other signal from that wire.
You can tell rpm from the 12 volt signal using the time scale. You can
figure dwell from the on/off times. Frankly, dwell and rpm meters are
a lot more convenient. The one thing a scope shows nicely is the
comparison between spark signals that can indicate a large variety of
problems.
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