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Since two cylinders fire at the same time on these motors and use the same ungrounded coil, both plug wires for these cylinders have to be connected in the circuit to check for spark on either cylinder.
When the plugs fire, either the top or bottom cylinders together, the spark travels out of one terminal on the coil, down the wire to the plug, across its electrodes to ground firing the plug, through the motors block to the other plugs electrodes and back to the coil completing the circuit. If either plug wire is undone or faulty, neither cylinder will get spark.
Here’s a short video showing how I checked for spark on a Zephyr. The ends of the wires were cut flush with just a bit of the core wire sticking out and I taped them together before spinning the motor over with a drill. You can see how the spark always jumps across the same two wires. This motor has great spark!