Quote:
Originally Posted by Y-Blockhead
If you look at the ignition system on 4 cylinder motorcycles with points ignition they have two coils, each with two secondary leads to two spark plugs, firing #1 and #4 cylinders together, and #2 and #3 cylinders together. They fire on every stroke, meaning they fire at ~TDC on the compression stroke and the exhaust stroke.
Both plugs seem to get equal spark every stroke, altho the cylinder with the exhaust valve open contributes no power.
I also had a 750cc Yamaha Twin Flat Tracker that had a Mag that fired both plugs at the same time.
I guess my point is that you would THINK the plug that saw no compression would fire easier that the one seeing high compression (in the case of the Yamaha, 13:1 compression ratio).
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Late model cars often use the same 'one coil per two cylinders' setup. They work differently than a typical Model A, etc., coil. With the coil type used on a Model A, the secondary windings are grounded to the coil housing or case. So when the plug fires, the current goes out the center tower of the coil through the distributor to the plug, jumps the gap to ground and returns to the coil through the engine and body to complete the circuit.
With the coil that fires two plugs at once (power stroke of one cylinder and exhaust stroke of the companion cylinder), the coil secondary windings are not grounded to the coil case nor to the engine. Rather, what would be the grounded end of the windings in a normal coil are instead connected to a second coil lead. So, to complete the circuit, high voltage current flows out one lead to a spark plug, jumps across the gap to ground, then through the engine's head to the companion plug and jumps from the ground electrode to the center electrode and back to the coil, thus completing the circuit. In this system, if the spark can't fire one plug the other one goes dead too. Thus, if one plug wire, for example, goes bad, two cylinders will not fire.