Stuart,
Your are correct with your understanding of the ballast resistor and the starting bypass. The ballast resistor drops approximately half the system voltage when coil primary current reaches the saturation level- about 4 amps for a typical contact-point ignition system. That means, with the key on, engine off, points closed, and 12 volts at the battery, the ballast resistor drops about 6 volts. The bypass circuit helps the ignition system when the starting load pulls the ignition system primary voltage down to 9-10 volts. With the ballast resistor in the circuit, the coil would get roughly half that voltage and the coil secondary output would be reduced, possibly to the point where spark plugs will misfire because of insufficient secondary voltage available, or the spark current will be insufficient to ignite the fuel mixture in the cylinder. The bypass circuit applies the full system voltage (cranking voltage) to the coil primary, which causes more primary current in the coil, which in turn, results in a hotter spark (higher spark current) for starting. Some coils have the ballast resistor built into the coil, and no starting bypass circuit. Vehicles that used those coils sometimes had starting difficulty in cold weather when oil viscosity and reduced battery capacity both caused a higher starting load and lower primary voltage.
The reasons for the hard starting when cold are first the fuel mixture in each cylinder is somewhat lean, not rich as is commonly assumed, and the cylinder compression is higher than at just about any other time- more on that if anyone is interested. Spark plug firing voltage is raised by both lean fuel mixture and high compression pressure. Contributing to high compression pressure is ambient air temperature, so in cold weather, air is more dense, so compression pressure is raised.
Six-volt electrical systems don't use a ballast resistor in the ignition system.
On the original question of the timing light, the advice on the fouled spark plug is correct. A quick check on the wires and spark plugs is to connect the timing light to another cylinder. If it works reliably, the irregular flash is the result of a wire or spark plug matter. On most six-cylinder engines you can time the engine on number six cylinder- half way through the firing order (one crankshaft revolution), so the timing marks are in the same place as they will be for cylinder number one. All you need to know for that process is the firing order for the engine and the specified timing cylinder, number 1, usually, then the first cylinder in the second half of the firing order puts the timing marks at the same place they will be for the 'original' timing cylinder.
Harvester
