Points Ignition
The ignition system is the group of electrical components that conspire to ignite the gas (or petrol) and air mixture compressed in a cylinder, and so make an internal combustion engine, like that in your car, run.
The majority of modern engines use an electronic ignition system to deliver a high voltage to each spark plug at just the right time. However, older engines used a points-type ignition which needs a little more care and attention to keep running.
The points themselves are like a switch that opens and closes to make an electrical circuit. They are found in the distributor, which is the black can-like cylinder that all the thick spark plug wires come from. The distributor is built around a central shaft that is driven from the camshaft and rotates at half engine speed. This is important because, if you recall the basics of internal combustion, each cylinder needs to get a spark on every second engine revolution.
This spark occurs across the terminals of the spark plug and needs a very high electrical voltage. (It might help to think of voltage as electrical pressure. Electricity can't flow through air of its own accord, but when the pressure is high enough it will punch its way through.) The high voltage is generated in the coil, which is effectively a transformer converting the regular 12 volts of a car's electrical system into the 20,000 volts or more that's needed to create the spark.
But voltage alone isn't enough to run the engine. The spark has to occur just before the cylinder has completed its upward compression stroke (known as top dead center, or TDC.) This timing is determined by when the points open, breaking the flow of electrical current in the low voltage, or primary circuit. The points are opened by a cam on the distributor shaft, and the timing is set by moving the points around the shaft. Typically, the initial timing is set so that the points open some 12 degrees before the cylinder reaches the top of its stroke.
When the points open the primary circuit is de-energized, and this allows the coil to release the high voltage that has been building up inside. This flows through the center of the distributor cap, beneath which is a shaft-mounted rotor arm which steers the charge to the appropriate spark plug wire. The voltage reaches the spark plug, jumps the gap between the terminals, creating a high temperature spark, and ignition occurs.
Another point to note with points ignition is the time for which they stay open. This is referred to as the dwell and is dictated by the size of the gap a larger points gap has a greater duration of dwell. This controls the duration of the spark. Dwell is measured as an angle and is typically around 30 degrees of engine rotation.
So, to keep an engine with points-type ignition running at peak performance, make sure that:
The points open at the right time, just before TDC.
The points gap and dwell angle are as specified in the manual for the engine.
There is no dirt between the points that would allow current to flow when the circuit should be broken.
