THE CAPACITOR

Only one component of the circuit needs some more explanation. That is the capacitor. It has two functions. When the contact points open, the current will try to jump the gap, giving sparks. These sparks are in the first place destructive for the points, but they also delay the current cut-off. That would reduce the strength of the self-induction peak. The capacitor works as a reservoir, and thus prevents this arcing across the points. The other function has to do with the 'sparktail'. After the first tensionpeak, the capacitor releases its energy into the coil, and because it is an open circuit now (points open) this current bounces up and down a couple of times, until it damps out. These oscillations are again transformed to the high-tension coil. The result is that the spark is maintained for a while, so it can reliably ignite the fuel.

Normal values for an ignition capacitor are between 0.1 and 1 µF. This is not very critical because it is a compromise anyway. At low rpm’s you need a bigger capacitor then at higher revs. So the engine designer chooses a value that is most suitable for that application. A fairly vague assumption.

More important is that the capacitor is functioning properly. Because they usually quit when you least expect it, and you can’t diagnose or repair them, you should carry a spare one. There are basically three modes of malfunctioning. A completely shorted capacitor prevents any spark to occur at all. A capacitor with an internal break can’t absorb the electrons anymore, so you will see a lot of sparks at the ignition points. A weak spark at the plug is the result. Finally a capacitor with an internal leak. It will probably work for a while, but especially when hot, the engine gets hard to start. When you have ignition troubles, and you don’t see something obvious amiss, it is a good idea to change the capacitor before delving into the points etc.