In a conventional rotating armature magneto, for each revolution of the armature, there are two positions where an optimum spark can be achieved. These positions, where the armature core leaves the pole piece, are exactly 180° apart and are usually referred to as the ‘break’ points. One of these break points is used for single cylinder machines where just one ‘bump’ on the cam ring causes the points to open and the spark plug to fire. Both break points are used with a two bump cam ring for twin cylinder engines. This is perfect if the twin cylinder engine configuration requires two plugs to fire at 180° intervals such as in parallel and opposed cylinder engine layouts.
However, V twin configurations are a special case. The cam ring for a V twin magneto has two bumps positioned to open the points at the unequal intervals required to suit the particular V twin engine. V twin magnetos always rotate at half engine speed so the angles between the cam ring bumps are given by the following equations:
180° – (Ɵ/2) °
and 180° + (Ɵ/2) °
where Ɵ is the V angle of the engine in question measured as an angle on the crankshaft.
As an example, a Vincent Rapide series B has a V angle of 50° so the angles between the bumps on the cam ring need to be:
180° – (50/2)° = 155°
and 180° + (50/2)° = 205°