Magneto spark

[Robert Leigh]

I have coil and battery, so this is not about Austin 7s. On another forum, selling a Matchless with a magneto, a poster has said that retarding the spark is not necessary and in fact that the magneto produces a weaker spark when retarded. I always retarded the spark on my motorcycles to avoid kick back. But more importantly, I don't see how retarding the spark would weaken it. Shouldn't the mag produce a consistent spark but retarding just causes the spark to occur at a different point in the engine's cycle?

Erich

Retarding the timing of a magneto usually moves the contact breaker backwards in relation to the armature. When the mag is assembled it is normally set so that the points open at full advance at the point of rotation of maximum magnetic flux change in the armature, which gives the strongest spark. when the points are retarded the magnetic flux change is lower when they open, so the spark will be weaker. This is not neccessarily a problem, because when starting there is less compression, so you can get a spark at the plug with lower voltage than when fully advanced. Even a fairly weak spark will start an engine, but it will not give good ignition at speed.

Robert Leigh