Flywheel cam

Good afternoon, I have been asked to look at a Ruston stationery engine with what seems like an electrical problem. each individual component check out fine but no spark. What I am surprised to see is that the points are closed for a very short time, probably around ten degrees of shaft rotation and then held open by the cam. Is the spark generated with the opening of the points or when they close? Thank you for the help regards Colin Boyles