An ignition source for initiating combustion is provided. The ignition source includes an electrical delivery conductor mounted in a delivery conductor mounting structure. An electrical ground conductor is mounted in a ground conductor mounting structure and extends from the ground conductor mounting structure to a point proximate the delivery conductor to define an ignition spark gap between the delivery conductor and the ground conductor. At least one of the delivery conductor and the ground conductor is mounted so as to be positionable with respect to the other one of the delivery conductor and the ground conductor to selectively adjust a width of the ignition spark gap.