Mechanics 6.0 Vee Angle & Arc Length of Timing Cam Lobe

The ignition timing of the four stroke, V-twin outlined below has one set of contact points, one capacitor (condenser), and a pair of spark plugs firing from a single coil each time the points open. The included angle between the cylinders is 45 degrees; common to Harley-Davidson, but would apply to 42, 47.5, 50, 55, 60, 72, or 90 degree twins delivering a "wasted spark". Although the cylinder splay of the final angle given would be more aptly called an "L" configuration. Furthermore, the method of producing spark voltage can be battery or magneto. The magneto can be either of two versions: rotating magnet (Bosch, Fairbanks Morris), or rotating armature (Lucas with its characteristic contact-breaker cam ring).

The timing shaft having a drive gear within the right side crankcase rotates at 1/2 crankshaft speed. A contact-breaker cam with two lobes of unequal arc length and hence, unequal duration that the points are open is attached to the shaft's end housed within the magneto or circuit breaker body. Both lobes open the points to the same air-gap width (0.014"-0.015" magneto & 0.022 battery). However, the rear cylinder lobe having the greater arc length holds the points open longer. The ramps of the two lobes are of equal length so the points remain closed for exactly the same time. This enables the capacitor voltage to build the same value before each firing when the points open.

The differential arc lengths of the cam lobes are necessary because of the unequal time between cylinder firings. The crankshaft, with its two massive flywheels rotates 405 degrees from the power stroke of the rear cylinder to the power stroke of the front cylinder. To complete the requisite 720 degrees of four stroke rotation, the flywheels rotate only 315 degrees from the front cylinder power stroke to the rear cylinder power stroke: 405 degrees - 45 degrees = 360 degrees & 315 degrees + 45 degrees = 360 degrees. Recalling that the timer shaft rotates at half crank speed the cam lobe with the greater arc length rotates 202.5 degrees from when the points open to fire the rear cylinder to when they open to fire the front cylinder. Subsequently, only 157.5 degrees of lobe rotation occurs from the time the points open to fire the front cylinder to when the rear cylinder fires: 202.5 degrees - 157.5 degrees = 45 degrees. Thus the lobe for the rear cylinder (with its greater arc length), keeps the points open 45 degrees longer than the front cylinder lobe, but each has equal time (equal ramp length) to build capacitor voltage.

Ignition timing on an older Harley must therefore be set so that he breaker-cam lobe with greater arc length opens the points for the rear cylinder. The shorter duration lobe, of course, should be set to open the points firing the front cylinder. Reversing the timing would alter ignition so completely that at best, the engine would run on one cylinder, if it even started. When restoring a motorcycle engine that uses a rotating armature magneto with a cam ring rather than lobe, e.g., a 50 degree Vincent, there must be a match between the ring and vee angle and also correct ring orientation. If careful attention is not paid to the latter then, in effect, one would reverse the timing with very frustrating results.