My friend has a 1959 BSA B31. Finding his 3 original wire 6 volt alternator had died, he decided to change the electrical system on the bike, to 12V Positive Earth . Most will recall this model had an emergency start, on the occasion of a flat battery.
He began by fitting a 2 wire Lucas RM21 12V Stator and matching Rotor, a 200W Regulator, 12V Battery, Lucas 12V DLB 102 Coil, 12 V Ammeter, and Bulbs.
As one can imagine, this required some changes to the original wiring loom/harness. The new Alternator via the Regulator/ Rectifier, produces a charged, to a full battery, of 13.9 volts with the engine running around 3000 rpm. Switching on the ignition only, the Ammeter shows a draw of around 6 amps. Unfortunately, after running a few miles the Ammeter still has not moved from its starting position, of minus 6 amps. I have checked the Coil primary / secondary, readings, they fall within the Lucas DLB 102 parameters. As the system is positive earth, the coil is wired minus to the switch side and plus to the points side. On the wiring diagram for the 1959 3 wire Alternator system, a wire runs from number 15, of the PRS8 Ignition Switch to the + side of the Coil. Can anyone explain the purpose of this wire? The bike starts on the first kick. Any contributions regards Amps draw and Switch to Coil wire, would be greatly appreciated.
