Old Britts – Troubleshooting Boyer Bransden MKIII Ignition Units
Download the complete PDF version using the green download button at the top right of this page for a practical Old Britts troubleshooting guide covering Boyer Bransden MKIII electronic ignition units used on British motorcycles. This technical reference explains how to diagnose common no-spark, weak-spark and poor-starting problems by checking the battery, fuse, plugs, coils, ignition wiring, kill switch circuit and the transistor control box.
The article provides a clear testing sequence for motorcycles that have suddenly stopped running, including grounding both spark plugs, checking for spark while operating the kill switch and interpreting whether one plug, both plugs or neither plug fires. It also explains how Norton applications differ from some other machines because the ignition unit may require triggering by turning the engine or disconnecting the yellow/black and white/black wires.
Further sections list common causes of ignition failure, such as low battery voltage, poor red-wire grounding, loose or corroded connectors, faulty coils, damaged coil link wires, short circuits and insulation worn through where wires rub against the frame or fuel tank. The guide also includes simple in-circuit transistor box tests and notes that hard starting with plug fouling can occur when coils are wired in parallel rather than in series.
Contents Include
- Boyer Bransden MKIII ignition troubleshooting overview
- Initial fuse, spark plug and high-tension lead checks
- Interpreting single-plug and twin-plug spark results
- Norton-specific triggering notes for ignition testing
- Bypassing ignition and kill switch wiring for diagnosis
- White, yellow/black and white/black wire test procedure
- Common causes of no-spark conditions
- Battery voltage, grounding and connector inspection
- Coil, link wire and short-circuit fault checks
- Transistor box in-circuit testing
- Parallel versus series coil wiring note for hard starting
