Old Britts

Troubleshooting the Boyer Bransden MKIII Ignition Units for British Motorcycles

If Your bike has stopped, follow the following troubleshooting steps:

  • Make sure that your fuse Is good.
  • Take both spark plugs out and ground them on the engine. Turn on the ignition and depress the kill button. this will collapse the field on the coils and you should get a spark on both plugs. The Norton unit would require triggering by turning the engine or disconnecting yellow/black or white/black wires.
  • If only one plug fires, check the plug or the high-tension lead.
  • If you get a shower of sparks at the plug when the key is on, check your battery for a bad cell.
  • If you get a shower of sparks at the plug when you depress the kill button, check for a broken wire in the points compartment.
  • If both plugs fire once at the same time, the trouble is probably In the rotor/stator unit, (In the points compartment) or your bike quit for some other reason, (e.g. out of gas.).

If there is no spark:

Run a wire from the negative side of the battery (remember. Norton is positive ground] to the white wire on the black box. This will bypass the Ignition and kill switches and associated wiring. Disconnect the white/black and yellow/black wires from their terminal at their stator unit in the points compartment. Touch them together, when you pull them apart, you should get a spark at both Plugs- If you do, the problem is in the wiring from the battery to the black box. Check the kill switch first (remember it winds through the headlight) or perhaps a terminal has come loose. If still no spark: Check the coils next. If you are running two or three coils, these are wired In series, so if one coil quits , they will all quit. Very common if you are running dual 12V coils. Otherwise, the black box her gone bad on you.

Common causes of no spark:

  • Low battery voltage - if the battery is less than approximately 9 volts, the black box will shut down.
  • No ground to red wire.
  • Loose or corroded wire connectors.
  • Bad coil.
  • The black wire connecting: the coils has come loose.
  • Black wire to coil shorting to ground (black box very hot).
  • Coil connected to black box shorting internally (black box very hot), common on Nortons due to coils squeezed by clamp.
  • Wires rubbing on frame or gas tank, wearing through Insulation.

Simple Tests for the Boyer Bransden MKIII Ignition Units

1. Switching the ignition on should produce a steady current through the ignition coils except for the Norton unit. This remains off until triggered by turning the engine or disconnecting the yellow/black or white/black wires.

No current through the coils could be caused by:

  • No power to the white wire. (Battery voltage low, less than 8 volts).
  • No earth to red wire.
  • Poor connectors. Tinning gone black or very corroded.
  • Coils or link wire open circuit.
  • Black coil wire shorting to earth, positive on Norton and some other bikes, (Black box very hot).
  • Coil connected to black wire shorting inside from primary winding to case (very common on Norton machines) (black box very hot).

2. Wires can rub through to frame, check by removing unit and look round all the transistor box wires.

3. All battery cells should be in good order as one poor cell will produce a high resistance supply to the ignition. This can make the ignition spark on switching lights or horn and in some cases may produce a continuous run of sparks.

4. Switching off ignition should produce a spark at all spark plugs. The Norton unit would require triggering by turning the engine or disconnecting yellow/black or white/black wires. (Firing on one cylinder only, one coil shorting to earth could be either coil).

If test No. 1 is correct but no spark is produced on switching off, the transistor box is faulty.

5. The Transistor box can be tested in circuit by disconnecting the yellow/black and white/black wires and with the ignition on they can be touched together and broken, this should produce a spark at the plugs. If not the box is faulty.

If your bike is hard to start and fouls plugs:

  • The Boyer Ignition Unit can cause this symptom if you have the coils wired in parallel instead of series.

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