Don't know the model at all, but if that "carburettor cable" goes to the automatic choke on the carb, then one or both wires to the correct plug will come from the stator or voltage regulator, my guess would be the yellow. If the carburettor cable is from an auto choke, check the black plug with a voltmeter across the two wires, it should be dead even with ignition on, but show an AC voltage as soon as you start the engine, don't run engine for long as choke will be on all the time. If the black plug isn't the right one, check the white plug.

 

Thank you very much.