The wire coming from the bottom of the carb. is the fuel shut off or tap, effectively...it needs 12v to allow petrol through. One of the wires from the engine will be the charging wire (maybe red) and the other one could be an ignition cut out (maybe black). There is a possibility that both wires can give out 12v though - depending really what the engine was designed for. Hope this helps!
John, March 2008