I suppose the YB uses batch injection, so multiple injectors inject at the same time for the same duration. If that is the case you can swap the injector plugs over (assuming the cables are long enough) to see if the problem then moves to another cylinder. If so, the fault is in the ECU. But I don't know if all 4 injectors inject at the same time on the YB.

..I just checked the YB wiring diagram and all injectors have their own link to the ECU. Actually that has to be if the ECU would be the problem. Though I still believe swapping injector plugs could be an option.