Have you opened the tap?
I had a similiar problem and when I opened the tap the spring was corroded and was not allowing the tap to open or close fully. Used a spring from an old carburretor which was the same size and it has been fine since.
The part number starts with 4KM which is the code for the xj900 which means it is likely the tap is the same as the early XJ900s. I know it is the same as the same BT1100 Bulldog (5JN2) because I checked it before I fixed mine.