Hydrocarbon (HC) is basically raw fuel, otherwise known as gasoline. The typical reason your Jaguar XJS would fail the smog check because of high HC would be defective spark delivery, but the fuel system has a lot to do with it as well. A "rich" or "lean" fuel mixture would contribute to high HC as well.
You should first inspect your Jaguar XJS's ignition system, ie. spark plugs, wires, distributor, ignition timing, and if everything looks ok, move on to the more complicated inspection of the fuel delivery system, ie oxygen sensor (s), engine coolant temperature sensor, mass air flow sensor. These components will require some auto emissions know-how and proper inspection tools. It is best left up to a smog check technician.
Follow the link below for a full list of High HC Causing Problems:
What Causes High HC?
