17l/100 is about normal but drive it real nice and you can squeeze it down to about 15l/100 but you only have to have someone in japan fart in the wrong direction and that goes out the window.

If its been run primarily on LPG (if it was fitted Im tippin it was) then the carby will be out of whack and overfueling as a start. It will also have the timing changed a bit to suit the LPG. Grab a manual that has the list specs for your carby (assuming its the original carby that would be the original workshops book) and retune the donk from the get go. If that doesnt give significant relief then its time to start faultfinding for overhaul of bits.