I finally got the XJS running and WOW! This thing is a blast! The only problem is the gas consumption. I drained the tanks to get all the old gas out,put 4 gallons of premium in along with some dry gas and some injector cleaner. I drove it around the parking lot of the storage facility I have it at and the car ran out of gas. I drove for a total of about a quarter mile. I checked the oil for possible bad rings and gas in the oil but the oil was fine. I looked for leaks but no leaks. I checked for disconnected hoses but everything is connected. I did notice I have no temp indication on the guage in the dash. Aside from that I am stumped. I am thinking of pulling all the injectors and having them cleaned. HELP!
I'm going to caution you not to overthink it. You said the tanks were drained. Remember that when the fuel is run out by the engine, there is almost always two to four gallons left that the system can't pick up. The car is "out of gas", yet some is still in there. The amount varies from car to car. That very easily could be what happened to you.
I wouldn't fret about fuel consumption until I could fill the tank, run some out then fill up and calculate that way.
Steven, I hope you are correct. When I drained the tank I used the fuel pump to empty the tanks. This is a Hess & Eisenhardt so I have 2 tanks plus the sump. I did notice while working on the car there was a big vacuum leak hidden under one of the intake manifolds. I am hoping that may be a cause of the consumption. My other idea is the fuel injectors since the car has not been started in 5 to 10 years. Also I have a fuel pressure regulator that bolts to the end of the fuel rail (the engine is a 92 that was put in after the former owner blew the head gasket on one side. He took out the Marelli stuff and put the Lucas stuff in it and it works). I tried to find one in case it was the problem and no one ever saw one like it. It is black, bolts on with 2 bolts, has a vacuum line and 1 fuel line on it. Thanks all.