Hello everyone! Well let me start by saying I am new to the forum and a newbie to Jaguars in particular. I recently acquired a nice 1974 XJ12 Saloon with only 50,000 miles for a very modest price. The car had been sitting for a number of years in the prior owners garage and had not been running for at least the last 2 years that i'm aware of. Anyway, I got it home, drained the tanks, changed the fuel filter, battery, added new fuel, checked the oil level, other fluids as necessary and attempted to fire her up. While she did not start right away you could hear her trying and with the help of a little starting fluid, she is now up and running.

Once up and running though, I began noticing a fuel leak coming from the carbon canister pipe under the left hand side of the car. I did a bit of research and concluded that there obviously should be no liquid fuel coming from that pipe and attempted to figure out why. I removed the fuel lines around the left carbs and checked each for leaks and didn't find any. I also removed the fuel pressure limiting valve and cleaned it. After re-installing the fuel pressure limiting valve the leak has stopped coming from the carbon canister pipe, but is now coming directly from both the carbs on the left side. I'm not certain if the excess fuel is supposed to return to the tank via a return line, or if there is a valve or switch that is supposed to regulate the fuel pump and slow its flow when the carbs are full. Has anyone seen anything similar, or perhaps can point me in the right direction to continue troubleshooting? Thanks in advance for any assistance.

 

OK, it has been 20 years, maybe more, so some thinking will need to take place, and maybe a stiff drink or 3.

YES, there is a return fuel line on the carby cars, so valving at the rear for return are the same as the early EFI cars, I THINK.

With the car sitting for so long, I would be changing all 4 needle and seat units in those carbies.

The fuel pumps just pump, no variation is supply, pressure or volume. The needle and seats close off supply when the bowl/s are full, and as said, the return goes back to the tank being used at that time.

The Strombergs were reknown for dribbling fuel, even from new, and many a V12 was converted to SU carbies, but that is not a task for the feint hearted.

Somewhere in my huge library of stuff, I have a plumbing drawing for that system, and I will look, and scan it here, if and when I find it.

 

Grant, Thanks for the reply and I will start looking into what it will take to change the needles and seat units on the carbs.

I did a bit more tinkering last night. I removed the left wheel and disconnected the three lines going into the carbon canister ( two from engine compartment and the return line to the tanks) and checked for leaks and all appear OK. I reconnected them and simply turned the ignition on (not running) so that the fuel pump would start to pump fuel through the system. As before, the carbs started leaking from the left side, where they are attached to the air filter cover. Slightly frustrated at this point, i disconnected the return line in the boot (at the inlet behind the back seat) and turned the ignition on, in an attempt to see if fuel was getting back through the return. What i was getting back was only a few drops and not a steady stream as I was expecting. I'm guessing that there should be a full re-circulation of any unused fuel correct? If so, I am starting to think that the return line may be plugged somewhere, which is forcing the excess fuel to overflow through the carbs.

 

Strombergs have an overflow in case fuel can't go where normally it would. I had a Triumph GT6 that the rear carb suddenly began pouring fuel out of the air filter side from the lower right. I took apart the carb, cleaned it, and some of the internals were stuck shut. Once those were back open and operating properly the car ran great. Get a Stromberg rebuild kit for your carbs. Converting to SU isn't too hard but it takes a weekend of work.

 

OK, a dribble from the return line is a stuck return valve, or maybe both. They are located inboard of each rear road wheel, and are electrically operated in relation to which tank fuel is drawn from.

Memory again. the LH valve is the return tor the RH tank, and the RH valve is for the LH tank.

I still have not found the S2 info, but I found the S3 fuel return schematics, and it is attached. The layout is identical.



The return line is under the LH side of the car, and the supply line is under the RH side of the car.

The pipe you have at the cannister that is coming from the rear of the car is in fact the vent line, NOT a fuel liquid line.

If the excess fuel cannot get back to the tank, due to blockage or whatever, the eventual pressure build up from the pump will dislodge the weakest needle/seat, and the exit of raw fuel is what you have.

New needle/seat assemblies are still strongly suggested, as are new diaphragms for each carby, as they will be rotted by now for sure.