Fuel leak at rear carb

hi all, a friend of mine has a series 2 daimler 4.2 xj6 with carbs. when he turns ignition off fuel leaks from rear carb overflow and continues to do so and wont stop. any ideas. the car has 1 tank with submersible pump and 1 with pump outside tank. thought it was over pressurising but it isnt. left both fuel caps undone and still does it.

Tap the float chamber area. If that doesn’t help, remove the float chamber and inspect the valve and float. The valves can stick sometimes. And the floats can sink.

I wonder why it happens when he turns the ignition off! The valve should seal against any pressure the pump can come up with…

it doesnt leak when running just when its off.?

You need to find out what causes it. Is a pump still running, is it happening when the ignition is turned on, when the engine is hot etc. because I can’t see it happen. Maybe looking at the cutaway drawings of the carbs helps a little.

if the fuel tank is full to the brim which is roughly the same height as carbs and the float valve is not stopping the flow could it cause a syphon and carry on coming out. perhaps.

Yes perhaps but it shouldn’t be able to run or run very fat if the valve isn’t closing at all…


from your description I assume it is SU HS8 carbs with separate float chambers.

Under normal circumstances the pumps are shut off with the engine and the supply of fuel to the carbs stops immediately. As one pump has obviously been replaced with a non-stock pump, maybe the electric connection is messed up?

Floats are known to deteriorate and “sink” failing to stop the fuel supply into the chamber. That would be my prime suspect - easy to check, cheap and simple to fix. Another thing is the electric fuel pump. Most people are unaware these days that carbs and floats can’t cope with the fuel pressure delivered by fuel pumps designed for FI. If mistakenly a high pressure pump is installed this may be o.k. for some time or under certain circumstances … but then fail at a later point of time.

Good luck


75 XJ6L 4.2 auto (UK spec)


You’re correct, I have had this happen to me,


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