Fuel pressure non return valve. XJ12 series One on Carbs

V12-Engine
1

hi Guys, just came to start the XJ12 Series one up for the first time in a while and all four carbs seem to be overflowing. I know the modern fuels can gum up float valves but unlikely to do all four at the same time. I see there is a non return valve / pressure limiting valve and wondered whether this is the culprit. Before I pull it apart can someone tell me how it works? I am assuming that unused fuel circulates back to the tanks, when the carbs are full and this valve may be the mechanism for controlling that. Would appreciate some advice. It will be the same as V12 E type I am sure. thx Andrew

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The fuel return valve is just a ball bearing backed by a weak spring.

The float bowls under the carburettors control the height of the fuel in the jet at the bridge of the carburettor. As the float bowl fills, the floats should rise and block off the flow of fuel into each carburettor.

kind regards
Marek

1 Like
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The fuel pressure shouldn’t cause overflowing but sunk floats can (I don’t like those tiny Stromberg plastic floats much) and stuck open needles can. Who knows what standing a long time does, when the fuel is done evaporating it will leave some ugly residue and that may keep the internals from doing their job. If you hit the carburetor with a soft mallet or handle, does it improve? Maybe it unsticks float or needle.

I don’t have the V12 parts book but is there a return feed to the tanks you can look at? I guess the pump only delivers a little more than needed and the valve limits the pressure? Imaginary numbers, the pump delivers 10 against a dead head and the valve bleeds off keeping some flow and the pressure at 5? Check the carbs first… only one as all four have the same issue of course.

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Thanks Gents. I have tapped the fuel bowls and after sitting over night having been started its solved the issue. I recall ( now!) this has happened before but when the heat from the engine works it way towards the carbs after a run it appears to dissolve the gum that is making the needle valve stick. Clearly the pressure in the float bowl is finding its way through the lower O ring. Looking at the staining under the carbs on the chassis rail its done it on and off for years. Anyway, thank you I think its sorted now. I was just unsure whether the return valve , if stuck could have been the culprit. It wasn’t stuck anyway when I checked. Andrew

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I don’t like the underside of the Strombergs. That being said I had my SU’s apart twice and every time both were wet in the throat area. Somehow this fixes itself after the first run and from then on they’re bone dry. Maybe the float valves need to reseat themselves after being open for a while.
Congratulations on solving the issue and on owning the XJ12. Keep driving so it doesn’t happen again :slightly_smiling_face:
David