Petrol Leak - Siphoning

Engine Oily Bits, Ignition, Fuelling, Cooling, Exhaust, etc.
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Carboy0
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Petrol Leak - Siphoning

#1 Post by Carboy0 » Tue Jun 04, 2019 1:08 pm

Rusty is stored in the level, but when we work on the car it’s usually on a 5 ish degree down slope by the nose.

I am finding that the fuel is siphoning out through front carb float overflow. Blocking the overflow just moves the leak to the front carb up through the jet and out through the carb mouth.

I have two questions

Is this Standard Triumph (joke) engineering?

Or

Is the fuel pump bu**ered?

All advice gratefully received.
Chris

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johnnydog
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Re: Petrol Leak - Siphoning

#2 Post by johnnydog » Tue Jun 04, 2019 7:22 pm

Hi Chris,
I assume you have Strombergs?
I think you will find the problem will be the float valves (also called needle valves) which determine the fuel level. They need to be set correctly so that they shut completely when the fuel reaches the determined level in the float. If the float chamber gets too 'full', the excess fuel will leak up onto the bridge area and into the airfilter box and drip onto the chassis leg.
This has happened to two cars of mine - I had fitted new Zenith needle valves from Burlen Fuels to both when I overhauled the carbs. I ended up refitting the old needle valves to both and the problem of excess fuel leaking stopped. It is as if the new valves now available aren't built to the the same high spec as originals. I suppose old ones too can wear but I would avoid fitting new ones whenever possible.
I tried several fuel pumps when investigating my fuel problem which made no difference.
There are previous threads on this issue if you search - possibly under needle valves / float valves or carburettor fuel leaks?
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Clifford Pope
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Re: Petrol Leak - Siphoning

#3 Post by Clifford Pope » Wed Jun 05, 2019 7:19 am

johnnydog wrote:
Tue Jun 04, 2019 7:22 pm
- I had fitted new Zenith needle valves from Burlen Fuels to both when I overhauled the carbs. I ended up refitting the old needle valves to both and the problem of excess fuel leaking stopped. It is as if the new valves now available aren't built to the the same high spec as originals.

I agree - the new kind with rubber valves are useless. I had repeated problems with fuel dribbling out of the top of the carbs until I dug out a pair of the old brass kind which worked perfectly.

The car is unusual in having the fuel tank mounted vertically behind the rear seats, and when full it hardly needs a fuel pump at all.
It's just occured to me that what it really needs is a master fuel stopcock just before the fuel pump - either mechanical or electrical perhaps. The latter could also serve as a security device, with a hidden switch.

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Re: Petrol Leak - Siphoning

#4 Post by Carboy0 » Wed Jun 05, 2019 8:18 am

johnnydog wrote:
Tue Jun 04, 2019 7:22 pm
Hi Chris,
I assume you have Strombergs?
No, I have twin SU HS6s.

Before fitting, the same problem was occurring, with fuel siphoning straight out of the end of the steel fuel pipe.

I take your point about the Stroms float chamber valves, that’s the same on the SUs and I will investigate both the needle valves and their adjustment, but it doesn’t answer the question about the fuel siphoning in the first place.

I hadn’t thought about the position of the fuel tank, I think the tank is/was close to full. I might put a fuel tap in the fuel line somewhere, that will solve the storage problem and provide an extra security measure.

Thanks guys, that gives me something to work on.
Chris

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Re: Petrol Leak - Siphoning

#5 Post by johnnydog » Sat Jun 08, 2019 3:15 pm

Carboy0 wrote:
Wed Jun 05, 2019 8:18 am
johnnydog wrote:
Tue Jun 04, 2019 7:22 pm
Hi Chris,
I assume you have Strombergs?
No, I have twin SU HS6s.

Before fitting, the same problem was occurring, with fuel siphoning straight out of the end of the steel fuel pipe.

I take your point about the Stroms float chamber valves, that’s the same on the SUs and I will investigate both the needle valves and their adjustment, but it doesn’t answer the question about the fuel siphoning in the first place.

I hadn’t thought about the position of the fuel tank, I think the tank is/was close to full. I might put a fuel tap in the fuel line somewhere, that will solve the storage problem and provide an extra security measure.

Thanks guys, that gives me something to work on.

Unless I'm mistaken, I don't think the floats and the level is adjustable in the HS6 carb, whereas the Stromberg has a bendable tab to alter the height.

Even with a full rank, fuel shouldn't siphon past the fuel pump, even on a downhill. I would be tempted to try a different pump, or at least remove it and check whether pressure is building up, both sucking and pumping in and out - your finger should suffice covering the inlet / outlet whilst operating the lever on the block side.
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Re: Petrol Leak - Siphoning

#6 Post by Alec » Sun Jun 09, 2019 9:25 am

Hello John,

the floats on a HS6 and HS4 are non adjustable, although the level can be lowered, to some extent, using extra washers under the jet.
It should be impossible for fuel to syphon back to the tank, as there are two one way valves in the fuel pump, one suction, one discharge. If one of those valves have failed so has the pump

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