Plumbing the last 18 inches of fuel line

[Rog13]

As a long term Stag owner and SOC member it’s high time I contributed something back to fellow members via this forum. I’ll start by offering a description of my solution for a bullet-proof system for delivering fuel over the last 18 inches to the twin Strombergs. I don’t know if it’s original but hope someone might find it interesting/useful.

Like most owners I’ve installed various combinations of rubber fuel hose and jubilee clips to the carbs in that hostile environment just above the block. And like many before me I experienced various leaks and failures of said hoses. It was clear that any tubing of organic composition (rubber, synthetic polymer etc) was in for a tough time due to the high temperatures and strong degradation and solvent properties of the fuel (possibly even more so since the addition of ethanol). A short life expectancy and un-predictable hazardous failure seemed, and too often was, inevitable.

So, with access to parts scavenged from scrap equipment at work and some experimentation, I established it was feasible to assemble an all-metal system for the last 18 inches of fuel delivery. The breakthrough came when I found it was possible to mount a Swagelok compression fitting directly on the ¼ in OD brass fuel pipe stub exiting the side of the carbs thus eliminating the need for rubber/jubilee clips. From these ¼ to 1/8 reducing adaptors I added 1/8 OD stainless tubing to a Swagelok tee piece and from there finally out to a tubing adaptor where the line reverts to conventional fuel hose, well away from the hostile area (see the attached pictures).

The use of a piece of 1/8 tubing from the tee to the existing hose might be considered restrictive to fuel flow but it was all I had to hand from salvaged parts. In practice I don’t think it has ever proved a limitation.

This arrangement has indeed proven bullet-proof in the 5+ years since I installed it. All the joints can be repeatedly dismantled and re-made if necessary and this type of SS fitting and tubing is rated to 100s Bar pressure (way in excess of what is needed of course). Other owners could replicate this system if they wished, perhaps using brass instead of SS or using ¼ tubing rather than 1/8.


P1000937.jpgP1000938.jpgP1000935.jpgP1000936.jpg

[V Mad]

I have not suffered from leakes etc with connections in this area, but I have suffered from fuel vapourisation (increasing Ethanol is probably exacerbating this problem).

I would avoid metal pipes for this reason (unless well insulated) and also be careful of metal fatigue; if the pipes flex due to engine vibtation you may be worse off than before.

To keep the fuel delivery cool I am thinking of using plastic covered with a braid as extra protection, and relocating the filter to a cooler spot.

[Rubce]

Why would that set up suffer from metal fatigue? Both carbs are mounted on the same plinth and so move together. The metal pipe connects those two carbs so the whole assembly of carbs and pipework will rock together with the engine. If he had tried to extend that metal pipe all the way upto the fuel filter mounted on the bodyshell then that would be a different matter!

Bruce

[73stagman]

mmmm.......... We have been doing some serious vibration testing at work over the last few weeks due to unprecedented metal failure and electronic component failure.

We have set up a rig with the electronic parts and the metal parts as they are assembled on the tools we build for subterranean drilling (oil, gas exploration and mining work) We then vibrated the rig at various frequency's, amplitudes (distance of movement) and "g" forces. It is amazing when looked at under a strobe light so that you can see the movement of individual components. The things you think cant move certainly do. The factor that appears to be important is the "suspended mass". If you look at the way the pipe work is holding the tee piece before crossing over to the other carb, the weight of that tee piece would certainly allow it to move of its own accord under vibration and stress would be seen on the bend piece of tube holding the tee in place. However the strength of the annealed stainless steel tube is very high and so the vibration can be handled by the tubing (I would guess that the tube used is instrumentation tubing as the fittings look like "Swagelock") I have worked with this type of tube myself and it is extremely strong and long life. (It is very small though?)

I dont think you would have the same success with copper tube for instance because it tends to work harden as it vibrates.
I think you can do things pretty much the same with suitably insulated rubber. (Goodridge hose with stainless steel overbraid is extremely protective and durable. You can go further and use "Fire sleeve" which is used to protect electric cables and hydraulic lines in foundries! It is also expensive! But not as expensive as a fire!

[V Mad]

Not the pipes between the carbs but the piece of metal pipe going over the cam cover to the rubber hose. If that was bolted down to the cam cover it would be safer.

[Rog13]

Thanks for the comments. I guess a little extra support never goes amiss. IIRC the original factory system had a fixture off the cam cover?

However from years of personal experience at work with instrumentation containing this type of fitting I know it is of the highest integrity. We routinely pumped hazardous gasses and fluids at up to 4000 PSI through similar kit and never experienced failures of the tubing.