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Webers and fuel pressure


Hamish

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Hi guys

i need guidance on fuel pressure for my twin 45 dcoe 

the past couple times I have switched the ignition on (activates facet red top pump) I have had floods of fuel out of the rear carb. This carb is first in the fuel line. It fills the carb throat and the opening above the first choke in the casting. 
I have a filter king pressure regulator/filter. 
I cured the first time with hammer tapping the carb body 

but the second time I have just left it. 
I will now clean out the carbs and ensure float height and jets clear.
 

I have bought a webcon fuel pressure gauge. Now to my question do I set the pressure with the car running ?

there isn’t a return line to the tank

or can it be just be on pump  pressure. I believe I need 2.5 to 3psi ?

thoughts welcome

 

IMG_0071.jpeg

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14 hours ago, Hamish said:

I have a filter king pressure regulator/filter

Shouldn't this be able to control the pressure to the carbs?

IIRC some Filter kings have a take off for a gauge, presumably to allow pressure to be set.

Ian

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You don't need a return line Hamish. You don't technically need one for injection either (there are other practical reasons with high pressure systems, like heat management).

Not sure exactly how the Filter King regulator works but I assume it's a basic diaphragm valve (like a gas regulator) in which case it should work with or without a demand at the carbs. If you think about it at idle/low load the carbs will be effectively closed at times anyway.

It will probably be easier to set though with the engine running simply because petrol is effective a non-compressible fluid and so it needs somewhere to leak down to as you try and reduce the outlet pressure.

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I had dellortos on a zetec in my spit. On the one and only autosolo I did, the fuel level kept rising in the float chambers, to teh extent the poor car was coughing/spluttering over the finish. I reduced the fuel pressure to 1psi, and that made little difference, butI never adjusted it back, and it ran perfectly.

I reckon carbs would run on almost zero pressure, as they rely totally on flow. But in the real world some pressure  is required to create the flow.

And feeling bad about saying it, but those tea strainer filters will be sapping a fair bit of power....

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As low as you can get away with. If the regulator is at the same height as carbs then pressure can be very low. The test is a good long full-throttle pull to make sure you’re still getting enough flow to keep up with demand.

For the original flooding problem you need to have a good look at the needle valves. Doesn’t take much dirt or varnish to make them misbehave in this way. Strombergs and SUs are similar.

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Thanks guys. 
the filter king is at the same level as the carbs.

the filter king does set the pressure but it’s adjustable and takes the pressure gauge as a temp test. Not to be left on. 
 
the tea strainer filters are removed for competition. just thought they may save me from that rogue road detritus causing havoc. But they do seem quite free flowing. 
 

i will start by a good clean and float height check - again.  then set the pressure as best I can with an analog gauge under the bonnet. (0-15psi)

thanks again for the guidance

H

 

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I run the triple Weber (40DCOE) on my GT 6 with 0.15 bar. Approx 2 psi? 
Fine when touring around and as well when bashed on the Nürburgring. 
Important  to keep the needle valves clean. 

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Original fault, cured by tapping with hammer, is generally accepted as a sticking float. Also worth checking the needle valve as this could also be sticking.  A further possibility is a leaking float resulting in a heavier float and consequent incorrect float level.  Can be checked by shaking the float to see if it contains any petrol.  Also check if both sides of the float are level.

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  • 2 weeks later...

Well I had the opportunity to work on the carbs this afternoon.

tested first and the petrol was still pouring out the rear carb like I could afford it !

so off with the covers filters and various jets, floats out no leaks or fuel in them the black “plastic” type. Ball bearing sprung loaded jets etc all good. Re-assembled and fitted pressure gauge. 2 to 2.2psi then same issues :sad: . It’s only the rear carb pouring out.

off with the cover again checked the float jet and float distances nothing untoward 

back together and no leak and pressure still the same.

pressure gauge removed and still no leak.

which is sort oh good - but I have had this before. And I didn’t find anything obvious that I “fixed”.

there is too much rogue fuel about to risk starting the car.

and I fear a good few egg cup fulls of fuel has washed down the rear 2 cylinder bores.

do I add oil to those bores via spark plug holes ?

will petrol evaporate out of sump?

is it something else to worry about as well as the rear carb leaking again?

I will be changing the oil for next season but I’m sure to be running the car before then .

need to keep testing the carb leak first though.

much nicer to find a cause and be able to fix something !

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Are you using the Weber plastic floats or the black plastic float copies of the original brass floats?  I had some of the latter and had the same problem which I put down to the plastic float catching on the float chamber body.  In the end I gave up with them and went back to brass floats and cured the problem.

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The Webers on the Spitfire would occasionally drip excess fuel back through the throats at idle. This was apparently independent of the float height/fuel pressure as far as I could determine and I put it down the fact that I was using the original accelerator pedal arangement with the solid linkage which tends to rattle the butterflys.

The Spitfire runs a cold air box with a large remote filter in front of the radiator and the box would collect fuel which would eventually start to drip out uncomfortably close to the exhaust manifold.

My 'cure' was to drill a hole at the lowest point of the air box and run a bit of 1/4" hose to a point below the chassis away from the exhaust.

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