Classic cars forum & vehicle restoration.
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bjacko
Joined: 28 Oct 2013
Posts: 527
Location: Melbourne Australia
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 Posted: Wed Jul 16, 2025 7:25 am Post subject: Fuel Filter |
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Maybe the filter needs to be mounted vertically? Or held vertically until it is full i.e. air gone then remounted.
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1938 Morris 8 Ser II Coupe Utility (Pickup)
1985 Rover SD1 VDP |
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Ray White
Joined: 02 Dec 2014
Posts: 7140
Location: Derby
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| Posted: Wed Jul 16, 2025 10:09 am Post subject: Re: Fuel Filter |
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| bjacko wrote: | | Maybe the filter needs to be mounted vertically? Or held vertically until it is full i.e. air gone then remounted. |
I raised the question with Google and this this was the response:
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Plumbing a Fuel System ? What You Need to Know First |
In a low-pressure fuel system for automobiles, the fuel filter should generally be placed after the fuel pump, but before the carburetor. This placement helps protect the carburetor from debris while also ensuring the fuel pump isn't subjected to excessive restriction from the filter." |
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MikeEdwards
Joined: 25 May 2011
Posts: 2713
Location: South Cheshire
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| Posted: Wed Jul 16, 2025 6:12 pm Post subject: Re: Fuel Filter |
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| Ray White wrote: | | bjacko wrote: | | Maybe the filter needs to be mounted vertically? Or held vertically until it is full i.e. air gone then remounted. |
I raised the question with Google and this this was the response:
"
Plumbing a Fuel System ? What You Need to Know First |
In a low-pressure fuel system for automobiles, the fuel filter should generally be placed after the fuel pump, but before the carburetor. This placement helps protect the carburetor from debris while also ensuring the fuel pump isn't subjected to excessive restriction from the filter." |
Nice of it to clarify that point.
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1976 Vauxhall HP Firenza, 1976 Vauxhall Sportshatch (x2), 1986 Audi coupe quattro, 2000 Audi TT |
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Ray White
Joined: 02 Dec 2014
Posts: 7140
Location: Derby
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| Posted: Wed Jul 16, 2025 7:18 pm Post subject: |
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One thing that has come to light is that a fuel filter should apparently be mounted with the outlet higher than the inlet; to avoid vapour lock. Mounting it vertically as Bjacko suggested may be the solution.
That seems to make sense to me. |
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Vintage Fly Guy
Joined: 27 Jun 2024
Posts: 179
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| Posted: Wed Jul 16, 2025 10:42 pm Post subject: |
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| Ray White wrote: | One thing that has come to light is that a fuel filter should apparently be mounted with the outlet higher than the inlet; to avoid vapour lock. Mounting it vertically as Bjacko suggested may be the solution.
That seems to make sense to me. |
I seem to recall that the early Range Rovers (code named Velar) when on test in desert conditions suffered from vapour lock, the cause was put down to heat from the engine bay, rather than the filter or pump being above the carbs (which on the V8 engine in a Range Rover would probably have needed a box on top of the bonnet to physically achieve!).
They cured the issue by fitting a fuel return system, with the fuel pump running full time and an additional fuel line installed to return the excess fuel back to the tank. Vapour lock cured, even when towing a fully loaded trailer across the Sahara.
So it's not just the position of the fuel pump, line and filter, things like the temperature of the fuel in the line (vaporisation) can also come into the equation. |
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Ray White
Joined: 02 Dec 2014
Posts: 7140
Location: Derby
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| Posted: Thu Jul 17, 2025 5:19 am Post subject: |
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In my situation the problem of a slight leak from the float chamber was only occurring with the ignition switched on and the engine not running.
Not a big leak ...but enough to make it noticeable in the garage even with the door open. |
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alastairq
Joined: 14 Oct 2016
Posts: 2121
Location: East Yorkshire
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| Posted: Thu Jul 17, 2025 9:03 am Post subject: |
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Nissan [Datsun] also used the 'return' fuel line to the tank, on their small 'world' cars [Cherry, Sunny, etc], back in the 1970's and '80's.
Enthusiasts in the USA today also use this addition to the fuel plumbing to overcome hot/ start problems with today's fuel, on older vehicles.
Smaller bore fuel line is used, or a small bore restrictor inserted into the fuel line...
Edit, to add, Facet fuel pumps now come with instructions to tilt the outlet upwards at an angle when mounting....so it is higher than the inlet.
Although I have to say, having used Facet pumps for 40-odd years now in various vehicles, I have always mounted them 'flat', on mini ex.dollies...and never had issues with lack of priming, or other issues. They even suck out of Austin Seven tanks...
On the topic of plastic fuel filters not 'filling up?' In my experience of plastic filters they always have appeared 'half full' when in use. A good indicator as to whether the pump is doing its stuff or not.
I imagine the filter body would only fill if the outlet was restricted in some manner....until the pump sensed the pressure increase and ceased pumping?
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Dellow Mk2, 1951 built, reg 1952.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces. |
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ukdave2002
Joined: 23 Nov 2007
Posts: 4242
Location: South Cheshire
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| Posted: Thu Jul 17, 2025 9:54 am Post subject: |
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Not sure I understand the original issue here?
SU carburettors work well with fuel pressure in the range of 1.5 psi to 3.5 psi at the inlet to the float chamber.
Pressure of 5 psi or higher will likely make the fuel overflow. The original SU electric fuel pump has an internal spring on the operating solenoid that effectively limits pressure to the intended range.
If there is no fuel filter between the pump and the carburettor, the pump will stop when its maximum pressure is achieved say 3.5psi, and this pressure will be maintained at the carb float valve.
If there is a fuel filter with air in, the only difference is that the pump will compress the air to 3.5psi...the pressure at the carb float valve will be the same; 3.5psi.
No difference! |
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