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  1. #1
    Join Date
    Jan 2009
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    Dronten, the Netherlands
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    345

    Sudden increase in fuel pressure???

    Dax with Rv8 3.5, standard S.U. carbs. Dismounted gastank last year and replaced with Dax fueltank and Facet electric fuelpump, low pressure, (6psi), no fuel pressure regulator. All was running fine for 2 years (about 5500 Km's) Then suddenly the car was not picking up anymore, eventually found out that it would accelerate fine after I shut of the pump, waited till carbs were empty and then switch on pump. My first thoughts were one of the needlevalves not closing off the fuel supply properly so I took both carbs apart and checked for dirt, callibrated the fuellevel at the same time when I had it open. After remounting the carbs still the same problem. With contact switched on, the Facet pumps fuel through the carbs, it is spraying out of the jets and running out of the overflow pipes.. Is there any way the fuelpressure delivered can have gone up, seems unlikely to me? What is the recommended pressure set-up for these carbs??

    Thanks IA, Theo
    Not for the faint hearted, not for the shy...

  2. #2
    Join Date
    Dec 2004
    Location
    Staines Middlesex / Abu Dhabi
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    64
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    1,083
    I'm sure someone better qualified than me will respond, but I understood that the basic SU only filled the float bowl from pressure and shut off the float and jets when full so as to avoid any such overflow when running normally. Similar as in your toilet cistern - fluid in until the float gets to the preset point and then restricts and shuts off the fuel inlet.
    So would seem one or both floats not working fully ? Are the floats in good condition and not leaking ? - you said you calibrated the fuel levels, but sure they are not sticking ?
    Personally not heard of pump pressure causing such a fault if the carbs are set up ok ?
    Chris Lutman -427SC Coupe / MG Tojeiro "LOY" replica

  3. #3
    Join Date
    Mar 2006
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    Location Location is a crap TV program
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    8,045
    An easy test would be to stick a pressure gauge straight onto the outlet of the pump so that the carbs are not even part of the system, if it reads 6 psi then you know that the fuel pump is still working correctly so the fault must be with the carbs. It would be worth finding out what the correct fuel pressure is or an SU carb, it might be less than 6 psi.
    I refuse to engage in a battle of wits with an unarmed opponent.


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  4. #4
    Join Date
    Jul 2004
    Location
    UK
    Posts
    8,103
    SU's need between 1.5-3psi to work best, above 3 psi the float needle can't cope unless its new, and will wear quickly with the pressure. Sounds like that's what happened here.
    Get a regulator on it (or a lower pressure pump) and dial it down to 2 psi and it will be perfect.
    Also check that your SU jet nuts move up and down freely and are not stuck (makes them run rich), and that the dash pot has light oil in to keep the dashpot from moving up too quickly (makes them run rich)

    SU's are lovely simple carbs, shouldn't be too serious to sort

    Good luck

    Steve
    Regards Steve

    Crendon #22

    452cu FE with top loader

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