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SU carb anti run-on valve

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  • SU carb anti run-on valve

    In a recent thread on SU carbs the subject of anti run-on valves was mentioned. About 40 years ago my Rover SD1 V8 had carb trouble which was traced to these valves. The garage man took the valves out and said that they gave trouble on many BL cars. He suggested I may feel an increase in performance because the valves were an interuption to smooth gas flow. I can't remember if there was an increase but now it's got me wondering if my 60bhp Marina engine would benefit from a "valvectomy". If so, what does one do to fill the gap, solder? Any ideas?
    Franklin, Leicester

  • #2
    Re: SU carb anti run-on valve

    The airfow through the carb will benifit by removing the valve. The best solution is to buy new plain butterflys from the sucarb website.
    The valves themselves are not for run-on. They were fitted to reduce emissions when the car decelerates rapidly (overun).
    Am I an expert? No - just been reading all about it as I am currently overhauling a pair of SU's myself. If you can get a copy the book "How to Power Tune an MGB" by Peter Burgess is a good read.
    I have chosen to rebuild my carbs completely as they were a bit of a mess.

    Cheers, Robin

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    • #3
      Re: SU carb anti run-on valve

      Originally posted by martinclan56 View Post
      The airfow through the carb will benifit by removing the valve. The best solution is to buy new plain butterflys from the sucarb website.
      The valves themselves are not for run-on. They were fitted to reduce emissions when the car decelerates rapidly (overun).
      Am I an expert? No - just been reading all about it as I am currently overhauling a pair of SU's myself. If you can get a copy the book "How to Power Tune an MGB" by Peter Burgess is a good read.
      I have chosen to rebuild my carbs completely as they were a bit of a mess.

      Cheers, Robin
      could this be the start of a magnum opus to add to my collection.

      To avoid confusion there are two completely different anti run on devices, both achieve a similar result of quickly reducing the vacuum in the carb itself stopping it from pulling excess Petrol into combustion.
      the poppet valves in the carb butterfly’s tries to minimise the pressure difference either side of the butterfly.
      there is on many MGB’s and other B series powered cars a valve connected directly to the inlet manifold. It’s intended to dump the vacuum when the ignition is turned off, it does not work on over-run though.
      MOC member since 05/97
      1984 Marlin Roadster SWB.
      1800TC, Unleaded ported head, stage 2 cam. Ford Type 9 gearbox, Dolomite Sprint rear axle fitted with MGF disc brakes.
      Three core radiator, Renault Clio vented front discs.
      The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.

      Loads of Marlin Reference can be found documents here or there.

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      • #4
        Re: SU carb anti run-on valve

        Thanks for the replies. I didn’t know there were 2 valves but mine is the poppet valve in the butterfly. I may have a spare plain butterfly somewhere or I know a bloke who has.
        Franklin, Leicester

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