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SU carb - poppet valve in butterfly

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  • SU carb - poppet valve in butterfly

    After 12 years I've just twigged that my 1275 Marina engine has a spring loaded poppet valve in the carb butterfly, an emissions requirement, as per many BL cars of the 70s/80s. I remember having trouble with an SD1 Rover about 30 years ago and it was down to these valves. I had them taken out and the hole soldered up. No reason to suspect that mine isn't working properly but it is a big obstruction to the air flow. Any thoughts on this?
    Franklin, Leicester

  • #2
    Re: SU carb - poppet valve in butterfly

    Hi Franklin,
    My first rebuild was a frogeye sprite which had minute twin SUs. I then rebuilt a TR2 with big SUs . after tese I had a TR4a and GT6 Mk1 both with twin Strombergs. All of these had nice plain butterflies. My wife , in 1977, bought herself a Spitfire 1500 which was a dog to keep in tune. This had Strombergs with the butterflies you mention. They destroyed performance at a stroke. I ripped them out and, after attempting the solder trick, replaced them with the earlier simple butterflies. I did need to re ject the carbs to get the mixture exactly right but the improvemnent was noticeable. Of note it came out of the factory with different jets in each carb! I would advise to bin your butterflies and replace with the older simple plates. You will have to retune the mixture afterwards though. You could solder them up but make sure that the surface is smooth afterwards and that the solder has "taken". Cheryl's Spitfire ingested a piece of solder when I tried that, No damage but a hole in the butterfly suddenly appearing in the cruise was "interesting. It's a quick job, Good luck !
    Marlin Berlinetta 2.1 Efi

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    • #3
      Re: SU carb - poppet valve in butterfly

      Franklin try Burlen Fuels (www.burlen.co.uk) They should be able to supply plain throttle discs for an earlier spec carb. Worth a try. if so get new screws as well.
      Best of luck Ben
      Ben Caswell probably not the last word on anything here!!

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      • #4
        Re: SU carb - poppet valve in butterfly

        Wasnt the purpose of the little valve intended to be an anti run-on device? My Twin carbs have them. I also have a Metro anti run-on valve fitted in the vacuum pipe between manifold and servo. It equalises the pressure on both sides of the butterfly, stopping extra fuel from being sucked through the jets.

        If you think the valve obstructs airflow, some other measures were to thin down or perhaps remove one side of the spindle holding the butterfly.
        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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        • #5
          Re: SU carb - poppet valve in butterfly

          Yes Steve that's right; as the butterfly shuts on the overrun the mixture could go very rich if the engine speed is still high, the poppet opens and weakens the mixture - so yes - its for emission purposes and also acts to prevent run-on when the hot engine is switched off. There are a couple of problems though, one is that the little spring wears out and so the engine is always running a bit lean and so hunts, or maybe ticks over too quickly for a while before levelling out and the other problem is that, as said above, the blessed thing occupies a fair percentage of the passageway. SOldering them closed or replacing with a plain butterfly solves these problems but can then lead to running-on and/or a higer tickover. I guess you decide which problem you prefer...never could imagine why everyone loved those SUs - fuel injection is so much nicer

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          • #6
            Re: SU carb - poppet valve in butterfly

            Hi All

            Just a quick question I have just fitted a single Hif44, not having a lot of knowledge about carbs, will I have the same problem?

            Adrian

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            • #7
              Re: SU carb - poppet valve in butterfly

              iirc the hif44 was quite a simple upgrade - i think i remember fitting them to minis back in the day. I dont recall there being as many problems but they are still essentially the same carb as other SUs. Werent they available in bigger sizes??? i'm sure someone will be on here soon to give advice. Oh yes, I remember, I had a twin carb 1275GT and I put a single HIF44 on it. Ran miles better and avoided the need for balancing. Was a long long time ago - before I got into Fiats and Webers!

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              • #8
                Re: SU carb - poppet valve in butterfly

                I must have been lucky with all my cars - I never had any one of them run on or pop. A properly set up constant vacuum (CV) carb shouldn't have these problems. The CV carb uses the pressure either side of the piston (or diaphragm in Stroms), which has tiny bleeds to meter equalisation of the movement. The damper must be in good vondition with the correct fluid too. Both bleeds are normally downstream of the throttle butterfly, one either side of the piston. The vacuum will increase with a slammed shut throttle but the airflow will also be greatly reduced too. If anything, I found that the little valves had the effect of making the carb think that the throttle had been opened a trifle thus causing run ons, spiky idle and lazy decel. The idea was to reduce CO emissions by allowing more air in with a slammed shut throttle but it was not a refined method. A lot of USA cars had an air pump in to the exhaust to sort this out. All bucket brigade tactics. The only general problems I had with SUs was the chance of vapour lock on a hot re-start. The correct heat insulators went quite a way to stopping this. The Stroms were similar but after a few years or so would develop pinholes around the edge seal of the diaphragm causing all sorts of unusual mixture changes! Didn't like them as much as SUs.
                Still, I've gone EFi now which does all this sorting out for us. (Until of course the impending, huge, solar flare b*gg*rs the electronics...)
                Marlin Berlinetta 2.1 Efi

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                • #9
                  Re: SU carb - poppet valve in butterfly

                  Thanks chaps, all interesting stuff. Keep it coming. I agree with Cameron - one big carb is far simpler than twins. Interesting that the subject has drifted into HIF carbs, this is something that has interested me recently. The only obvious fault on my original Marina carb is that there is a lot of play between the throttle spindle and the carb body which I know can lead to problems. It has been like it since I bought the car. I put a tight rubber O ring on the free end of the spindle and shoved it up tight to the body, then smeared a load of grease round it.
                  Franklin, Leicester

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                  • #10
                    Re: SU carb - poppet valve in butterfly

                    I once tried machining a groove in my sloppy spindles and put a small O ring in the slot which was inside the carb body bearing area. this worked OK for a while. However, the O ring can rot and decide to jam the throttle. This was quite a surpirise in my GT6 on the A19 near Midllesborough in about 1972 when it stayed open at MANY MPH. OOps. I got the spindles and bearings properly sorted after that!
                    Marlin Berlinetta 2.1 Efi

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                    • #11
                      Re: SU carb - poppet valve in butterfly

                      Originally posted by Franklin, Leicester View Post
                      Thanks chaps, all interesting stuff. Keep it coming. I agree with Cameron - one big carb is far simpler than twins. Interesting that the subject has drifted into HIF carbs, this is something that has interested me recently. The only obvious fault on my original Marina carb is that there is a lot of play between the throttle spindle and the carb body which I know can lead to problems. It has been like it since I bought the car. I put a tight rubber O ring on the free end of the spindle and shoved it up tight to the body, then smeared a load of grease round it.
                      Several topics here;

                      1. Most engines had a single 1 1/2" SU from a Marina or twin 1 1/4" SUs from an MG Midget - late 1.3 Metros & Maestros used a HIF 44 which is 44mm or 1 3/4" on an alloy manifold. I saw some flow data that showed the single HIF44 flows more air and hence fuel than twin 1 1/4" SUs or the single 1 1/2" SU. Twin 1 1/2" SUs can be had for the A series, BL Special Tuning made several thousand sets in the 1970s, but I have never seen a manifold.

                      2. The brass spindles do wear the aluminium carb bodies. Burlen Fuels can supply rebuild kits with a reamer and oversized spindles. A word about Burlen Fuels - they own the rights to all SU materials and remake lots of old SUs, dating right back to the 1920s. SU (Skinners Union) was owned by BMC then BL and then Austin Rover - when they went t**s up, Burlen Fuels bought all the rights and designs for SU. If you buy SU bits from anyone else, they were originally supplied by Burlen Fuels, they have a brilliant parts catalogue website and shop.

                      3. Butterfly poppet valves - Peter Burgess (B series tuner) reckoned that they can reduce air flow by up to 10%. If you buy the oversize spindles from Burlen Fuels, buy some plain butterflies and screws at the same time.

                      Chris
                      Last edited by c.weedon; 09-04-12, 08:37 PM.

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