Fitting SU carbs to DS
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Thread: Fitting SU carbs to DS

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    Fellow Frogger! badabec's Avatar
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    Default Fitting SU carbs to DS

    Hello, one daft idea that keeps popping into my head is to fit twin HIF SU carbs to my DS.
    I know if you change one thing, five other things will need changing.
    But, the SU is a lovely carb that doesn't warp its flanges, gives good economy and plenty of power.
    Talk me out of it someone...

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    While you are at it, fit the ever-reliable AED (auto choke) device from a Jaguar of the same period. That should complete the picture.

    Twin carbs obviously need balancing, so that's added work and will be a repeat exercise as they move out of tune/balance. How are you going to choose the best needle for the job? Also, I believe the damper pot springs are calibrated to the application. So, there may be some trial and error to avoid flat spots. Is something like an MGB a good starting point?

    The only thing better I can think of is a set of four Strombergs to equalise. Some UK cars sold to the USA had Strombergs instead of the SUs - emissions strangling most likely. People often change them to SUs when the buy these cars from the USA. And then there are the people that fit Webers or similar to get away from SUs. it's all a bit confusing. If you want to play, how about Megasquirt???

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    Fellow Frogger! badabec's Avatar
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    Hello David S, added to the problems you have outlined, there are also the throttle linkage, breathers and air box.

    Stick with the Weber I think, for now.

    Thanks for the input

    Peter

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    Quote Originally Posted by badabec View Post
    Hello David S, added to the problems you have outlined, there are also the throttle linkage, breathers and air box.

    Stick with the Weber I think, for now.

    Thanks for the input

    Peter
    Not sure if this as appropriate comment Peter but back in the old days when I was playing with Fiats
    and Lancias there was a variety of different Webers available.

    Have you considered upgrading the Weber it self?

    John

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    Pancake filters and a breather feed via a balance tube between carbs would solve those two problems. Linkages shouldn't be that hard and you could always fit a flexible cable as used on an EFI DS. The manifold would need to be bespoke and a pair of twin branches would suit. The HIF has an integrated float chamber, so you should have more room to work with, but is there actually enough clearance between the head and sphere / coil to fit them on a decent manifold with air filters?

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    What advantage?

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    Fellow Frogger! badabec's Avatar
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    Quote Originally Posted by moulton2speed View Post
    What advantage?
    Because I don't take my own advice to leave it alone and constantly want to fiddle and change things! I like a challenge and it gives me something to think about when I sitting in interminable traffic jams. It may (probably) come to nought.

    There was a proprietary conversion to SU in the 60s.

    Thanks everyone for your input

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    Twin SU on parents Triumph were nightmares with tuning each day to get the car not to stall rev up to high.

    A dash pot dampener never did understand why the oil disappeared out if it so fast. Maybe worn out but I hated those carbs.

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    Tuning twin SUs is a walk in the park. Easy as...

    I ran them for many years and looked at them about every 8 months There is little to touch. The easiest way to get the pair running with the same airflow is to use a small water manometer. There's still one buried in my junk somewhere, as well as an exhaust gas meter.

    A major problem I see has been raised - the needle is designed for the engine - as it rises the diameter changes.

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    I've done both twins on the Cooper S and triples on the E-Type. They are reasonably easy, with recurrent balancing issues only a problem when there are rubbish linkages between them. The wavy pressed steel clamps used between shafts can cause issues if worn. Triples take a bit longer to get right for sure. I still have the flow meter I bought when I was 18.

    The comment about the dashpot spring was also important as spring rates are specific to the tune (rated by colour from memory; I've seen blue and red). But if there was a conversion back in the day, the info is probably out there somewhere about which needles and springs were used.

    I actually quite like the SU but then I'm fairly familiar with them and read David Vizard's Mini book till it was well worn.
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    One performance mod with SU's is too remove the dashpot spring, as this gives better throttle response. The dashpot damper is exactly that!
    1963 Morris Cooper 997
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    Quote Originally Posted by OnSafari View Post
    One performance mod with SU's is too remove the dashpot spring, as this gives better throttle response. The dashpot damper is exactly that!
    NO NO NO. As already said the spring is an integral matching part of the selection to match the carby to the engine. It is selected to get the piston opening to match the air flow so it fully opens just before full airflow. The piston also controls the needle position which controls the mixture.
    The difficulty I would see is getting some needles these days. There were tables of the needles and their diameter at each distance over their length so you could estimate changes to make. That book by Visard springs to mind. I never found the to need for constant "tuning". Most problems are caused by crap linkages and non matched flogged out carbies and air leaks. The basic settings of spring and needle never changes in use.
    Jaahn
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    Presumably, removing the spring is intended to allow the piston tom move more rapidly and therefore add more fuel. The choice of oil for the dampers must also have an impact, but can't have been too critical in most road applications where the manuals just advised to fill with 'light engine oil'.

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    Besides damping piston flutter, the oil also causes a delay to additional air via the piston at sudden throttle openings, to slightly enrichen the mixture for more instantaneous throttle response (to prevent a flat spot) - a similar function to the pump diaphragm and jet on a Weber.

    There are two types of needle; the fixed ones which are centred on installation don't usually wear because they don't touch anything, but the spring loaded ones are biased to one side of the jet and wear both the inside of the jet and the needle through friction.


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    Hi
    I would liken the removal of the spring to, say, removing the venturie from a carby because it will let more air in Why bother with the theory just look on the internet
    In my day we actually looked in a book written by someone who had a reputation achieving performance or the makers manuals.
    With an SU the spring, or even a zero spring? is a particular setting. The damping is controlled by the 'dash pot' oil not the spring, and actually works as an enricher similar to an accelerator pump. But hey let's not let the theory get in to the discussion.
    jaahn
    PS in the last days of carbies and for pollution control some major manufacturers used the constant vacuum principle of an SU in some of their carbies to get better outcomes. They did not look like an SU but had the same working principle. So the idea is good.
    I see Stuey has posted before me

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    bob
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    Default SU or Stromberg CD

    G'day,

    personal experience shows Stromberg CD was the less frustrating of the two - over the long haul. For nice new units there is nothing to pick between them, unless you have parts data for your particular engine. As has been mentioned, there must have been hundreds of needles in the list.... Are the needles you want still around ?

    Fixed choke carbs are generally much easier to play with, and more forgiving about being moved to somewhere where they were not designed for. In olden times, before the nanny state and atmospheric issues and design rules, if you wanted more carby you butchered an old manifold to accept two or three of the same - sometimes a manufactured manifold was available. Or tried something designed for another engine altogether, triple monster webers on an old FJ motor was not unusual - along with big overlap cams etc....

    I have had a Stromberg fixed choke ex a flat head Ford v8 fitted to a Standard 10, it ran much better and gave similar economy to the tiny Zenith[?] it replaced.

    Balancing multiple units was a much overrated black art, I had a Triumph 2000 with triple Stromberg CDs, the only criteria was to ensure that all the butterflies were fully open at full throttle - perfect idling was of no interest....

    cheers,
    Bob
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