I have an SU carb downdraught, horizontal damper piston, I'm planning to fit but I'd like to see how it should be done. The SU site has useful info on the carb but not on which cable goes where and how they are arranged. Does anyone have pics of an SU in situ which they could post please?
Ta

I have one of these on the Ulster Duncan - very good it is too!

What size is the one you have?

I will try and take some photos for you.

I have one on my Ulster too! I will take a photo to show how I have connected mine up. I like the set-up as the carb is well away from the hot exhaust and is very easy to get at.

Duncan, I have asked Ruairidh to do his magic with the photos as it is well beyond me!
I think my SU is a 1". It is mounted on a Dante manifold fitted with an aluminium adapter plate to turn it through 90 degrees. Fuel supply is from an SU electric pump mounted on the bulkhead. The hardest bit to do was the choke cable as on the Ulster the petrol tank is in the way. I turned the choke operating lever through 180 degrees and the cable is routed over the engine to the pull knob on the dash under the steering wheel.

By the way I tried to post on "Sales & Want" for 1 1/4 period S.U.s but can't see the result. Can't see any ad in fact...
Renaud

Location: Brittany

Renaud

Your advert (both) are there.

IIRC it's 1 1/8" and I have a Reliant manifold. It's all at Keith Dobinson's as it took it with the engine for reference so I can't check immediately. I looked at the SU site but there were no pics of the horizontal one nor any of the control cables. I'm sure it can't be that difficult but a reference to other installations would be helpful.

If its a 1 and 1/8" carb it should be ideal. I ran my Arrow special with the D/D SU on a Reliant manifold, again with an adapter to turn it through 90 degrees. It worked very well. SU's are good to play with as you can alter the control levers etc very simply.

Here is the setup on Malcolms car - mine is very similar, the choke coming to the passenger side on my car.

Thanks for the pics. That seems to be a very long choke cable? Is the throttle operated by the standard linkage? (which I do not have)

Location: Ripon

The choke cable is longer than I would have liked but works okay. The throttle shaft assembly is similar to original, made up out of bits and pieces, using a proper return spring.

Location: Brittany

Renaud, I can't see the ads using Firefox, but I can see them using Internet Explorer!

Location: Bonnie Galloway

Strange to relate chaps, I can see all the 21 Ads both using Firefox 43.0.4 and Internet Explorer 8.

Ian Mc.

Location: Shropshire

Hi all,

I'm using 1 1/8" D2 carb on my nippy and it works well.

I have a question about needle.
Could you tell me what type of needle using?

Now I am using with "V3" needle and red spring but it is too lean for my engine. (idling-rich, high rev.- lean)
So yesterday, I bought "3" needle. It is richer than "V3" needle.

Engine specification is ;
1 1/2" pressure-fed crank, big IN valve, a little bit sports cam.

Regards,

Junichi Sugimoto

Location: Japan

I run an 1 1/4" DD SU on my Ulster Junichi.

After struggling for some time to get it to run cleanly Ian McGowan advised fitting a 3 needle.

The car now runs very well indeed.

The one I have came from eBay described as " just removed from my Ulster, ready to fit" so I siuppose I'm going to have to find out how well it's set up relative to my motor.
I have found the SU website very useful so far...

Hi Ruairidh,

Thanks for the advice.
I think D/D carb needs rich needle. My friend's Ruby uses H1 S/D carb with "GG" needle.
It work fine but too lean for my nippy.


Junichi

Location: Japan

Ian McGowan

Strange to relate chaps, I can see all the 21 Ads both using Firefox 43.0.4 and Internet Explorer 8.

Ian Mc.

Location: Brittany

Ruairidh,

I saw SU needle chart, "GG" needle is very lean type.
Anyway, I will change "3" needle and checking. Thanks a lot.

Junichi

Location: Japan

I have single 1 1/8" semi-down draught SUs on two of my Sevens and both are best on GG needles. Lots of trial and error or a rolling road session with someone who knows what they are doing are the only, real, way to determine what's best for your particular engine. Both my SUs are early post war with bronze pistons and, therefore, no spring.

Both engines are similar specification. Pigsty Trials cam, big valves, lightly ported block, Ricardo head, four branch exhaust and alloy 'log' type inlet manifold. Honda pistons on one, Quarry Engineering on the other.

Steve

Location: North Yorkshire

Hi, all down draught users. May I suggest you check you have lock washers under the dashpot cover/top holding screws. On the way to Berkley Castle Rally one year I stopped at the traffic lights in Worcester. The engine continued running smoothly but wouldn't rev up. On opening the bonnet I found the screws had dropped out, and the carb top had upended itself on the top the cylinder head. It was intersting trying to find small screws at 8am in Worcester on a sunday morning. However on entering the autojumble at the rally, the first thing I saw on a stall was a complete 1 1/8 th DD carb on a stall. Guess what I spent my pocket money on!!

Location: In the warm

I am surprised by the frequent mention of dd SU carbs. I would expect them to be very rare. As a matter of curiosity what do they come off? About the only car I was aware of was the large post war Wolsley but presumably they used large carburettors.

(I gather conventional SUs do not work well in speedboats. From the pitching of Sevens on some surfaces the same limitations might apply!)

Location: Auckland, NZ

I cannot recall the last Austin Seven competing regularly in trials where I saw anything other than an SU carb, and believe me I have driven and watched a few motors pitching more than any boat. Curiously, most but not all these cars use side-draft SU carbs.
Interesting choices of needles: many of us resort to the same identified group of SU needles for the common 1", 1.125" & the slightly optimistic 1.25" carburetters.
We all seem to like the sensible higher mixture velocity of the two smaller choke sizes. We like GG; EB; MOW; M6; and the slightly longer number 3 needle in the bigger carb. It isn't as mythical to remedy the weakness or richness as people think. The needle chart is most useful once you understand how much the piston lifts at various throttle settings. Basically, it should fully lift at full throttle.
That piston ought to move freely but you should feel resistance from the oil damping. The piston should close promptly with the throttle closing. Keep it free from carbon & that ultra-fine grit we see in 'used' carbs.
Please never reshape needles to solve a minor problem (that will come back and haunt you or the next owner who reads the needle code). If you have never seen a new needle, these have a microfine annular spiral surface, not the silky smooth finish it may feel like (without looking through a lens)!
Sort out what part of the throttle setting is weak or over-rich and ask for alternatives to try. The most common error in understanding your carb is incipient worn throttle spindle. It weakens the mix by weeping air into the flow. This air-leak is not always evenly added, as throttles are spring-loaded and change shape in differing ways.

1930s Rovers used D-D SUs. The Rover 10 uses 1-1/4". It has '10' stamped on the flange.

Location: Melrose, Scottish Borders

Down draught SUs do seem to be relatively uncommon.It may be of use to know that apart from the fairly large ones fitted to Rovers they were also fitted to Singers,10hp 1936-46 and 9hp 1939-?.I am currently rebuilding a 11/8 specimen of unknown origin[Dremel,epoxy,brass pins].John Barlow has provided a D/D manifold and flanges to turn the thing through 90 degrees.As its on my track car choke operation will be under the bonnet an easy cop-out.Throttle by cable.The question ;is it worth it? Anecdotally they do go well,I used one on a previous special but never really pushed it.Bill Wiiliams recommends them in his book,which incidentally I love,but as usual there is no reason given.Mind you he also advocates twin SU s for the ultimate in performance but most experts I have spoken to say they are a waste of time....but I have a pair of OM 1" off a J2 MG in my box of bits and the temptation is hard to resist.My second question ; why not?

Location: The Pits,Leicester

I seem to recall that when car makers in the 1930's started fitting downdraught carbs to their side valve engines, it was always advertised that this resulted in more power than previously. I suppose they would say that wouldn't they?
I like the D/D SU for its accessibility, (less bending of the back), and the float chamber is away from the heat of the exhaust manifold.

I seem to recall that when car makers in the 1930's started fitting downdraught carbs to their side valve engines, it was always advertised that this resulted in more power than previously. I suppose they would say that wouldn't they?
I like the D/D SU for its accessibility, (less bending of the back), and the float chamber is away from the heat of the exhaust manifold.

I like the notion of a cooler float chamber,hence I plan to use 30degree floats and 6inch inlet stubs.Any objective evidence for lagging exhaust pipe,in the same vein?

Location: The Pits,Leicester