larger updraft carbs?

[Ian Williams]

Jon if you use an H series rather than an HS, they don't look too out of place, with the H series you also then have the option of an 1 1/4", 1 1/8" or 1" choke bore. If you are using a decent cam, head and ported block I would strongly recommend you consider the looking at choke size and needle I suggested earlier as your starting point. The 1" OM is fine if you are using a std or mildly tuned engine, the problem with using an SU which is too large for the state of your engine tune is that you severely limit piston travel which in turn limits the mixture available from what ever needle is fitted to a very narrow range and subsequent reduction in performance. It is NOT enough to simply set the jet on an SU so that you apparently have a good cruising mixture, I can guarantee that following route will not give a good mixture through out the rev range and performance WILL suffer. To really make things work well you would be well advised to consider size/area of the entire inlet tract from Carb to piston; manifold, port, valve, valve lift and transfer port. Of course to complicate matters for you the exhaust path also has a bearing on how well the inlet path works, fortunately on an A7 most things have been tried, so match what you are doing in its entirety with a well proven set up not just repeated folk law from someone who has read something somewhere. Just because the fastest car on the track has successfully used a 1 1/2" SU with an OMG needle does not mean it will work for you if the rest of your set up is not matching the set up in the car you are copying. Going back to the manifold why not use steel tube as per an Un-Blown Ulster, possibly not the most efficient design that ever existed but period authentic if that matters to you and plenty good enough unless racing a a high level.

[JonE]

I'll definitely use steel tube for the last bit but have located a damaged log/ALR dd and will at least try that first.
I'm conscious with all this that crank will be my limiting factor in all of this - i.e. I'm using an old, and standard, one.
The other reason I'd thought 1" was precisely because of the piston travel. I'd read the ?Hazzard SU book (you'd recommended some years ago, I think) and thought I'd got my head round the issue of piston travel. This engine will again be crank-limited, standard cam, ported a bit and radiused tappets - and just rely on being as light bodily as possible. It worked well with the Woodie with a standard ruby manifold... and 26VE which is only about 17mm choke? (So this should fly with a bathtub on top of the chassis and retaining the lighter uncoupled system).

[Ian Williams]

Yes you would probably benefit from the 1'' by the sound of things, the manifold I made for you should work inverted in that case provided the Ulster exhaust has enough upsweep to clear the dashpot. I was made to fit an OM to a std car so I don't think the bore restriction in the inlet runners will matter in an engine as you describe

[JonE]

What has been interesting is seeing how eminently understandable all the Solex late 1920s/early 30s catalogues are for assessing the correct carb choice, jet choice and so on. This section shows the differences gravity makes for moving from updraft to sidedraft (on 30MOVertical and MOHorizontal) using the left hand column determinant of car's engine size in litres x max rpm - so say a mild Austin 7... c.3750 as 0.747x5000 yielding 19mm optimum choke size as horizontal, but 21 when updraft - with consequently larger jet. So presumably the later downdraft sees a further efficiency here. 
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[Bob Culver]

Solex venturis are less obstructed than Zenith so can be smaller. And the innards of the 1" Su seem the same as the 1 1/8 which is far more common. With large carbs the throttle becomes very non progressive. SUs are supposed to acheive full lift before full power so that enrichment occurs, but users here report that on stock cars the 1"/1 1/8 dashpots do not fully lift. However as Seven owners are no longer concerned about economy matters little.
I would expect updraught to be smaller. Fuel needs be very atmised to survive the lift. Was the venturi more obstructed with the updraught carbs?. Or does the table recognise that the higher revving updraughts were generally sporty vehicles?

[JonE]

Bob, if the index on the left is a factor of cubic capacity and revs, then the latter can't be correct unless they were tiny engines? How does the statement play with the comparison of Zeniths in Austins, for instance... are there any examples where there are similar conditions at play allowing one to just focus on choke and partially-linked jet size in the two variants? With Sevens, there don't seem to be any direct comparisons available as the cars were getting heavier and engines changing when things moved from vertical to horizontal.