I have personal experience of a variety of different carburettor & manifold combinations on both
standard and mildly tuned road-going
Austin Seven engines and my findings are summarised in the following notes.
However, I do not include details of the selection, adjustment and maintenance
of specific carburettors, because there is already a wealth of information
available and indeed whole books written on the subject. In my experience it pays to ensure that manifold passageways are generally smooth with no sharp steps or corners. It can sometimes be useful to remember our basic fluid mechanics theory here, that tells us sudden enlargements are a greater source of friction loss than sudden contractions of similar proportions. An interesting fact and perhaps somewhat counter-intuitive.
It is often necessary to remove material to achieve reasonably smooth passageways and this can be achieved with small grind stones in a hand-held drill or ‘Dremel’ style flexible drive. It is also important to remember that gaskets might need trimming to ensure they don’t interfere with the smooth flow of gas.
I believe the inlet tract is more important than the exhaust and I am not convinced that a polished finish is necessary in a touring engine. The photo shows a reasonably smooth gas flow path in a current A7 engine that performs remarkably well.
Many Chummy owners claim surprisingly good performance with the Updraft 22FZ Zenith carburettor in conjunction with standard early Austin cast-iron and alloy manifolds. However, I believe this might have more to do with the light weight of these cars, than efficient breathing of the engine, because the gas path is unarguably torturous.
My experience is that a side-draft 26VA Zenith mounted on the Ruby type standard Austin cast iron manifold does offer a slight increase in power. This is especially noticeable if the narrow throat of the inlet is opened up from the standard 0.75” Dia, to say 0.95” Dia and the internal corners nicely rounded.
However, I have a personal preference for SU Carburettors. They seem to suit Austin Sevens rather well, are often easier to set-up and being based on a variable jet principle, they enable any slight blockage to be easily overcome by tugging briefly on the choke. Back in the Sixties, many A7s sported twin SU carburettors but unless you plan to de-siamese your inlet ports, it now seems widely accepted that a single instrument will be more effective and of course a good deal easier to set-up. Whilst on the subject of jet blockage, I always employ a fuel filter. My preferred type has a fine brass mesh above a small glass bowl in which I often find small amounts of debris which are easily removed. One purchase of petrol in rural France on the 2017 Eurotour caused the glass bowl to become completely full of fine grit particles but happily, nothing reached the float chamber.
So what size and configuration of SU carburettor do I favour? Well, I have had good results with a 1.00” Dia (measured at the butterfly) horizontal sidedraft SU fitted to the standard Ruby style inlet/exhaust manifold; which seemed to give a further improvement in performance over the 26VA Zenith on the same manifold.
However, a more tasty option is a 30o 1.125” Type H1 SU mounted on an ALR (or similar) alloy inlet manifold with 30o flange, in conjunction with an ‘bunch of bananas’ steel exhaust manifold such as the one shown here made by Ian Bancroft.
This combination has worked extremely well for me for some years but fitting it in-place is not entirely straightforward. Firstly, there is no room for the centre manifold stud, so this must be blanked-off, which means the other studs need to be in really good shape. I have previously mentioned that Sir Herbert’s offerings seem a little frail at ¼” Dia. so I always make mine 9/32” Dia.
Access to several of the manifold nuts is also severely restricted with this particular exhaust arrangement, which makes the length of the studs somewhat critical. Even then, there is insufficient clearance to accommodate a conventional socket for tightening the nuts. So, I have resorted to an old box spanner with some material ground-off its outer surface. Clearly, for structural reasons, it is important not to overdo this grinding or let it get too hot which would destroy its temper. Happily, old box spanners are often made of excellent steel and my ground-down version has lasted many years.
Box saloons (and I guess Rubies) have plenty of room to accommodate a ‘bunch of bananas’ exhaust manifold but with a ‘Chummy’ it is more of a squeeze. An easier option in this case, is a swept four branch exhaust manifold such as that supplied by Pigsty Racing and a fine example can be seen in the photo on the left of one of our Club cars.
Clearly, with sports exhaust manifolds you have to make your own arrangements for connecting the exhaust pipe because standard exhausts will not fit directly. Actually, I have found that they sometimes don’t fit terribly well even in unmodified cars.
Another interesting option is a Nippy manifold. Interesting, firstly because it will accommodate a standard exhaust downpipe and secondly because it seems surprisingly efficient. This efficiency might derive from a masterpiece of theoretical Longbridge calculation but some argue it is more likely a happy accident of design. Never mind, it works really well.
Unfortunately, the correct Nippy downdraft Zenith carburettor (the 30 VE1) has become an endangered species, the last one I saw for sale about five years ago was severely damaged and still had an asking price of well over £200. However, several other similar Zeniths make excellent substitutes and often come-up for sale at reasonable prices. There is also plenty of available information on suitable choke and jet sizes. 26
For those of us that prefer a variable jet in our carburettors, it is fairly straightforward to fit a 30 degree aluminium adaptor to a Nippy manifold as shown in the photo. Standard adapters are available but the ones I have seen are designed to take a 1.25” HS2 SU, which I believe might be slightly too big for my road engines.
However, with a little ingenuity, these adapters can be modified to take a 1.125” H1 SU and I am currently enjoying excellent results with this configuration. As a point of interest I am currently using a GG needle which seems about right.
The photo also shows my oil catch tank. The valve chest cover breather holes are blanked-off and an adapter connects (via a length of old outer speedo cable) to an old Brasso tin that has a ring of small holes in its lid. It works surprisingly well.
Since this photo was taken, I have added a short home-made inlet trumpet, fitted with a coarse gauze to prevent any unwanted large lumps of grit from entering the engine. It seemed a good idea and there is just enough room under the bonnet - but it has had no noticeable effect on performance. At some stage I propose to experiment with some form of air-cleaner cum silencer but for the moment, I shall continue to enjoy the noisy roar of the engine, especially noticeable on wide throttle openings.
Therefore
unless the linings are obviously fairly new, I invariably replace them when
building an engine. Austin Seven clutch linings are not wildly expensive, they
are dead easy to fit and it gives enormous peace of mind. Similarly, a set of
new clutch springs is also a good idea if yours look a bit compressed compared
to new ones which are typically 1.375” long. Incidentally, although some people
recommend them - I have never felt the need for double clutch springs, even with
fairly pokey engines. Maybe clutch slip is more frequently caused by oil
sneaking into the clutch? 
able
condition. New replacement items are available, mostly at reasonable prices
except that a set of three levers will set you back approaching £30. No wonder
it’s common to apply weld to the worn faces and then file them to shape. Play in
the lever pivots can sometimes simply be removed by replacing worn pins with
lengths of 5/16” silver steel. Some books suggest removing wear with oversize
pivot pins and reaming the pillars but this will do your reamer no favours
because the pillars are hardened steel, therefore new items might be a better
option. Woodrow tells us the pivot pins should be a press fit in the levers but
I have found that this is not always the case. 
many of us might be driving around with our fan-belts too tight. The suggestion
being that a tight belt will try to climb-up the flange rather than sit nicely
on the peak of the convex pulley where it should be. Mine has definitely shown
this tendency for some time and causes one side of the belt to wear rather too
quickly. I therefore experimented with a much slacker belt and noticed an
immediate improvement in its alignment without any noticeable loss of traction
to drive the fan which might otherwise cause overheating. 
