FITTING AN SU CARB TO A 1932 A7 RP BOX
This turned out to be another job I had been putting off for many years, but which turned out to be incredibly straight-forward once I had all the bits assembled.
Which SU carburettor?
The intake hole in the Box’s manifold (and all
standard Sevens, I think) is about 1 inch. Whilst 1¼“ SUs are readily
available at autojumbles and can be fitted without a problem, clearly a 1” or,
even better, a 11/8” is preferable. The 1” can
easily be identified as having no outside flange whereas the 11/8”
does. I was lucky to pick up a 11/8” at Beaulieu so
the rest of this article will be about fitting it.
Will it bolt straight on?
Unfortunately, not! The holes in the inner
flange of the SU to go over the
studs are vertically placed whereas the studs in
the manifold are horizontal, so we need a 90o adapter. This is
easily made on a home lathe or is available from the usual A7 suppliers (e.g.
Tony Betts at Seven Counties Austins and
David Cochrane at A7 Components).
If you’re making one yourself, note the small cut-out section which is necessary
to accommodate the throw of the throttle lever. See dimensions at end. However,
with the adapter, yes, the SU then bolts straight on.
Do the A7 accelerator lever and choke cable
fit directly?
Yes!
Choke: The SU carb has its own fixed
arm and sprung choke lever so the A7 stiff wire choke cable simply goes through
the hole in the fixed arm (just for guidance) and pulls directly on the choke
lever, in the correct direction. The only thing needed is something to grip the
wire to stop it pulling out of the choke lever. You may be able to use the
existing one on the Zenith, but I had a spare in my box of bits. If you
have a flexible choke cable in a sleeve, then the hole in the fixed arm will
accommodate the nipple-end of the outer cable.
Accelerator arm: The Zenith carb has
a ball and pin which bolts to its throttle arm and fits in the vertical
accelerator lever of the car. The SU has an exact hole to take the ball
and pin and it can easily be adjusted vertically on the throttle spindle to get
the right height. I was warned that I might need to organise the A7
accelerator lever to have a bigger movement to fully operate the SU butterfly,
but the 11/8” seems to be fully opening and closing with
the standard A7 accelerator mechanism. (Perhaps the 1¼ “is different.)
In order to remove the ball and pin from the Zenith
carb, I first removed it from the vertical throttle arm. This nearly ended
in disaster! First, remove the split pin which is preventing the head of
the accelerator arm from rotating and unscrew the head using the screwdriver
slot at the top. This is where the danger arises – under the head is a strong
spring with two tiny cups which surround the ball end of the pin. The spring
won’t suddenly jump out until the ball is removed by pushing it down and pulling
it sideways but you may a) lose the upper cup lodged in the head and b) lose
your eye when the ball does come out and the spring is released. For me,
the bottom cup went spinning through the air, right over the car and far away
only to be retrieved 20 minutes later. I now wonder if it was possible to remove
the pin from the Zenith without taking the head of the accelerator arm to
pieces. The Zenith can now be removed from the manifold.
Fitting the SU:
Because the inner flange of the SU will be a tight fit
on the outer face of the adapter, you can no longer use the existing studs but,
instead, have to use hexagon socket -headed, countersunk screws (5/16 BSF 1 inch
long) that screw down into the threaded holes left when the studs have been
extracted. My two studs came out relatively easily but a little heat on
the manifold may be needed. Be careful, of course, not to shear-off a
stud. Making sure the two, new bolts, facing outwards to engage the SU are
in place, the adapter can then be firmly screwed to the manifold with a suitable
gasket and goo. The SU can now be slipped over the protruding bolts with
another suitable gasket and goo and the nuts tightened down. The lower nut
gave me the most grief of the whole job as the nuts cannot be put into place
with the SU pushed home. However, with a lot of patience and a lot of
fiddling the SU was firmly bolted in place.
The ball pin is easily bolted in place, with the nut
gripping the spindle left firm, but not tight at this stage, so the levers can
be adjusted to fit. Now for the tricky bit – getting the ball back into
the head of the accelerator lever on top of the spring and lower cup by pushing
the cup
down from above with an electrical screwdriver whilst pushing the head
over the ball. Once in place, pressure is taken out of the system and the
top cup and head can screwed down and the split pin replaced.
With the spindle nut a firm fit, you can now push down
the throttle connecting lever so that the butterfly is fully closed when the
accelerator arm is at the bottom of its travel. After the trial run, I had
to lift the accelerator arm very slightly so that its spring held the butterfly
more firmly shut – when I lifted my foot off the accelerator, the butterfly
wasn’t closing properly and the revs didn’t die away enough.
The choke cable is an easy fit but ensure that it is
fully home before tightening the keeper.
And that’s it! Mine
started first time and, after allowing the engine to warm up on a fast
tick-over, I adjusted the mixture by ear to produce the fastest setting (rather
like setting the timing by ear) and then took it for a good run. (For SU
novices, the large nut at the bottom of the SU is easily screwed upwards by hand
to weaken the mixture and downwards to enrichen it. The setting is done at fast
tick-over.) Checking the plugs on return, I got a grey electrode and black
surround which is probably about right. Vince swears by ColorTune but my
old one only has a 14 mm thread so I’m waiting for an 18 mm adapter from E-bay
(just type in “Spark
Plug Thread Adapters 18mm down to 14mm”) to
see how close I’ve got by ear.
The actual job took only a day having got all the bits
assembled but, whilst the Zenith was off, I took the opportunity to check the
tappets and try to seal the valve chest cover a bit better to stop the annoying
dribble of oil after every run and cleaned out the inlet port with a wire brush
and smoothed a raised part of the casting with a rotary file.
Joining up the fuel pipe:
The SU banjo is much larger than the one on the Zenith
and so is its input nozzle requiring a larger diameter pipe (5/16” o/d) than the
existing one (1/4” o/d) and a larger nipple at the fuel pump end (both available
from Seven Workshop). However, to my surprise, the larger nipple was a
petrol-tight fit to the fuel pump which, therefore, needed no alteration to the
output port. I could have used the existing pipe as a model for the new
one but took the opportunity of rotating the top of the float chamber to take
the banjo, and thereby the petrol pipe, further away from the manifold and
exhaust downpipe to help reduce vaporisation in the pipe. The only points to
note about soldering the pipe into the banjo is to make sure you’ve got the
alignment right if you’ve already bent the pipe to shape and, more importantly,
that the indented groove that takes a recessed washer is facing outwards towards
the large nut. (I got it wrong and had petrol squirting everywhere until I
worked out what the problem might be and had to re-solder the banjo rotated by
180o.)
Needles:
Ian Mason-Smith has done
quite a bit of research on which SU needles are appropriate for A7 engines – see
our website
http://www.da7c.co.uk/technical_torque_articles/su_needles.htm
and there is a very useful interactive webpage (http://www.mintylamb.co.uk/suneedle/.)
where one can type in up to 3 needles at a time by name and a graph will compare
their running characteristics. My SU came with an MO needle so I started
from the premise that if it was good enough for the previous owner, it was
probably good enough for me. On the Burlen’s website (http://sucarb.co.uk/technical-jet-needle-recommendations),
one can search for which needles were fitted as original equipment and my MO was
fitted to 1933 Morris 8s and Austin Healy Sprites, so I’m going to stick with
the MO. Ian uses EB (which looks to be almost identical to my MO and was
also used on Austin Healy Sprites) and AN needles but in 1¼“ carbs with springs
(the 11/8” doesn’t have a spring). With
his
11/8”, Bernard uses an M..
Burlen’s:
The SU was in a rough old state when I bought it so
there were three options – a) leave it alone but just clean it up; b) buy a kit
of parts and do it myself; or c) get Burlen’s to completely refurbish it.
I chose the latter.
There are now two sides to this, and you will have to
choose – the total cost was an eye-watering amount but the bag of old bits I got
back contained every screw, spring and split pin; spindle, butterfly, needle,
float, float needle, jet etc etc. In other words, everything had been
changed except the body and piston and the body was polished to look like
chrome. I even got a new banjo and filter thrown in as mine was missing. Was it
worth it? Only you can decide. I’m absolutely thrilled, albeit a little
bit poorer.
Running:
ALL IN PLACE PHOTO
Everyone who has made the conversion used the phrase
“smoother running”. It’s difficult to quantify this but I have to agree.
The car did run much more smoothly, with an absence of having to double-declutch
and, whilst there was little improvement in top speed (is this the limitations
of the whole car rather than just carburation?), there was a distinct
improvement in torque giving quicker acceleration and more pull up hills. It
also now starts “on the button” without having to prime the fuel pump or wind
the handle. Am I pleased I made the conversion? You bet. Early days yet
but, at the end of the day, everything can easily be reversed if not.
David (Bodger) Whetton