After losing
many of you with the theory of how the Zenith side draught carburettor works, I
will deal with the specific details of the 26VA version fitted to the Austin
Seven.
Unfortunately because of the design of the side draught version it is impossible
to portray the three-dimensional carburettor as a two-dimensional drawing so the
picture below is of an up draught version of the same carburettor. If you look
carefully you will recognised the float bowl and emulsion block layout as
identical to the Austin Seven carburettor, the only real difference is the
layout of the slow running air regulation screw and passages although the actual
working of this section of the carburettor shown is identical to the 26VA used
on Austin Seven's.
As you will
see all the features are there but some in different positions!
In
normal running the main
jet (11) is doing
most of the work supplying a flow of petrol, relying on the ‘suction’ from the
engine. The level of fuel in the float bowl is such that the emulsion block
beak (15) is just above the fuel
level which means that fuel should not flow out until lifted out by the
flow of air past the beak (15) Immediately you will see from this that if the
fuel level is too
high in the float bowl not only will it drip out it will also provide too rich a mixture when the
engine is running. Conversely if the level is too low then the amount of fuel
lifted by the flow
of air through the choke tube will be less leading to a weak mixture. The
correct way
to adjust the fuel
level is by
altering the thickness of the aluminium washer between the fuel float needle valve (6) and the
body of the carburettor.
When the
throttle is first opened the air flowing into the engine is initially slow and
causes less petrol than required to enter the air stream, therefore the
compensating jet (12), capacity tube (5) and the three minute holes in the
emulsion block all contribute to provide extra fuel, temporarily making the
mixture more rich until the air flow is more constant and the flow from the main
jet is sufficient.
We now turn
to one of the most misunderstood parts of the carburettor, the slow running jet
(3) and air regulation screw (4).
The amount
of fuel to achieve slow running is controlled by the slow running jet (3). The
air flow in the slow running system is controlled by the air regulation screw
(4). If you screw the adjustment in more air is being sucked from the engine
side of the system which in turn lifts more fuel from the jet (3), making the
mixture richer. If you unscrew the control then more air from the outside is
sucked in, less from the engine therefore less fuel is lifted from the jet (3)
making the mixture weaker.
Now the
misunderstood part! If you look at the engine side of the slow running
passage-way you will see it has two branches, one finishes near the engine, the
slow running outlet (1), the other finishes at the butterfly. When the butterfly
is closed (or virtually closed) there is more ’suck’ on the engine side of it
than on the carburettor side therefore the engine sucks through slow running
outlet (1), making it work at its most efficient lifting fuel from the slow
running jet. When the butterfly is open then the pressure (or amount of ’suck’)
is the same on both sides of the butterfly and in both of the two passages
therefore the ’suck’ hardly exists and the slow running jet is no longer
actively providing fuel. The slow running adjustment, therefore, only works when
the throttle is closed, it has no effect on the mixture when the throttle is
open, or substantially open.
In normal
use the carburettor should give you very little trouble, however, due to the
design of the jets in the float bowl any condensation or dirt can very easily
block the main or the compensating jets in the bottom of the float bowl. These
are simply unscrewed (using one of the bowl fixing screws which should have a
square key end) and cleared by blowing through, do not use a wire unless
absolutely forced to. Don’t forget to make sure that water or dirt is not in the
passageways under the jets. These can be cleared by blowing down through the
emulsion block beak. If you cover the other jet holes then most of your
blow will come through the open hole.
If you find
the slow running adjustment is doing nothing then under the carburettor you will
find a little brass plug screw which, when removed, will reveal a jet, the
progression jet (2) which is most likely blocked.
Other than
the throttle stop screw which sets tick-over speed there are no adjustments
designed into the carburettor. However, if you have a modified engine then
adjustment may be necessary by changing the size of the main jet.
This article, written by Malcolm Watts, originally appeared in Seven Focus Jan 2007 pp13-15.
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