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.
1 slow running outlet
2 Progression jet 3 Slow running jet
4 Air regulation screw
5 Capacity Tube 6 Fuel float needle
valve 7 Fuel Filter
8 Fuel Inlet bolt
9 Fuel inlet connection 10 Float
11 Main Jet 12 Compensating Jet
13 Emulsion block 15 Emulsion block
beak 17 Choke tube
18 Progression jet outlet
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.