How it Works - Carburettors (part 2)

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.

carb jet 2

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!

carb jet 4In 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.