SU Variable Choke Carburettors Explained

The S.U. carburettor is one of the simplest carburettors ever invented! Unfortunately, following discussions with many people, the operational principles of this carburettor appear to be poorly understood. SU carburetors have a minimal number of moving parts and are easy to tune, providing of course you understand the operational principles! It is the intention of this article to help clarify those principles in the hopes that many owners will be able to tune and maintain their SU carburtetors aided by better knowledge and less blind experimentation.

The correct technical name for the S.U. carburettor is VARIABLE CHOKE or CONSTANT DEPRESSIONcarburettor (see note). This document will refer to the S.U. as a variable choke carburettor.

The variable choke carburettor inherited its name because a sliding piston, acting under the effect of engine manifold vacuum, automatically adjusts the size of the throat orifice (choke) causing the air velocity and the vacuum over the fuel jet to remain constant.

The letters "S.U". are actually stand for a brand name and represents the words "Skinners Union" named after three brothers who, back in the early 1900's, invented, patented and began the manufacture of this type of carburettor.

There have been a number of S.U. variable choke carburettors manufactured over the decades. However, the basic components that make up the carburettor and the principles of operation are the same.

The variable choke carburettor consists of the following major components:

§ The fuel bowl housing the fuel float and fuel flow regulating needle. In later S.U. models (HIF series) this is integral with the carbie body.

§ The carburettor body which houses the main jet, the butterfly and the cold start enrichment device (choke).

§ The piston (air valve) which includes the fuel metering needle and dash pot.

§ The piston (suction or bell) chamber containing the piston, piston spring and damper.

The Hitachi HJG46W carburettor installed in the pre-pollution Zeds is based on the S.U. model HS6. The later model 260's (the GS and GSR-30) utilised the Hitachi HBM46W variable choke carburettor highly modified to accommodate the stringent anti-pollution laws introduced in the 1970's. Unfortunately, this later model carburettor was not as reliable as the earlier model. I found this out after purchasing my GSR 30.

The later model carburettor is difficult to tune. Instead of adjusting the air fuel mixture by adjusting the jet height, similar to the HS6 S.U., the jet height is fixed. Additional air bleeds into the side of the carbie (just behind the piston) via an adjusting screw causing the air fuel mixture to run lean or extra lean. Unfortunately, the GS series Zeds, released in Australia do not have hardened inlet valve seats that don't like lean mixtures terribly well and tend to pit. This also is the reason that the later model Zed's should not be fed with unleaded petrol (ULP).

The component parts, which together, form the variable choke carburettor are detailed in Figure 1 and defined below:

The Variable Choke Carburettor

1. Air valve suction (bell) chamber
2. Air valve (piston)
3. Air valve (piston) spring
4. Air chamber to manifold vacuum connection (air less than atmospheric pressure)
5. Suction chamber air inlet (air at atmospheric pressure)
6. Dashpot
7. Dashpot piston damper assembly
8. Fuel needle
9. Jet
10. Jet retaining nut
11. Fuel bowl
12. Carburettor body
13. Throttle butterfly
14. Throttle lever (not shown)
15. Fuel float & fuel supply regulating needle (not shown)
16. Idle speed adjusting screw (not shown)
17. Choke cable (not shown)
18. Choke lever lowers jet to enrich mixture (not shown)
19. Choke cam - slightly opens throttle butterfly so not to starve engine of air when choke is used (not shown)
20. Choke return spring - ensures jet remains in the normal running position – choke is inactivate when engine is warm (not shown).

PRINCIPLES OF OPERATION

Most automotive combustion engines are effectively a reciprocating pump. The volume of air they draw over a given duration in time is proportional to their speed.

Example: Assume we have a four-stroke engine with a volume of 1 litre (61 cubic inches). If the idle speed is 1000 rpm, the volume of air flowing through the engine will be 500 litres (17.66 cubic feet) per minute.

At 5000 rpm, the volume of air will be 5 times that at 1000 rpm or 2500 litres (88.29 cubic feet) per minute.

Assume the engine to which the carburettor is supplying fuel is running at idle speed. The jet, fuel needle and piston spring are sorted to match the performance requirements of the engine. The dashpot assembly is filled with oil of appropriate viscosity. I’ll explain the meaning of “appropriate” later in the text.

The butterfly (throttle) is partially open under idle conditions (no air, no fuel – no combustion).

There will be restricted airflow into the engine and a large pressure drop across the butterfly when compared with that of the surrounding atmospheric pressure. (The lower pressure in the inlet manifold is a vacuum when compared with atmospheric pressure).

The vacuum on the engine side of the carbie throat connects to the top of the piston chamber via an orifice located in the underside of the piston.

The underside of the piston connects to atmospheric pressure via an orifice located near the throat on the air inlet side of the carburettor.

When the throttle is partially open the vacuum within the inlet manifold is allowed to communicate to a greater degree with the air/fuel mixture between the piston and butterfly. This in turn will draw air out of the bell chamber above the piston, causing a partial vacuum within this chamber. There is now a pressure differential across the piston (Low pressure above the piston, high pressure beneath it). This pressure differential will cause the piston to rise, pulling the tapered needle out of the jet, allowing more air and fuel to flow through the carburettor throat and into the engine causing the engine speed to increase.

	When the engine speed is constant the air velocity (vacuum) past the jet is directly proportional to the volume of air passing between the bridge and piston, ie. piston height regulates the air speed over the jet, and piston height is proportional to engine vacuum. Figures 2a and 2b indicate the piston and butterfly positions at low and high engine revs. In both instances the engine under load and is running at a constant speed. The air velocity over the jet in both instances is the same. How can that be? If the engine is revving at a higher speed then the air velocity over the bridge in figure 2b will be higher as well, correct? Not So! The opening between the piston and bridge in figure 2a is small when compared with the overall throat diameter (piston in its optimum position). Though the engine’s air demand is small, the velocity over the jet is high because of the small choke area. The high velocity air draws the fuel out of the jet.

	In figure 2b the engine's speed has increased considerably, as has the engine's air deman and the piston has risen to its optimum position for the given speed. The opening between the piston and bridge is now much larger, but the air velocity over the jet is the same as it was at the lower speed (the air velocity is proportional to the throat area). There is now more fuel drawn out of the jet due to the reduced diameter of the fuel needle. Where the air velocity does change is in the fixed pipe diameters of the induction system, ie. the inlet manifold.

PISTON SPRING

The spring within the suction chamber loads the piston in a downward (closed) position. The tension of the spring is selected such that the piston reaches its fully open position when the engine reaches its maximum air demand, that is maximum brake horse power output, not maximum RPM, at any given engine speed up to maximum BHP:

§ If the spring is too weak, the piston will be elevated to a level higher than its optimum position causing the engine to run too lean.

§ If the spring is too strong, the piston will not rise to its optimum position causing the engine to run too rich.

Why is this so? Surely, if the piston is higher than it should be, the engine will run richer, and if the piston is lower than it should be, the engine will run leaner?

Wrong! Remember the volume of air the engine draws is proportional to its speed.

So… If the piston rises too high the throat area will be larger than optimum, the vacuum between the piston and the bridge will be low. That is the air velocity across the jet will be low, drawing less fuel, thus causing the mixture to run lean.

The opposite occurs if the piston does not reach optimum height the throat area will be smaller than optimum… The air velocity will be high between the piston and bridge causing a larger volume of fuel to be drawn through the jet assembly causing the mixture to run rich

There are a number of springs available for the SU type carburettor each with a different compression loading. They are:

I am unsure if Hitachi, via Nissan, released their variable choke carburettor with a range of different compression rate springs. I believe however, that the HS6 SU springs will fit the Hitachi carbie if a substitution is required.

I have not measured the spring compression rates of the Hitachi carburettor therefore I am unable to compare the ratings between the recommended spring used in the SU carbie and the Hitachi spring. Red springs are recommended where 13/4” SU carburettors are bolted onto the inlet manifold of an unmodified L24 or L26 engine.

DASH POT/DAMPER ASSEMBLY

The dashpot and damper assembly are also located within the suction chamber. The damper assembly, which is actually a one way valve, is contained within the dashpot filled with oil. The valve and oil work such that they impede the lifting of the piston, but allow it to fall rapidly once the speed of the engine decreases.

The dashpot serves two purposes. Firstly, it acts as a damper to prevent the piston following air fluctuations at low engine speed thus keeping the piston steady. Secondly, when the throttle opens it prevents the piston rising in unison with the opening of the throttle. If the oil in the dash pot assembly is too thin the piston will rise too quickly causing the air/fuel mixture to lean out.

Air and petrol, in a hydraulic sense, are both fluids and air is less dense than petrol. Therefore, air has less inertia than petrol. So when the throttle is opened more, air will be sucked into the carbie but the petrol will take a little longer before its flow rate catches up with the new air flow rate.

By damping (retarding) the piston movement with oil an accelerator pump action occurs, ie. as the throttle is opened, the movement of the piston is retarded a sufficient amount to cause a momentary enrichment of the mixture, enabling a sharp pick-up in engine speed.

What type of oil should be used?

Too often people use light duty (sewing machine or general purpose) oil in the dash pot assembly. This type of oil does little if anything to impede the upward movement of the piston as the throttle opens.

Engine oil can be too viscous (depending on climate). After 2 hours of driving it ends up in the bottom of the piston, the majority of it sucked into the engine. This happens because it is too thick to pass through the damper as the piston falls causing the oil to flow out of the top of the dashpot.

I use a mix of 20W-30 to 20W-50 and sewing machine oil. The ratio is three parts engine oil to one part sewing machine oil.

When you use the aforementioned oil mix if you attempt to raise the piston when the engine is cold you will find that a lot of force is required to move the piston to its uppermost position. When you release the piston, it will drop to the bridge quickly (less than half a second).

THE FUEL METERING NEEDLE & JETS

There are literally hundreds of needles available for the SU carburettor, the majority with profiles manufactured to suit particular vehicle engine systems. There are two types of needles available for the SU carburettor, biased and unbiased.

The biased needle has a collar and spring attached to the top of the needle. To eliminate droplets of fuel forming on the needle it is located within a bushing located in the underside of the piston. The needle is a loose fit within the bush and is loaded by the spring in a downward direction causing the needle to lightly contact the side of the jet. All anti-pollution SU carburettors are supplied with biased needles.

Figure 3 shows the two needle types, the biased needle being the one on the right.

The first two dimensions (1 and 2) govern the idling mixture. The next five dimensions: 3 to 7 govern the pick up in fourth gear, from 30 to 70 kph (approx 20 to 40mph). A cruising speed of 60kph (35 mph) will lie somewhere around the fourth dimension, a cruising speed of 80kph (50mph) will occur around the sixth dimension. The dimensions from 8 to 13 affect top end rev range of the engine. The last 3 dimensions, with 13/4” diameter carburettors, do not actually take part in the fuel metering process.

According to the S.U. Fuel Systems Catalogue, the following needles are the most suited to the 240 and 260z:

OTHER FACTORS AFFECTING THE TUNING OF THE ENGINE/CARBURETTOR

An alteration in the size of the carburettor should only be necessary if the breathing capacity of the engine has been altered substantially. This situation may be necessary if larger inlet valves are utilised simultaneously with alterations to the head, ports and cam, and/or an increase in engine capacity.

I installed a camshaft that gave additional lift to the valves and increased the inlet and exhaust valve duration.

I found that the red springs were inadequate, that is the engine speed would never reach redline, and the bottom end power was inadequate. At around 5500 rpm, it would miss-fire excessively.

I then uprated to yellow piston springs and found that the engine would rev to around 6500 rpm and then start miss firing. The bottom end power increased somewhat.

By finally changing to green springs the engine revs out well past red-line, about 95kmh (or about 60mph) in first gear (I have a 3.36 diff and a 2.9 first gear), and has plenty of bottom end torque. I did not need to change the fuel needles.

I don't have access to a chassis dynamometer so, the spring changes were done by trial & error. Had I made other internal changes to the heads, ie. larger valves or lumpier cam, I may have needed to replace my existing carburettors with twin 2” or triple 1³/₄” carbs.

AIR FLOW INTO THE CARBURETTOR

The medium, through which the air passes and enters the carburettor throat, can greatly affect the air fuel mixture entering the engine.

Air filters tend to reduce the airflow and lean out the air/fuel mixture entering the engine. The density of the filter element reduces airflow.

There are two types of filter element available on the market at present. These are the paper element type and the oil impregnated foam element type. Each of these have advantages and disadvantages.

Paper element filters tend to give less air flow restriction for a given element surface area but can allow more micro-fine dust particles into the carburettor which can build up inside over extended periods of time.

Oil impregnated filters tend to give greater air flow restriction for a given element surface area but are better at filtering out the smaller dust particles, provided of course they are maintained properly. Therefore, by fitting an oil-impregnated filter to the inlet side of the carburettor it may be necessary to enrich the mixture to accommodate the change of filter type.

Ram pipes, also known as ram tubes or velocity stacks, are horn shaped devices that can be fitted to the inlet side of the carburettor to improve the performance of the engine.



Figure 4: The length and shape of the ram pipe determines the rev range over which the engine's power curve is affected. More air flows into the engine due to the following factors:

§ the difference in the cross-sectional areas at the inlet and engine side of the ram pipe;

§ the cross-sectional shape (taken along the centre axis of the ram pipe);

§ inertia of the air entering the ram pipe (hot day = air less dense = less air into engine, cold day = air more dense = more air into engine).

The following explanation refers to Figure 4.

Air is passing through the ram pipe into the carburettor and into the engine. A vacuum is present at the mouth of the ram pipe when compared with the surrounding atmospheric pressure. The air velocity at the mouth of the ram pipe is low when compared with the air velocity at the mouth of the carburettor. The volume of air available at the mouth of the ram pipe is large when compared with the nominal throat diameter of the carburettor.

As air is drawn into the engine, it passes from the mouth of the ram pipe (A) into the carburettor its velocity increases due to the reduction in pipe diameter. The air also has more inertia due to this increase in velocity. The inertia of the air passing through the carburettor enables an increase in engine performance.

Air is compressible, so let's consider what happens within a theoretical cylinder inside an engine as the piston reaches the bottom of the inlet stroke and the inlet valve closes. The engine is fitted with a carburettor only.

Say the volume of the cylinder is 250cc. Ignoring friction losses, a normally aspirated cylinder without a ram pipe will suck in 250cc of air/fuel mixture, depending on valve timing. The air/fuel mixture within the cylinder at the bottom of its stroke will have a density approximately equal to the atmosphere surrounding the engine. The inlet valve then closes and the air/fuel within the cylinder compresses and becomes denser as the piston rises in the cylinder.

Consider the same cylinder/engine under the effects of a carburettor fitted with a ram pipe. The higher velocity air/fuel mixture also has greater inertia, as the piston reaches the bottom of its intake stroke and before the inlet valve closes a greater amount of air/fuel will enter the cylinder. This may only be a couple of cubic centimetres (cc's).

Effectively the higher velocity air, due to its inertia, is pushing more air/fuel into the cylinder, creating a Ram Effect. As the inlet valve closes the additional volume of air/fuel is trapped within the cylinder. With more air/fuel mixture in the cylinder, the engine develops more power. I have fitted a pair of two-inch ram pipes to my Zed and have found that the vehicle's performance improved over the rev range 3000 to 4500rpm.

Last Updated March 1, 2004

THE SU VARIABLE CHOKE CARBURETTOR

Introduction

PRINCIPLES OF OPERATION

PISTON SPRING

DASH POT/DAMPER ASSEMBLY

What type of oil should be used?

THE FUEL METERING NEEDLE & JETS

OTHER FACTORS AFFECTING THE TUNING OF THE ENGINE/CARBURETTOR

SIZE OF CARBURETTOR

AIR FLOW INTO THE CARBURETTOR

Ram Pipes

MULTIPLE CARBURETTORS

§ The 240 and 260Z firing order is 1, 5, 3, 6, 2, 4