March 1, 2004
SU VARIABLE CHOKE CARBURETTOR
Principals of Operation
Article & Illustrations by
© 2004 Mal Land & ZParts.Com
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.
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.
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.
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.
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
The component parts, which together, form the variable choke carburettor
are detailed in Figure 1 and defined below:
Variable Choke Carburettor
and Flow Paths
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
5. Suction chamber air inlet (air at atmospheric pressure)
7. Dashpot piston damper assembly
8. Fuel needle
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).
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.
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).
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
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.
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.
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
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.
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,
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?
Remember the volume of air the engine draws is proportional to its speed.
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.
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:
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
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
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.
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.
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.
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.
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.
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).
FUEL METERING NEEDLE & JETS
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
shows the two needle types, the biased needle being the one on the right.
needle profiles are measured at 3mm (1/8 inch) intervals along the centre axis as indicated in Figure 3.
vehicle speeds given below are a generalisation and assume that
the throat area of the carburettor will not restrict the airflow
therefore affecting the volumetric requirements of the engine (piston
fully open at max brake horsepower)
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
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
SIZE OF CARBURETTOR
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
installed a camshaft that gave additional lift to the valves and increased
the inlet and exhaust valve duration.
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”
AIR FLOW INTO THE CARBURETTOR
medium, through which the air passes and enters the carburettor throat,
can greatly affect the air fuel mixture entering the engine.
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.
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
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
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
You will probably need to readjust the engine idle speed to accommodate
the change in air fuel mixture. Once completed the improved performance
characteristics of your vehicle should be noticeable under acceleration.
Why do the
variable choke carburettors connected to the Zed engines have a pipe linking
the inlet manifolds?…
pipe connects the inlet manifolds to minimise air pulsations through the
carburettors caused by engine cylinder firing order.
240 and 260Z firing order is 1, 5, 3, 6, 2, 4
The timing diagrams in Figure 5 depict (approximately)
the air pulses through the front & rear three cylinders of a theoretical
engine six cylinder engine with no balance pipe installed.
the balance pipe the piston follows the fluctuations in manifold vacuum
in turn affecting the air/fuel mixture feeding the respective cylinders.
This situation is more noticeable at low engine speeds
The balance pipe enables the presence of a continuous vacuum in the inlet
manifold reducing the pulsation effect caused by the opening and closing
of the inlet valves.
Note: With a balance pipe installed between the two inlet manifolds
piston fluctuation will be minimal for both carburettors.
Zed owners should not be concerned
about the pulsation effect
because if the engine is idling at 750 RPM each inlet valve opens/closes
350 times/minute, or 5.83 times/second. There will be 35 pulses per second
in a six-cylinder engine. At 3000 RPM the pulse rate will be 140pps.
engine speed therefore reduces the pulse effect.
©2001 Mal Land - The reproduction
of this document (on any form of media), without written permission of
the author, will incur legal action.
§ Tuning S.U. Carburettors 4th edition
by G.R. Wade
Published by Speed Sport
Depression = Vacuum