SU Carburetors Explained | (2022)

Fig 2bbutterfly open

SU Carburetors Explained | (2)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.


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 ismaximum 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:

SU Carburettor Springs

Blue2.5 ounce
Red4.5 ounce
Yellow8 ounce
Green12 ounce

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 1
3/4” SU carburettors are bolted onto the inlet manifold of an unmodified L24 or L26 engine.

SU Carburetors Explained | (3)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. Thevalveand oil work such that they impede the lifting of the piston, but allow it to fall rapidly once the speed of the enginedecreases.

(Video) SU Carburettor Explained - Everything you need to know (and possibly more)

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 openedmore,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.

SU Carburetors Explained | (4)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).

SU Carburetors Explained | (5)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,biasedandunbiased.

The biased needle has a collar and spring attached to the top of the needle.To eliminate droplets of fuel forming on the needle itis located withina 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.

The needle profiles are measured at 3mm (1/8inch) intervals along the centre axis as indicated in Figure 3.

(Video) SU carburetor choke mysteries explained

The 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 1
3/4” 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:









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.

(Video) SU carburettors, what you should know before tuning

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.

SU Carburetors Explained | (9)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

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.

Fig 4 Ram Pipes

SU Carburetors Explained | (11)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 toFigure 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.

(Video) SU carburettor Baseline & Initial Adjustments

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 aRam 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.

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.

SU Carburetors Explained | (12)MULTIPLE CARBURETTORS

§Why do the variable choke carburettors connected to the Zed engines have a pipe linking the inlet manifolds?…

This pipe connects the inlet manifolds to minimise air pulsations through the carburettors caused by engine cylinder firing order.

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

Fig 5

SU Carburetors Explained | (14)The timing diagrams inFigure 5depict (approximately) the air pulses through the front & rear three cylinders of a theoretical engine six cylinder engine with no balance pipe installed.

Without 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.

: With a balance pipe installed between the two inlet manifolds piston fluctuation will be minimal for both carburettors.

Zed owners should not be concerned
aboutthe pulsation effectbecause if the engineisidling 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.Higher engine speed therefore reduces the pulse effect.

(Video) Side Draft Carburetors - How They Work

©2001Mal 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


SU Carburetors Explained |? ›

SU carburetor is an example of a constant vacuum type of carburetor. It consists of a single jet in which a tapered needle operates. The area of the throat is varied by means of a piston that slides up and down. The tapered needle is connected to the accelerator.

How do SU carbs work? ›

SU carburettors featured a variable venturi controlled by a piston. This piston has a tapered, conical metering rod (usually referred to as a "needle") that fits inside an orifice ("jet") which admits fuel into the airstream passing through the carburettor.

What does Su mean in carburetor? ›

The initials S.U. stand for Skinners Union. From these beginnings the S.U. Company grew into one of the major manufacturers of car fuel systems and components in Europe, its carburetters being widely used on both touring and competition vehicles. S.U.

How do you identify SU carbs? ›

The easiest way of course to identify one is if you have the identification tags which are on the carburetors and usually they will be on this case a HIF44 usually up on the dash pot or on an HS4, one of these, or on the side bolt. Usually the earlier ones are the small triangular-shaped ones.

How do you set the mixture on a SU carb? ›

To correct the mixture, move the jet adjuster nut one hexagon flat at a time. Screwing it up - anti-clockwise as seen from above - makes the mixture weaker. Screwing it down - clockwise makes the mixture richer.

Are SU carbs any good? ›

The SU's are great carbs if you understand them and are good at carb set up. That being said, I love my Holley carb and edlebrock intake. You couldn't pay me enough to get me to take the time on the SU set up to get it running right. I had SU's on MGB as well it now has a weber, huge improvement.

Who made SU carbs? ›

6. S.U. Carburettor, made and sectioned by George Wailes & Co, 1908. While carburettors were being produced, fitted and tuned under Carl's general directions, Herbert Skinner was busy with questions of patents.

Do SU carbs have an accelerator pump? ›

The 'accelerator pump' of the SU. Damper oil temperature fine-tunes control of this. Its high cold viscosity slows the piston further causing greater enrichment when cold, easing as it gets warmer.

What oil do you put in SU carbs? ›

The first is SU Damper Oil, which is a straight 20 grade and comes in a handy 125ml bottle. This oil is for use where the carburetter is fitted with a damper inside the piston and can be used on all SU, Zenith and CD carburetters.

What is the best type of carburetor? ›

Burnett (Holley): The classic 750 Double Pumper is the hands-down choice here. It was made for a combination like this—even if the vehicle is heavier than 3,100 pounds. Smith (Edelbrock): One of Edelbrock's Performer Series 800-cfm carbs is perfect for a mild big-block.

What are the 6 carburetor systems? ›

To provide for engine operation under various loads and at different engine speeds, each carburetor has six systems:
  • Main metering.
  • Idling.
  • Accelerating.
  • Mixture control.
  • Idle cutoff.
  • Power enrichment or economizer.

Which carburetors is used for high speed engine? ›

The downdraught carburetor is placed at a higher level than the inlet manifold and as a result, in this type of carburetor, the air and the mixture flow a downward course. In such carburetors, the mixing tube and throat are largely resulting the engine speed to be high even when the vehicle is running at a high speed.

How does choke work on SU carb? ›

The principle of the variable choke carburetter is to employ a means whereby the effective choke orifice will expand as the demand increases, and contract when the demand diminishes. Such a variation in choke area will achieve a constant air velocity and depression over the jet.

What oil do you put in SU carbs? ›

The first is SU Damper Oil, which is a straight 20 grade and comes in a handy 125ml bottle. This oil is for use where the carburetter is fitted with a damper inside the piston and can be used on all SU, Zenith and CD carburetters.

Do SU carbs have an accelerator pump? ›

The 'accelerator pump' of the SU. Damper oil temperature fine-tunes control of this. Its high cold viscosity slows the piston further causing greater enrichment when cold, easing as it gets warmer.

Does carb cleaner spray work? ›

Clean-R-Carb - How To Use Our Carburettor Cleaner Spray - YouTube


1. 342 MG Tech | Fitting and Tuning H Type SU Carburetors on Your MG
2. SU carburettor tuning - Part 1 - The Basics
3. S U Carburettor
(Bharath Raj)
4. S U Carburettor
(All About Mechanical Engineering)
5. Origin of S U carburetor explained by Edd China
(Henry Maynard)
6. How to Set Up and Tune SU Carbs in Easy Steps
(Resto Nation)

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