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SU Caby Tune


This section does not go into how SU carbs actually work but I would very much recommend reading the following which beautifully illustrates how these carbs operate.


Before we get into the detail, it is worth saying something about a few myths and legends.


· "SU Carbs go off over time ..." actually not true unless the carb has worn spindles for example. The main reason that an SU carb appears to go off tune is because the engine condition changes or there has been a change in the fuel used, maintenance is poor or the temptation to fiddle has not been resisted.


· "Don’t use dashpot oil / no dampers / no piston springs" The SU carb is a precision instrument and every part of it is present for a specific and important reason. For example, the spring ensures that the piston is pushed downwards against the upward force on the piston of atmospheric pressure. This keeps the piston at a specific height exposing a specific section of the needle. Dashpot oil has a number of functions including slowing the upward movement of the piston (as the vacuum depression by the engine increases with engine speed), temporarily increasing the richness of the mixture. It also dampens out what would otherwise be a constantly oscillating piston. Like nature, each item on an SU carb is there for a reason


· "See what others recommend as the right needle for your car" True if the car is unmodified but so very few cars of 40 plus years old will be in the standard factory specification. By all means ask what needles people with a similar set up are using but then work logically from that needle to the needle you need for your setup (more details below).


· "if you want more power and torque, go for Webers" Whilst Webers may ‘look the part’ they cost a lot more than twin SUs and are a lot more complicated to set up. I have read that a well set up SU is more than a match for most Weber set ups.


First check what parts your carbs are using. A surprising number of carbs are modified over the years or use non-standard parts, to the blissful ignorance of their current owner.

MGCs had twin HS6 SU carbs which are:

. 1 ¾ inch diameter butterfly


· 0.100inch diameter main jet


· ST needle as standard

Original carb specification for the MGC was:


· UK - 1967/1968 Specification


AUD150F (front)


AUD150R (rear)


Needles – ST (Rich SQ, Weak CIW)


Spring - Yellow


· UK – 1969 Specification


AUD341F (front)


AUD341R (rear)


Needles – ST


Spring - Yellow


· US – 1967/1968 Specification


AUD287F (front)


AUD287R (rear)


Needles – KM


Spring - Yellow


· US – 1969 Specification


AUD342F (front)


AUD342R (rear)


Needles – BAD


Spring - Yellow

A number of methods exist for tuning SU Carbs with various levels of complexity. This section concentrates on the HS Type carb found on MGC but the principles are equally applicable on all SU carbs.


I can’t stress this enough. Before even going near your carbs make sure that you have checked at least the following:

. A good spark reaching the spark plugs (remove one and hold it tight against the engine block with something to insulate you from the spark)


· Spark plug gaps are set correctly


· Ignition timing is set appropriate to the engine and level of modification. A hotter cam may require greater advance than a standard cam. By way of example a Kent AH2 mild road cam at 278 degrees requires a static timing advance of around 14 degrees in my experience. Make sure you are using static timing figures for static timing and not static figures for strobe timing.


· Points are not pitted. If using electronic ignition, this will not be an issue of course.


· Distributor cap is in good condition – that is that the contact surfaces inside the cap are not pitted or black. Also check that the cap itself is clean inside to avoid conduction across the surface.


· Ignition leads – check these are fully home and not cracked or damaged.


· Vacuum line – make sure this is attached to the distributor (if applicable) and that the servo take off is tight and the rubber is not perished (leading to leaks).


· Gaskets – it is not unknown for these to be put on upside down which means that the vacuum transfer holes (which allows atmospheric pressure to push the piston up to its correct height) at the front of the carbs are blocked by the gasket. This will ruin all attempts to properly tune the carbs.


· Valve clearances are correct


I always recommend changing the fuel filter (if you have one) and air filter elements before tuning. If you have K&N’s give them a wash in the recommended fluid. If you have time, I would also clean out the fuel reservoirs.


Nothing will mess up all attempts to tune a set of HS carbs like an air leak. Make sure that all of the gaskets are in good condition and all bolts and nuts are tight but not over-tight.

And if there is even a hint of sloppiness in the spindles then replace these and rebush if necessary. Worn spindles will make setting idle impossible. You will set it to 800rpm, blip the throttle and it will then be at 55rpm or 1300rpm. I would also check that the throttle linkages all work and work smoothly.


One which is easy to miss. The level of fuel in the fuel reservoir dictates the level of the fuel in the jet (just like the water at two ends of a hose pipe full of water will be at the same level). If one is higher than the other, tuning will become confusing and difficult, as effectively different parts of each needle are exposed.

With the float assembly resting on the needle valve (upturned float chamber lid), the gap between the float lever or the float itself and the rim of the float chamber lid should be between 0.126 and 0.189 inch (3.2mm to 4.8mm) and ideally the same on each carb. With a steel float it is possible to carefully bend the steel float arm to set this. With a plastic float, it is necessary to add or remove washers from under the needle seat.


Remove the three small screws holding the dashpot to the body of the carb. Be careful not to lose the screws as they are almost impossible to find in your engine bay. I also lay them out in the same relative position on the bench.

The dashpot can be removed by hand but take care to hold the piston inside it and lift the whole assembly out vertically, so as not to bend the needle. Note how there is only one way for each dashpot to go (one of the screw holes dictates this) and make sure you know which is the front and which is the rear carb dashpot, as well as which piston goes with what dashpot (by convention the front carb is the carb nearest the radiator). I tend to rest the piston, with the dashpots over them, on the edge of the opening of a long glass coffee jar (long enough so that the needle is not resting on the bottom of the jar).

Clean the piston carefully and clean it right to the top inside. Don’t whatever you do use wire wool or a wire brush and any sandpaper OR ANY OTHER ABRASIVE MATERIAL. SU pistons and dashpots (also known as suction chambers) are matched assemblies with a defined and specific clearance between the piston and the suction chamber. Anything abrasive will ruin this and make tuning impossible. I use a degreaser followed by Brasso to do this. Ensure everything is meticulously clean and dry and then lightly coat the rod in the centre of the piston with WD40.

Don’t stretch the spring inside the piston and make sure you put it back on when reassembling (easy to forget). With both pistons cleaned, reassemble and tighten the small screws gently and evenly. The piston locates in only one orientation inside the carb due to the groove mating with a corresponding section on the piston. The piston should fall freely within the dashpot. If you tighten the dashpot screws too much or unevenly, then the piston may get stuck.

Having removed the air filters, it is possible to see the end of the piston through the front facing air inlet hole. With the engine turned off, gently insert a finger into each and pull the piston upwards to the same height, holding both pistons with separate hands. Release each piston at the same time. As the piston hits the bridge inside the carb at the end of its travel it will make a dull clunk.

If both pistons hit the bridge at the same time, this is perfect. If the pistons hit within a fraction of a second of each other, this is also probably okay. More than a second and I would investigate why they drop at different rates. Below sets out what to do if the pistons are not falling in sync.


One I would recommend doing. Holding the piston carefully, look at the needle from all angles to ascertain whether it is bent. On some HS carbs including the HS6, the needle is rear biased which means that the needle is angled towards the rear of the carb, in effect pointing towards the engine.

The picture below shows a spring loaded needle on the right and a fixed needle on the left (the reason for the numbered ‘stations’ is explained below). The difference between these types is that a fixed needle is solidly fixed into the piston via a screw whereas spring loaded (often called biased) needles are held in place within the piston by a collar and a spring and can move by virtue of the spring.




· Fixed needle – needs to be precisely centred within the jet to within a few thousands of an inch. If it is not centred or is bent, the needle will rub / bind against the side wall of the jet, wearing the needle and impairing performance if it sticks. To centre a jet, remove the dash pot and piston and adjust the jet so that, when looking inside the carb, it is flush with the top of the bridge (the bridge is what the piston falls on to at its lowest point). Refit the dash top and piston and let the piston fall slowly on to the bridge. If the piston sticks just before the bridge, the needle is not centred and is fouling on the main jet. Loosen the large nut under the carb, which allows the jet to move, with the piston sitting now on the bridge re-tighten slightly, lift the piston up and allow to drop again. Repeat this until the piston drops easily then tighten the nut fully. Check again to make sure that the piston drops positively on to the bridge, then adjust the jets to a starting point (see below)


· Spring Loaded needle – around the time of the MGC, SU came up with a new mounting style for needles specifically a soft-mounted spring-loaded needle tipped a few degrees to one side which forces contact with the jet wall but only a light contact without binding. The reason for all this was emissions. A binding and worn fixed needle leads to poor emissions. A biased needle will also lead to wear but only over time. In effect, all biased needle set ups wear but this was supposed to be better than a few very bad emissions from a few fixed needle carbs. With what were seen at the time as limited lives and servicing, the biased carbs was thought to be a reasonable step forward on total emissions.

Important – if sourcing replacement parts the jet bearing for fixed and biased needles are different (the neck of the jet for a fixed needle is smaller in diameter to allow for centre-ing). When inserting a needle into the piston, whatever type you are using, the needle should sit in the cylinder with the base of needle flush with the base / face of the cylinder. Reinsert the piston carefully into the carb, making sure that you don’t bend the needle.



What a lot there is written about this issue!

The oil in the dashpot is needed to restrict the upwards motion of the piston under acceleration so that the mixture becomes enriched. It is also dampens out fluctuations in the movement of the piston that would otherwise occur at low engine revs (the piston would move around constantly ‘fretting’ as the air fluctuated).

Whatever story you believe on what is best, the presence of the oil and the damper does restrict the upwards movement of the piston which in turn does mean the mixture is temporarily richer (higher velocity air movement over the jet). It also means that the mixture is not too lean as the engine vacuum increases as the throttle is opened.


I would not recommend water but the options include:


· 20W50 (in the manual)


· A mixture of 20W50 and sewing machine oil (3 engine oil to 1 sewing oil)


· Hydraulic lift oil


· Automatic Transmission Fluid (again not sure why people recommend this)


Again from what you read on the net, you would think that these springs did nothing and had no effect. They do.




· Springs hold the dashpot "down" against the vacuum. The strength of the springs will determine where the pistons sits at a given level of vacuum from the engine.


· Even more important than the strength of the spring is whether it is equal to the spring in the other carb. If the springs are different (or old such that they have different strengths now), the piston will be "held" at different positions in each carb meaning that each carb will be showing a different part of the needle profile at a given engine vaccum.


· The paint colour on the end of the spring should tell you which spring you have but may well have worn off over time.


· If you want to check a spring performance or compare two springs, then using a light-weight set of scales and a piece of cardboard folded along its length and inserted into the middle of the spring, compress the spring to that length and then read the associated spring weight on the scales. Standard springs were yellow. If damaged, replace (and I would recommend doing so as a pair for the above reason).


Yellow 8oz (load) 2.750 (length inches) (Part AUC 1167)


Light Blue 2.5oz 2.625 (Part AUC 4587)


Red 4.5oz 2.635 (Part AUC 4387)


· If you want to look at using different types of spring for modified engines, I would recommend buying "The SU Carburettor High-Performance Manual" by Des Hammill. This is a great book with a wealth of detail you can’t easily find elsewhere.


There are a couple of ways of doing this.

The first is to adjust the jets so that they are just flush with the top of the jet housing (with piston removed) and then turn the adjusting nut 12 ‘flats’ clockwise. It is important to do exactly the same on the other carb, so as to keep the mixture adjustment in sync.

The second is to use a vernier measurement gauge to adjust the jet so that the main jet on both carbs are 0.050 inch / 1.25mm down from the top of the bridge within the carburettor housing. The engine will start although these settings are very unlikely to be correct. The advantage of this method is that you know the jets are in sync. It is also close enough to be able to start the engine.

Now run the engine until it reaches normal operating temperatures.


Simply this means that each carb is able to act independently of the other. It is important to adjust each carb’s airflow whilst they are not linked in any way.

Now some people will tell you that you only need to undo one of the nuts / clamping bolts at one end and that there is no need to undo both ends. This is true if you are just trying to balance the air flow at idle. But once the accelerator is pressed and the throttle linkages act so that the carb comes off idle, it is important that each carb picks up equally. That means undoing both side’s nuts and setting them precisely and in sync.


With the engine up to operating temperature, now set the air flow so that the flow is equal through both carbs at idle. There are a number of ways of doing this, all relying on turning the idle screw clockwise to adjust the airflow, achieve a balanced airflow at each carb and a sensible tickover speed. First back off the throttle stop adjusting screws so that they are both just touching their respective stops and then turn each of them exactly 1 and and a ½ full turns. Then:


· By ear using a piece of pipe placed just on the lip of the carburettor edge next to the visible end of the piston , move back and forth on each carb, compare the ‘hissing’ sound and adjust until the sound is the same.


· Use a cheap tool and get so frustrated with it that you buy a more expensive one.


· Use a tool like Unisys or Motometer to accurately balance the air flow into each carb (the middle section rotates to allow the air flow into the glass tube to be adjusted). Swapping back and forth between carbs is very quick and easy.


To adjust the flow, turn the idle screw clockwise in small increments and be sure to open / blip the throttle to ensure that it finds the new level.

Once the carbs are in balance, set the gap between the pegs on the ends of the linkage between both carbs until there is an equal amount of free play on each side – typically around 1/16th of an inch (0.15mm). I do this using a feeler gauge inserted in between the peg and the linkage and then tighten all bolts and nuts. Then check that when you pull the accelerator cable each side picks up at exactly the same time. My experience is that this takes a few times to get right.

It is also worth checking that the pegs are far enough in. This sounds odd but it is possible to have the pegs too far towards the middle of the space between the carbs meaning that when you brake or accelerate, one peg no longer operates the mechanism.


Balancing the above airflow only checks that the air flow is in sync at idle.

It is also worth checking to see if the pistons rise at an equal rate and do not stick or otherwise travel unequally as the engine revs rise. It is possible to use a homemade tool to do this but I would recommend the cheap but effective SU Toolkit which consists of two rods that are inserted where the (removed) dampers used to be with two perpendicular lengths of thin wire which act as pointers. As the engine revs rise, the pointers should rise equally; as the revs fall they should fall equally.

If it doesn’t rise and fall freely, this may be either that the jet is not centred correctly, the dashpot or piston needs cleaning or the suction chamber screws are not equally tight, assuming of course that the pick up by the accelerator mechanism is in sync as noted above. Just possibly you or someone might have swapped the pistons over (they are made to match the dashpot) in which case it might be worth trying them the other way around. Also check that your needles are not bent. (Note that the shaft of the SU Tool can also be used to check the gap in the fuel chambers between the hinged level and the underside face of the fuel chamber lid.)


Looking down from the top of each carb, turn the adjusting nut on the underside of the HS6 clockwise to richen the mixture (jet moves down exposing a thinner part of the needle) or anticlockwise to weaken the mixture.

Again there are a number of ways of checking the adjustment of the mixture at tickover. The principle used here and for all of these and the standard tuning methods is the implicit assumption that if the mixture is adjusted correctly at tickover, then it will automatically mean that the mixture is correct across the whole rev range. This is true if the engine is standard and unmodified and using the standard recommended needles.

So by way of example, using ST needles in an unmodified C engine with original air filters and adjusting the mixture to be correct at idle should mean that the mixture is correct right the way to the rev limit. Obviously where you have made mods such as, in my case, Kent AH2 cam, K&Ns, unleaded head, fast flow exhaust, modified inlet manifold, 123 ignition, the original standard needles will not be the best needles for my set up, even if they can be made to idle nicely.


. The ‘By ear’ method. Sounds imprecise but in the hands of an experienced tuner, often very close to perfect.


· ‘The method in the manuals’ – where the mixture is adjusted until the engine RPM just begins to drop. If you did the 12 flats clockwise originally, then adjust by turning anticlockwise (weaken) until the engine speed just starts to drop. Then turn clockwise until engine speed starts to rise, followed by one more "flat" (on the nut) clockwise. Then, using the lift pin on the underside of the dashpot mounting flange, press up until you feel contact with the piston and then lift slightly.


· If the engine RPM increases when the pin is lifted, and remains elevated, the mixture is rich – weaken the mixture by turning the adjusting nut anticlockwise.


· If the engine RPM drops when the pin is lifted your mixture is too lean - richen the mixture by turning the adjusting nut clockwise.


· If the engine RPM initially raises as the pin is lifted and then settles back to the original RPM or very slightly above, the mixture is right.


· ‘Tailpipe Emissions’ – it is possible to use emissions testing equipment to adjust the mixture. If you have one exhaust pipe at the rear then obviously by the time the exhaust gases reach this point they will have mixed completely. So you could have a front carb very rich and a rear carb very lean and it appear to be correct. The way to avoid this is to keep them in sync. Don’t turn the front carb without turning the rear carb the same amount (if you have twin tail pipes work out which one relates to which carb). The ideal air fuel ratio is between 12.8(4.3% CO):1 and 12.1 (6.3%CO):1 but at tickover it is recommended to go leaner than this i.e. the leaner the better for the engine whilst this allows for a smooth even idle. With only a visual test for emissions, the level of CO emissions is not important for the MOT but it is for your bank balance (wasting unburnt fuel) and the environment:


· 2.5% CO – Lower end of ‘ideal’at tickover


· 3.5% CO – Upper end of ‘ideal’ at tickover


· ‘O2 Sensor’ – See below for more details


With the mixture adjusted correctly, the tickover may need to be adjusted. Again, it is important to undo all of the bolts and to turn each idle screw exactly the same amount to keep the carbs in sync.


Take the car for a test drive and if necessary adjust the mixture. Cs have a tendency to like to run slightly rich.


Pull the choke until the jets are just at the point of moving but have not moved. Screw both fast idle screws in until they just touch the fast idle cam. Alternately turn each fast idle screw until the idle speed is increased to 1,000 RPM with the engine warm.




The huge difference here is that you are no longer simply tuning at tickover. By mounting an onboard Air / Fuel monitor , I would of course recommend only doing this in such a way as to be able to remove all wiring once tuned i.e. temporary mounted), it is possible to see the air fuel (AF) ratio across the entire engine rev range. These work by using an O2 sensor inserted into a welded in boss in your exhaust pipe connected to a monitor. ‘Wideband’ monitors are the only way to go. It is also possible to wire two sensors and switch between them (or just use one and swap it over).


Maximum power for a petrol engine is in the range of 12.1:1 to 12.8:1 Air to Fuel (AF) ratio, depending of course on your engine, its mileage, the cam, the exhaust etc. Typically a C likes to run slightly rich.


By measuring the AF ratio whilst driving the car at various revs and engine loads, it is possible to build up a picture of whether the engine just needs adjusting but has the right needle or whether a needle of a different profile is required.


For example, a particular needle may give 2.5% CO at 800rpm, an AF of 12.8 at 2500rpm but an AF of 11.5 at 4000rpm indicating that a needle with a thinner profile at that section of the needle is required (i.e. the section that "equates" to 4000rpm).


A thinner needle (smaller diameter) will allow more fuel through (richer); a thicker needle (larger diameter) restricts fuel from getting through (leaner).


All SU needles have one end which is thicker than the other i.e. they taper. And that taper is specific to the particular needle. There are for example 354 needles, all using a three letter code, just for the .090 jet and all of these needles have different tapers.


Each needle is divided into what are called ‘stations’. Each station appears at 1/8th inch further down the needle with station number 1 being the thickest and 12 (or higher, up to 16) being the thinnest part of the needle. Station 1 & 2 equate to the needle thickness at "Idle". See above needle diagram.


Obviously as the main jet is wound downwards below the level of the bridge, the station on the needle that is ‘seen’ by the carburettor also moves down i.e. becomes perhaps Station 2 and also as a consequence a Station further down the needle comes into play and is also ‘seen’ by the carburettor. The important point is to establish which stations on the needle are actually ‘seen’ by the carb. It may be Station 1 to 12 or 2 to 13 or 2 to 14 etc.


Recall that the original SU needle for the MGC was ST as standard, rich SQ, weak CIW in UK:




Station 1 .100 .099 .099 .099


Station 2 .095 .095 .0955 .0955


Station 3 .0915 .0925 .093 .0924


Station 4 .088 .0895 .0905 .0893


Station 5 .0854 .087 .0875 .0865


Station 6 .083 .0852 .0856 .0834


Station 7 .0812 .0831 .0836 .079


Station 8 .0794 .0805 .0819 .0774


Station 9 .0775 .0787 .080 .0743


Station 10 .0757 .077 .078 .071


Station 11 .0738 .0753 .076 .068


Station 12 .0719 .0737 .0752 .065


Station 13 .070 .071 .0746 .062


Station 14 .068 .069 .074 .059


Station 15 - - - .056


Station 16 - - - .053


My C currently has BDL needles in it but with a slightly modified profile. That is of course specific to my engine and its characteristics. If you do intend to change your needle there are a few points to note:




· Each needle you use in the 0.100 inch jet must have 0.099 as the first station number. This limits the vast number of needles you need to consider.


· When choosing a needle bear in mind that, even though there are hundreds of needle profiles available off the shelf, all of these are for a specific application. Modified engines either by cam or manifold or in any way non-standard are unlikely to have an exact needle for your application. Again I would recommend buying "The SU Carburettor High-Performance Manual" by Des Hammill if you want to understand how to modify a needle. His books is excellent on this aspect with lots of practical tips and photos.


· Be logically about it. If you engine is too lean in the midrange, try a needle with a thinner profile at that point. And if it is still too lean, use this knowledge to influence your next needle choice. Put simply, don’t just randomly try needles but build on knowledge gained by the each needle you try.

All of the SU needles details are set out in the SU needle chart, allowing you to potentially find the needle you need. It is worth trying http://www.teglerizer.com/suneedledb/index.html to play with station numbers or you can try http://sucarbs.com/programs.html which is just great.

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Creation date: Aug 2, 2017 4:21 pm     Last modified date: Dec 26, 2018 12:04 pm   Last visit date: Nov 20, 2019 11:53 pm
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