Edited by T from original submissions by John (Arsewipe) November 19th 2006;
Some additional info and images by named contributors:
Submission by T Nov 21st 2006:
SU Carburettors (Skinners Union) were patented and designed by George Skinner in 1905.
The SU Carburettor was invented in an era when the ignition system was a car's greatest weakness. Unlike the uncompromising High Energy Ignitions of today that can spark under almost any circumstances, turn of the 20th Century cars had ignitions that were not capable of managing even the light changes in Throttle position which controlled the Manifold Pressure without missfiring occurring.
S.U. Tag AUD503
Other Mariner SU tags are as follows "added by Slayer208":
===SU Carburettor Identification:===
There are a Few types of SU carburettors that are available. H Series, HS Series, HD Series, HIF Series & CD Series "Stromberg". For a common Holden six, running either 2 or 3 of the HS6 is the most popular choice.
'H' Means Horizontal.
'S' Means Side-draught.
'D' Means Down-draught.
'CD' Means Constant Depression.
'E' After HIF44 means the carburettor is running on an Electric Choke Device.
'AED1' After HIF?? means the carburettor is running on an Auto Enrichment Device "AED".
'Th' Means the choke on the carburetter is thermally operated.
'HIF' Means Horizontal Intergral Float Chamber
Listed below is the common types of SU carburettors and their appropriate sizes:
Carburetter Type: Carburetter Size:
HIF38 38mm or near 1-1/2"
Added by slayer208 on 4/03/2010.
The Spark Plug Fouling that occurred as a result of the primitive oils in use and poor Piston Rings was yet another problem. The 1953 film Genevieve demonstrates this admirably.
During normal operation the more the Throttle opens, the more spark is needed to bridge the Spark Plug gap and fire the mixture. In the early days if the Throttle opened too rapidly the Voltage needed to spark the Spark Plugs and fire the charge could exceed the ignition's ability to provide the necessary spark. As a consequence a missfire, gag or flatspot would occur.
A Carburettor that could automatically react to the delicate response of those early ignition systems, by only feeding an increase in Manifold Pressure as the ignition system could respond, was needed and the SU filled this niche.
End of T's submission.
The SU and CD (Constant Depression) carbs basically work by sucking fuel out of the jet by the suction of air passing over it. The suction is determined by how much air flow is concentrated directly at the jet. The needles on both carbs vary the amount of fuel supplied (much like the rochesters) by the tapered needles sliding up and down inside the jet which varies the jet size.
===Comparison With a Fixed Choke Carburettor:===
Submission by T Nov 21st 2006:
Unlike an ordinary Fixed Choke Carburettor (like the normal downdraught Stromberg found on Grey and Red Holden Engines), Constant Depression Carburettors like SU's are sympathetic to weak ignition systems.
This sympathy occurs because the oil dampened piston gradually increases the manifold pressure only as the ignition system is able to fire each charge.
Under rapid acceleration, the slow rising of the piston generates a temporarily rich mixture since the restriction of airflow increases the fuel flow from the jet until the piston can rise to the new height. At the new height the air/fuel ratio becomes balanced again. This is the SU's way of simulating the function of the accelerator pump in a Fixed Choke Carburettor.
Since weak ignition needs a rich mixture to fire properly this process doubly aids in smoothing the Engine's acceleration and combatting Flat Spots.
Even though a Fixed Choke Carburettor interacts with the ignition through the Power Valve and by the degree of Main Jet exposed by the Throttle Butterfly, it largely expects the Engine to instantly fire any mixture sent to it.
As a consquence a Fixed Choke Carburettor can gag or have a flat spot under conditions that a CD Carburettor will not.
End of T's submission.
SU Animation by T Feb 22nd 2007:
===Fuel Pump and Fuel Pressure Regulator:===
Fuel Pressure Regulators Submitted by oinks308 on Tue, 17/05/2005 - 10:48.
The idea of the Pressure Regulator is to cut down the pressure fed to the SU's which run at a maximum pressure of 2.5psi. Anything over that normally forces the fuel past the needles and seats and then the fuel will flood from the bowls on the SU's. As I know it the Holden six fuel pumps operate at up to 5.5psi, which will guarantee flooding. The early Needles and Seats were brass but the newer neoprene type Needle and Seats do a far better job at stopping fuel from flooding, but only to a point. This is why it is essential you run a Fuel Pressure Regulator, even if you run the 1.5" or 2" SU's, they all run at the 2.5psi limit. Because the Regulator is a valve of sorts it will always allow fuel through and won't cause any damage to the Diaphragm in the fuel pump while the fuel is still flowing. I even run one on my 308 in my VK, reason is that I still have the 6 cylinder fuel lines fitted up so on a warm day it vapour locks, so beside the tank I have a 9psi Carter Electric Fuel Pump to keep a constant pressure to the mechanical 5.5psi L34 Fuel Pump. The Regulator is fitted beyond that so in the event that a Diaphragm may tear in the Mechanical Pump it wont bypass it and then pump 9 psi directly to the Rochester which doesn't use the return line back to the tank as the original VK 308's did. This is what concerned my mechanic. 9 psi would flood it, hence the Regulator set is at 5.5psi the same as the L34 pump.
A little complicated mind you, but it all works fine. Run SU's without a Regulator and major flooding with fuel pouring out of the bowls onto your exhaust/ extractors = an expensive lesson, this is why they were originally made... The different pressures I ran was just an experiment to see if I 'light footed' it could I gain any better economy? It appeared so, but whether that was because of the pressures ran back then or if one week I drove up more hills than the previous who knows? lol When young we do try some strange things lol. End of Oink's submission.
===Dash Pot Oil:===
The amount and viscosity of the Oil used in any SU is determined by the requirements the SU sees.
The Car manufacturer will specify this info.
Otherwise it will depend on the circumstances.
submission by John
PLEASE NOTE: Information supplied here given by me, is not entirely pure written law, it is my own experience and knowledge gained from others that has helped me along the way,, 'T' has done everyone a great service by compiling all of this information for us all to help with a complex, not easily understood tuning technique > balancing multiple carbs,, please treat it as such, helpful information, not pure written law,, thankyou,,,,:) Other than the oil in the dampers, there really are only three main adjustments that need to be made, throttle balance, idle speed and the fuel. Reference should be made at this point that the float levels need to be accurate to make correct adjustments to the fuel. Manufacturers specifications will describe the settings, but for ease, I generally turn the top housing of the float bowl (remove the three screws) upside down and set the top of the float so that it is approx. level with the housing, this adjustment is made using shims (soft alloy washers) under the seat (needle and seat). Before you start, make sure all your timing etc is correct and the car is at operating temperature. Next, you need to make sure that all carbs have the correct idle speed set, usually a special vacuum gauge is fitted to the throat of each carb to measure this, but since these are an extremely rare item you can get a very close adjustment by listening to the suction through the carb throat using a piece of hose pipe (hard to judge without years of experience) and listening to it. Factory set up's usually have a balance tube between each set of inlet ports to help even up any imbalance between carby settings. Next you need to ensure that all the throttles are opening at the same time, some factory fitments of multiple S.U's require that the throttles open at different intervals (throttle shaft wear can affect this too), and these would be mentioned in manufacture's specs. Each linkage between carbs will have an adjustment for this (set with clamps on the shafts) and this is VITAL for correct balance. This can be checked using feeler gauges but I find sight is the best measurement for this, especially when the linkages are worn. The final adjustment is fuel. On the base of the carb is a large nut (usually 5/8 in size) the correct way to tune the fuel is to remove all the suction chambers with pistons as well (MUST NOT BE MIXED UP). There are many different types of damper springs (normally different colours to donate different tensions) these should all be identical. There are also different jet sizes as well as needle sizes and lengths. These should be number stamped and jets are either stamped or different colours (ALL MUST BE IDENTICAL). With all the chambers removed next you adjust the jets up to have them all flush with the top of the jet housing in the carby. This gives you a level starting point for all carbs so they are equal (very easy to get lost in your tuning if you don't have a common zero point from which you start).
Normally the car won't idle with the jets wound up to the point at which they are now so start by turning each jet down '2' flats each. Now start the car and get it up to operating temperature (NOTE: a correctly tuned SU carb will not even start a cold car without a choke, let alone run while cold so be aware.
If you want the SU tuned correctly, then you WILL NEED A CHOKE. With the car warm you now need to assess what the idle will be like. If the car stalls then it will be too lean. Now each adjustment you make will need to be made to each carby indentically and just adjust the mixture one flat at a time. From now it is a slow painfull mix of adjusting the fuel and you may find that if your carby's were well out of adjustment that you might have to adjust idle speed as well. Sometimes you can be lucky and have a central adjustment for all three, but alas the norm is to adjust all 3 individually and use a balance gauge (or hose pipe) to match them up, painfull and slow but worth the effort. When you think you have mastered the tune you can now check it. On the side of each carb is a piston lifting pin, (located on the same side as the float bowl) With the car idling, lift the pin on the first carb and hold it. This raises the piston about 1/8 > 1/4 inch. If tuned correctly, the idle speed should rise slightly and then settle back down to normal. If it splutters or stalls, then it is too lean. If the revs rise and stay up - too rich. Now make your adjustments and do this for each carb, and keep note of how many turns you make so that you can have a base to start from next time you tune. Balancing multiple carbs is a time consuming and delicate practice that is quite hard to master, but to really know if your carbs require tuning, if you can start it without a choke, then they are way too rich. As for oil, generally manufacture's recommend engine oil, but I like Auto Transmission Fluid as it is a bit thicker than machine oil which some people use. If you want high revs from a high powered car engine oil will cause you to use more fuel. The basic principal of an S.U carb is that air drawn accross the jet draws fuel from it. If the piston can't rise enough then more suction is placed directly at the jet which will draw more fuel. Transmission fluid will allow the piston to rise quicker and give you a larger vollume of air/fuel mix, but it depends on what jets you are running as to whether or not you get the correct balance of air/fuel, v's power/economy.
===Further Mixture Adjustment:===
Submission by VHBunky Nov 29 2006:
The SU needle is fixed to the piston which when rises will let more fuel in yes, but its the actual jet in which it seats that determines fuel mixture at idle. Lowering the jet will actually richen the mixture and raising it will weaken it. When the choke is applied the jet slides down to enrich the fuel then back up once it's going. For getting the mixtures right at anything but idle through the rev range I think it really depends on the needle profile and varying taper across the whole needle's length. Buying different needles can be challenging. You may go through a few before you find a set that's perfect, and modifying them is way beyond my comfort level.
End of submission by VHBunky Nov 29 2006:
Submitted by Arsewipe on Tue, 28/11/2006 - 19:43.
There is no idle circuit as such, normal carbies have idle, cruise and power circuits, SU's have just one that varies for all it's needs. The mixture is adjusted by raising or lowering the jet, but this varies the mixture right across the entire range of the carby, not just at idle. The varying of fuel is controlled by the raising or lowering of the needle.
Chokes are very important for starting a cold correctly tuned SU, because you will not start a cold car without one, and yes they lower the jets, but this has the effect of raising the float level, by lowering the jet, it changes the point of depression and effectively floods the carby with fuel, (remember that the float level determines the level that the fuel sits in the jet, the higher the fuel level, the easier it is for the fuel to be sucked out of the jet and vice-versa). I have heard of guys machining their own needles, but this is just way over the top for me too.
Submitted by VhBunky on Tue, 28/11/2006 - 16:41.
The piston pin is used to lift the piston from the outside of the carb. The Piston Pin is lifted a specific amount (cant remember how much exactly), to allow you to determine whether your mixture is correct based on the rpm differences when lifted. It should be located just under the base of the piston chamber. Look for something that resembles a pin and gently push it up. If it moves I'd say you found it! End of submission by VhBunky on Tue, 28/11/2006 - 16:41.
Submitted by Arsewipe on Tue, 28/11/2006 - 19:43.
The piston lifting pins are located (as you look at the carby directly in front of you from the side of the car) under the base of the carby, on the same side as the float bowl, just behind the air cleaner.
End of submission by Arsewipe on Tue, 28/11/2006 - 19:43.
From memory, the needles are what differ greatly, not the jets, so the ones in the kit should be ok. From memory again the kit would only roughly contain a jet, o-ring for the jet hose seal, a needle and seat, a gasket for the fuel bowl, and a shim for the needle and seat, *These are important because they determine your float level*. Unless the needles are bent, or different from each other, they are useable. When setting the needles in the pistons, make sure that they are all set level with the bottom of the piston using a straight edge.
If you do a search through Old Holden you'll see that I have posted a few times; threads on setting up and tuning multiple SU's.
My very first car was an HR van with super hot 179HP and Tripple inch and 3/4 SU's ... So I learned how to tune them, then I spent many painful years working on and owning BMC, twin su's became my forte, soon everyone that knew me that had multiple carbs were knockin on my door, it is a fine specialty and normally you will need to fine tune SU's every few days, and constantly keep oil up to the dampers, (auto oil is the best BTW) ... I used to love tuning V12 jags, all done by ear and a screwdriver, love them, top carbies ... but your bonnet will be up every other day.
===Cold Starting SU's:===
Submitted by T on Mon, 16/04/2007 - 22:44.
SU's don't have a choke butterfly like a Stromberg does. For starting, the jet is pulled down on one carburettor which richens up the mixture. End of submission by T.
Submission by T Nov 22nd 2006:
When you add extra Carburettors to an Engine you are making the Engine breath better. As a consequence the Engine's Compression Pressure will increase for any given Throttle setting. The result is that more Voltage will be required to spark the Spark Plugs. It's recommended that you uprate your Ignition System to HEI before adding extra Carburettors if you haven't already done so, otherwise Flat Spots on acceleration can result. End of T's submission.
====Dashpot Oil Viscosity:====
Recommended grade of oil for general use is equivalent to a 20 grade motor oil. Could be substituted for 2-stroke oil. Singer sewing machine oil is also acceptable, if you are wishing to increase throttle response.
Neer ever use AutoTrans Fluid. Many people use this, and it's not the thing to do as this stuff is not good on engine components. If you want to use a mix of Singer sweing machine oil and 20 grade, that is fine. Using way too thinner oil will give you a faster throttle response, but can also allow too much air to flow though your engine and make a lean mixture, whereas too thicker oil will restrict your engine and likely give you a much richer air/fuel mixture.
Submission by Circlotron: Putting thicker oil in an SU or CD has much the same effect as increasing the Accelerator Pump squirt in a normal carby.
End of submission by Circlotron.
Submission by John (Arsewipe) cont'd It would make it run richer all round.
===Jet Sizes and Relevant Info:===By Slayer208 4/03/2010
OA6 was the factory replacement jet pin size, where the Stromberg CD-type carburettor was replaced with the HS6 SU carburettor for use with the Holden 186 & 202 motors. The Stromberg CD-type carburettors on the torana XU-1 manifolds "including aftermarket" were often replaced with HS6 carburettors due to their reliability, simplicity, easy tuning any the availability of replacement parts.OA7 was more commonly used for the smaller displacement 173 & 179 motors.
OA8 was used for even smaller displacement holden sixes, such as 149's and 161 motors.
another commonly used jet size was the SL jet pin. It has been noted to have similar properties to the OA6, but top-end has been known to be considerably richer.
You can download an SU needle program at:
You will find this program more than useful, and it can take a hell of a lot of the hassles out of searching for jet sizes. This program is very detailed, and will probably tell you exactly what you need!
By Slayer208 on 4/02/2010
There's really not that much that can cause SU carburettors to flood. These are listed below:
1] Fuel pressure getting to the carburettor. These beauties were only designed to handle 1.2PSI each as a maximum, so it's crucial that you have an in-line fuel regulator.
2] Check your float level, needle and seat.
3] Check the baffle hole on the top of the fuel bowl cap. If this is blocked, your carburettor will flood. remove the baffle and blow air though it.
4] Check the float itlsef. Make sure that it doesn't have any holes or leak fuel into it.
The amount of wear on the jets makes them become oval shaped from constant side loading.
Q: Setting the needle further into the piston is the only way for my motor to start as well as run nicely. Why is that and how do I fix it?
A: Check your float levels, they determine the height of the fuel in the jet. Either that, or you have the wrong needles or a blockage in the jets. Sometimes the rubber seal can collapse around the intake of the jet if it is installed incorrectly.
Check that the pistons are rising as they should. Personally, I would strip and inspect them. Then set the whole lot up from scratch, ideally, this is the best thing to do to a set of carbs that you are not sure of or are having problems with.
This is the best way to ensure that you have everything set equally, and gives you a zero base before you start tuning. It is very easy to become lost with all the settings if you do not have a zero starting point....