SU Carburetters (British spelling) - R. Kwas 3 - 2004,   Revised Feb 2015 (added detail on Damping Valve Assy)

(This article is also "in progress"...I plan to add to it, as interesting bits come up, and as the spirit moves me really....)

The information presented is from my experience, it is not intended to take the place of an SU manual, or even just the Carburetter section of the shop manuals out there, but should be used in conjunction with their content. 

Quick Links: 
SU Books available
"Tuning" and adjusting SUs:
Damping of Dashpot
Damping Oil
Damping Valve Assembly
SU Carb Applications
Common SU problems:
SU Upgrades:
Brickboard Reprints on SUs:
Hot starting an SU equipped vintage Volvo:
Link to separate article on Choke
Link to SU Carb Service - Lite and Expanded versions.

Probably because they are not electrical in any way (and therefore exempt from the Lucas curse), the SU carbs of a vintage Volvo are, in my opinion, one of the few things the British car industry got really right, bless them...(contrary to what you might hear in the automotive world)! 

History.  SU carbs were allegedly developed working with plumbers (air and fuel "flow" through them, so that makes sense, sort of).  It is well known that SU stands for Skinner's Union, my careful research has uncovered however, the less well known fact, that they were actually developed by British pork butchers, and that SU was actually shortened from SSU, standing for the Sausage Skinners Union.  Below, an earlier development of the Sausage Skinners Union.  The SSU, SG5, featuring "Spiral Venturi Design" (SVD). It was said to be the inspiration for the first SU side draft 1902...the family resemblance is apparent.

SU SG5 with SVD (and an integral Velocity stack!)



After a lot of "product development", this is what they came up with.  It's a beautiful thing!

SU HS6 Exploded Diagram for reference. 
Later version, with Secondary Throttle Linkage (48), floating Metering Needle (17,18,19),
and Fuel Bowl Cap (36) with provision for Vent tubing is shown.  
Source:  Green Factory Manual,  my markups.

In a wonderfully simple SU carburetter., the varying exposure of a tapered "Metering Needle" (19) as Dashpot (12) vertical position varies as a function of air induction, is the mechanical equivalent of the electronic engine management system's lookup table or "engine map".  Naturally, in the modern electronic engine management system, there is a lot more optimization going on, but the SU had the really important issue - correct metering and dispensing of fuel for the volume of air drawn through - very well under control.

The SU carburetter is of a constant Velocity, variable choke design...that is, by varying the position of the Dashpot  as a function of the volume of air the engine draws, the cross-sectional area of the throat of the carb is varied, which keeps the velocity at the venturi point constant...and by keeping the velocity constant, it is fairly simple to meter the fuel for a given volume of air induced, although this leads to somewhat of an oversimplification in fuel metering, since the fuel should be metered as a function of mass of the air being ingested and not just volume (as density of the air, which can vary due to things like weather, season and altitude, is not compensated for).  That is why I like to adjust the mixture for summer and winter.  Other than that, they seem to thrive on neglect, I honestly don't understand why some owners seem to continuously need to fiddle with them!

An exception to the constant velocity occurs during changes in the throttle Acceleration...more on this under Damping Oil following.

SU Books available:

"Tuning S.U. Carburettors, published by Speedsport Motorbooks 1975, ISBN85113-072-0" Link to Library.


Haynes manual.  (not nearly as detailed, but perfectly adequate for normal maintenance) 



SU Carb Applications  Here's a handy chart relating an engine cc's and HP range with the size and number of SU carbs the case of a 2 liter Volvo engine with two 1 3/4" HS6s, it would seem that we have plenty of carburetion from the factory!

Source:  Tuning S.U. Carburetters  Published by Speedsport Motorbooks

Dimensions of Metering Needles used on SU Carbs fitted to vintage Volvos:

(...measured in 0.100 increments starting from the shoulder.)

"Tuning" and adjusting SUs:

Centering the jets:  The metering needle (19) and jet  (22) may not come into contact.  If they did, wear would surely result to these precision parts, which should stay preciseLink to Service Notes Page on centering carb jets.

"Synchronizing" the carbs.  This impressive sounding term, which sounds as though it was borrowed from NASA, is the source of much confusion, profit to purveyors of unusual looking "special" tools, and more discussion than New England weather.  I don't see the big seems intuitive that the two carbs need to be doing the same thing at the same after assuring that the linkages act upon the throttles at the same time, the carbs should be dynamically "synched" (an even more impressive term!), that is, throttle idles adjustment screws (10) should be adjusted such that at idle, the induction of the carbs is as close to equal as possible.  This will result in a nice even, smooth idle.  A number of devices are available to help with this equalization, and I too own a couple of them...the Unisync and Tuning Tool Set...but I find that these NEVER make it out of the toolbox anymore!  I prefer instead to use my own original equipment for tuning...and I have this with me ALL THE TIME!  I use my eyes and ears! I look at the height which the dashpot is raised, and I might stick my head down between the two carbs to better listen and compare the wonderful sucking noises. 

* (Hypothetical questions: What's the difference between a visual indication by a little indicator button being sucked up the tube of a Unisync, for instance, vs. the dashpot of the carb being sucked up? Why would you trust a separate indicator tool more than the actual carb?)

Link to Volvoniacs thread on Carb Synching:

Synchronizing Tools

Unisync  A little red "float" is sucked up the the graduated indicator tube as a function of the amount of air pulled past its own variable venturi. 

Unisync...removed from its resting place in the toolbox for the
first time in more than a decade for dusting and pictures! 


SU Tuning Tool Set with "Pointer wires".  Its resting place is right there in the tool to the Unisync!

This picture is left as a large file to retain the detail of instructions.
The jet centering pin is seen at lower left.  


Carbalancer  I don't have one of these (I'm concerned with the possibility of being arrested for possession of drug paraphernalia),  but it looks like it works in the same way as the Unisync, except the indicator is separate from the venturi. 

Picture source:



Maintenance  SU carbs, once set for satisfactory operation don't really need a lot of attention...sure the linkages should be clean and (must be!) lubed, and the damping oil should be checked occasionally.  I also find that a half turn richer for the cold (and flu) season also gives better throttle response, but owner/mechanics who insist on needlessly fiddling with their SUs should consider getting a girlfriend, or maybe taking up video games, or knitting, or some other hobbies less harmful to the way their vintage Volvo runs.   

Damping of Dashpot

Damping oil.  Function of the damping oil is three-fold.  First, to lubricate the precision fit and the never-ending movement of the Dashpot (12).  Second, to dampen rapid pulsing of the dashpot at low RPMs, and thirdly, to slow dashpot rising response when throttle opening is increased, effectively enriching the air-fuel mixture (an accelerator pump effect). 

Picture source:

It's amazing what "information" is out there on what type of oil to use in this application...all the way from water to SEA90 weight!  Experimentation or custom tuning notwithstanding, it should nominally be a straight 20SAE weight oil (Reference 1), and since it is (quite) slowly consumed as droplets are slowly liberated from the valving mechanism by the vacuum, and sucked out the holes in the bottom of damper piston, dashpots reservoirs should be checked and topped up** a couple of times a year (I do it when setting the Summer/Winter mixture).  Fluid prices range from $5.75 to $8.95  for 125ml of official SU oil in a container with an official SU label, or $2 for a whole quart of automatic transmission fluid (ATF) or about $10 for a gallon of Marvel Mystery Oil...the choice is the reader's, but I know what I prefer!

** "Topping up" is a misleading term...topping up suggests that the reservoir should be filled a lot more than it should be...the damper tube filled to the top is way too full...about a thimble full  should be enough (or a teaspoon, or 2ccs, a quarter of a shot glass etc. ...whatever's handy!)...the important point is to to have the one-way-valve (those brass parts on the bottom end of the rod coming down from the Screw-On cap (15)) totally submerged in it at all times and at all vertical locations of travel as the dashpot moves.   And to prevent those telltale "blood-spots" on the underside of the hood from overfilled oil shooting out of the vent, I just snug on the Screw-on cap, hold a rag over the vent-hole, and (air filter removed) lift the dashpot as high as it will go...excess oil will be shot out into the rag...if none shoots out, I add just a bit more and repeat until it does (kind-of like:  lather, rinse, repeat...except:  add oil, check for squirt, repeat)...and by checking the damping oil level in this manner, we are also checking for probably the most important function of an SU carb:  The free motion of the dashpot throughout its entire range of travel.

Damping oil fill level.


The only quantitative test I've heard of for checking the Dashpot shaft:  

The "5-7 Second Test"  [listed under Fit of vacuum piston]   (Reference 2)

Quote from the manual:  "The fit can be checked by plugging the air holes in the pistons [dashpots, 12] with for example small corks, placing them in the suction chamber [no separate number on diagram but it's the other big part of the 11 sub-assembly], and holding the parts upside down.  The damping piston should be fitted not not filled with oil.  The vacuum piston spring should not be fitted.  The piston should normally sink to the bottom from the position shown [maximum dashpot travel] in 5-7 seconds."

Link to Volvoniacs thread on Damping oil:

Link to Thread: SU dashpot damping effect on mileage?

Damping Valve Assembly (15)  This is a one-way (check valve) consisting of four components show below, which assures there is damping action when Dashpot(12) is rising against counterforce of Spring (14), and no damping action when Dashpot is falling.  It does this in conjunction with the Damping Oil. 

Damping Valve Function  (Reference Graphic below). 

Dashpot Rising:  When Dashpot/Fluid are rising (left), floating Ring (Graphic, 1) is on Valve Seat (a miniscule [I measured this at 0.025" ] but important gap is located below Cylinder(2), between cylinder and Circlip(3)), valve is therefore closed, and no fluid (other than the bypass past Valve-Seat and Cylinder to Dashpot tube wall -see:  The "5-7 Second Test") is allowed to flow.  Here is where Damping Fluid viscosity determines flow resistance and thereby rate at which Dashpot is allowed to rise. 

Dashpot Falling:  When Dashpot/Fluid are falling (center), Ring is lifted off its seat (gap is then located above cylinder and Ring, between Ring and Valve Seat surface of Rod, and Damping Oil is allowed to flow past seat without restriction, through Cylinder and Circlip.  As the oil encounters very little resistance, dashpot falls quite freely to bridge.  

 Transition area:  Dashpot can dither as it transitions between rising and falling, and Gap relocates from below to above Cylinder. 

Link to British Volvo Forum Thread "HS6 Slippin?"


Common SU problems:

Overflowing fuel bowls  The fuel feed is a low pressure (2-3 PSI) system with a simple float and needle valve controlling the flow into the fuel bowl.  Unfortunately, and more often than just occasionally, almost every owner has seen the bowls overflow (caused by contaminants or possibly moon phase, holding open the bowl valves)'s hell on fuel mileage when this is pumped out onto the street, to say nothing of if some of the vapors are ignited by sparks at the brushes of the generator etc. 

Fuel feed pressure, that the carbs are exposed to, is (indirectly) lowered by spacer under the mechanical fuel decreases the excursion of the pump-lever by the camshaft lube, so it is critical to replace this spacer when replacing fuel-pump.  Caution!...spacer is made of brittle bakelite.  When installing a fuel pump, torque fuel-pump bolts evenly, and do not over-torque to prevent cracking this spacer.

Fuel flow blockage due to blocked jet tube.  Even with a pre-pump and post-pump fuel filters installed,  I've seen enough miniscule particles of (?) in the bottom of the fuel-bowl, that I don't know for certain, but swear there was enough contamination to affect the (vacuum motivated only) fuel delivery, as the vehicle exhibited fuel delivery symptoms only at high fuel demands, and the symptoms were cured after cleaning out the fuel bowls.

False air.  Worn throttle linkages can cause "false air".  False Air is defined as air which enters the engine by a path other than through the carb and past the venturi, where it is "measured" such that the corresponding correct amount of fuel may be dispensed.  High mileage SUs typically develop wear-gaps where the steel linkages bear and wear against their light alloy bodies.  This allows false air to be ingested.  False air causes a lean mixture at idle...less so at elevated RPMs, it also causes inconsistent idle and difficulties in tuning (typical compensation is to richen up the mixture at idle).

Cold-starting  Link to Check your Choke tech article

SU Upgrades:

Polished Dashpot domes.  Shiny is always faster!

"Velocity Stacks". Velocity stacks decrease the turbulence of the air ingested by the carbs...and presumable improve the atomization at the venturi...air filters should still be prevent sucking in random debris (with decreased turbulence)!  Stacks really only make a measurable improvement in the upper RPM range and wide open throttle (WOT), at low revs, so it seems to me that they follow the "silicon breast-implant theory" explanation of their existence (don't do much, but look great).  On the Yamaha developed engine in the Ford SHO Taurus of a few years ago, an servo-motor system controlled flaps as a function of RPM and varied the intake runner length from the long ones at low revs. to additional, parallel shorties at high revs. for additional optimization (and probably widening of the torque-curve).   

Examples of stacks:

My freshly cleaned-up 1800 engine compartment showing those impressive stacks (even with filters!).


A couple of other, very different, but equally impressive use of stacks!  We don't need no stinkin' filters.  Suggested captions welcome!

From the count of the ignition wires, is that really two V12 motors in there...and, are there
three more carbs on the other side?  Wow! 
Picture source: (I don't remember, but I'm pretty sure it wasn't the Smithsonian's site!)


Picture source:  Uncle GŁnther's 68th birthday party.


Air-cleaners.  For better breathing, and/or higher change intervals, use the "fat" stock "Late-Production" air cleaners.  PNs:  672281-3 (Rear),  672280-5 (front, with PVC fresh air fitting).  These have three holes for the later three hole carbs, but can be used, with suitably longer bolts, in two bolt earlier carbs either with separate nuts, or after modifying the carbs with threaded inserts (see below)  Link to threads :

Block the third hole with a dab of fuel compatible gasket silicon (Room Temperature Vulcanizer, RTV) to prevent rattles, and unfiltered air from entering carbs.  For use with the velocity stacks, I fabricated a custom installation of those reusable foam types.

Threading Air-cleaner bolt holes.  After dropping the semi-hidden nuts of the air-cleaner bolts for the nth time when trying to install them, one might consider upgrading the holes of the earlier carbs with threaded inserts...Heli-coil or Pem inserts both work nicely.  These make air cleaner removal or installation a one-hand, instead of two-hand-(plus occasional profanity) operation.

Placeholder for photos and details on upgrades here.



Brickboard Reprints on SUs:


From Brickboard, in response to "Carb screws loosen?" thread posted by someone claiming to be "Drivin' in the Zone!" on  Wed Feb 25 15:31 UTC 2004

Link to entire thread:

DitZ *;

The fibre washer (16) should be in place and provide enough friction to prevent backing off from vibration when the (Bakelite) dashpot screw is HANDTIGHTENED only (use tools, and you will surely break the Bakelite). I find it very unusual that Bob had success without them!

If fibre washers are not in place, threads are worn, dashpot screws are not tightened enough, or some combination of the above, this can allow vibration to back the screws that case, try teflon plumbing tape or a little gasket goop of your choice on the threads to snug them up (NEVER Loktite!). Use of incorrect (too viscous) oil, AND a backfire can blow the damping mechanism pins right through the Bakelite (not so good)...but you have stated that you are using (one of) the recommended damping oils, so this cannot be the cause. Damping oil level (overfilling) is not likely to be the cause either.

Question: Does you engine have excessive vibration issues?

My recommedation: Assure your fibre washers are in place and not oil soaked, or damaged and not sitting flat, snug up the dashpot screws just a little tighter, if problem persists, use teflon tape or gasket goop on threads. Worst case replace daspot screw/damping mechanism assy.

[Exploded Diagram of HS6 was located here, see above]

* it seems that you picked a name, whose acronym inspires less than confidence!



Hot starting an SU equipped vintage Volvo:

From Brickboard, in response to hot-starting difficulties and suggestions of installing an electric ignition system to improve.


I'm not going to take exception to the info you give about 60s ignition systems because its all true and correct...I will however take exception to redesigning the ignition system on ONE car when thousands out there work just FINE.

Some failed component notwithstanding, the problem sounds like it MAY just be the common difficult hot-start (or start after heat-soak) situation, which just about all vintage Volvos have. When you have a computer with sensors all over the place, the computer knows how best to start under those conditions, but for ALL of my vintage Volvos where I'M the computer, the hot-start procedure is: Ignition ON, No Choke, throttle(s) wide open and kept there, then crank until it starts...back off the throttle as it starts and then blip it a few time to clear the EVERY TIME! Problem? What problem!



From Brickboard, in response to questions about configuration of SU carbs


SU carbs are highly configurable in order to be suitable for just about any vehicle you can imagine. The AUD XXXX numbers are the designations for the different components which can change...for example, according to my SU book, the jet for all HS6 carbs were AUD9148 (front), and AUD9149 (rear).

Once delivered to Volvo and installed those little tags (which probably served to ID the carb configuration when inventoried) didn't do much any more, so were usually lost the first opportunity...

Common vintage Volvo needles specifications (measured in .1" interval starting from the mounting end shoulder.  Needles are made of soft brass, so measuring them with a hard-faced measuring instrument like a micrometer or caliper is asking to scratch them, so not recommended!...if a close visual inspection does not reveal any damage such as scratching, and rolling the (nontapered) base on a flat surface, while watching the needle tip for wobble or bending, the are probably fine):

KD: .099,.095,.0908,.0883,.0856,.0830,.0775,.0740,.0705,.0670,.0635,.0600,.0565,
ZH: .099,.095,.0915,.0893,.0870,.0847,.0820,.0795,.0775,.0755,.0735,.0715,.0695,
idle / pick & cruising / top speed it looks like the KD is richer starting just off idle, compared to the ZH, the other common metering needle used in Volvos.


See also:



I agree with George on that...KA is a needle which I have NEVER seen or even heard of in a Volvo sounds like that carb may have originally been on another SU book calls for KA as the metering Needle for: ...I just checked briefly and it doesn't call for this needle for ANY stock HS6 equipped vehicles!!!...but from checking the needle data (see below), it does show, that the needle is more lean all throughout its metering range than the KD.

Following, are common vintage Volvo needles specifications (measured in .1" interval starting from the mounting end shoulder. Needles are made of soft brass, so measuring them with a hard-faced measuring instrument like a micrometer or caliper is asking to scratch them and not recommended!...there's nothing wrong with used needles if a careful visual inspection does not reveal any damage such as scratching or bending.



Comments and constructive criticism on this article are welcome.  You can keep the complaints and bitching...didn't your mother ever tell you:  "If you don't have something good to say, don't say anything!"



1.   [Specifications, carburettors, oil for damping cylinder] 1800s Owners manual, Page 24

2.  [Fit of vacuum piston] 1800s Owners manual, Page 11


Thread:  Wohin kommt das Daempfungsoel im SU?

Suggested on "SU dashpot oil"


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The correct grade oil for the dampers in SU carbs is SAE20 which we
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Why not try Penrite Damper Oil for SU and Stromberg Carbs?  I
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German site with good SU info:

CVI...good selection of SU spares.


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