SU Carburetters
(British spelling) - R. Kwas 2004-2023,  revisions On-Going.  [Comments added !]

The information presented is from my experience, it is not intended to take the place of an SU manual, or even just the Carburetter  (British spelling), section of the shop manuals out there, but should be used in conjunction with their content.  I have tried to use terms like Dashpot, Dome and Bowl Valve (opposed to Needle valve) to refer to specific components, so reader may notice some inconsistency with factory terminology...

SU Carb History 
SU Books available
Comparing Metering Needle Dimensions
"Tuning" and "Synchronizing" SUs
Mixture adjustment
Initial Mixture Setting After Jet replacement
Damping of Dashpot
Damping Oil
The "5-7 Second Test"
Damping Valve Function
Damping Valve Assembly
SU Carb Applications
     Metering Needles
Common SU problems:
False Air
SU Upgrades:
Brickboard Reprints on SUs:
Hot starting an SU equipped vintage Volvo or Starting after a Heat-Soak:
Long Cranking during Starting Attempts after Extended Parking Times
Reference Information
    Metering Needles used on various engine configurations
        Comparing SU carbs HS vs. HIF
Links
        Tom Bryant
Links to Videos
Grose Jet Product Sheet
Fuel Bowl Venting
ECB18
Webers?!?

SW-EM Links to related Tech Info not on this page:

Carburetor Additional Notes
    Mixture Optimization and the Gunson Colortune
    Wideband O² Sensors
Carb Spacer/Thermal Isolator Notes and Observations
Fuel Delivery Components and Considerations
Separate article on Choke
SU Carb Service - Lite and Expanded versions.
    includes:  Check and Adjustment of Fuelbowl Floats
SU Jet Bearing and Supply Tube Notes
SU HS6 Jet Supply Line Seal
SU Carb Throttle Plate Early vs. Late including Throttle Overrun Valve
SU Carb.  Adding Threaded Inserts for Air Cleaner Securing

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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 from guys who only know downdraft carbs...)! 

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 carburetter...ca. 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!

Setting the SU History Record Straight

Exploded Diagram of a (late) SU HS6 Carb as used on Vintage Volvos.  Referenced Components are Highlighted. 
Dome assembly(11), Dashpot (12), Dampening Valve (15), Fiber Washer (16), Guidetube and Spring (14), Idle Screw (10), Fast Idle Cam (55) and Fast Idle Screw (49), Throttle Linkage/Plate (43, 44, 46), Jet assembly (22 - 30), Lifting Pin and Spring (5, 6),  Fuelbowl lid (36), Lidscrew and Washer (38, 39), Bowlvalve Assembly (40), Bowl Float and Float Pin (41, 42).  Links shown in this list point to stand-alone Notes or Tech Articles. 

SU HS6 Exploded Diagram for reference.  Later version, with Secondary Throttle Linkage (48),
floating Metering Needle (17,18,19), as well as Throttle Overrun Valve and
Fuel Bowl Cap (36) with Bowl-valve (40) and provision for Vent tubing is shown.  
Source:  Green Factory Manual,  author's 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, and this is controlled by the driver's control of the Throlle, the cross-sectional area of the throat of the carb is varied, which keeps the velocity at the Venturi 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, or one would have to make adjustments to compensate for large changes in altitude.  Other than that, I find 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 does occur during changes in the throttle position...like 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.

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

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SU Carb Applications  Here's a handy chart relating an engine cc's and HP range with the size and number of SU carbs recommended...in 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 in SU Carbs fitted to some vintage Volvos:

Metering Needle dimensions. measured in 0.100 increments starting from the shoulder.  Note that when used with a 0.100" Jet, and with Dashpot in the lowest position, that would be a mere 0.001 of clearance...not very much at all!... and making the point of how important clearance and centering of the jet is. 

Link to Reference Information:  Workshop Bulletin, Metering Needles  (showing a more complete listing)

Comparing Metering Needle Dimensions:

I have recently run across this tool which allows selecting any MN and it draws a graph which shows the dimensions and draws a cute little chart allowing an instant comparison.  I have entered the two common stock MNs of KD and ZH to generate the following graph where it is immediately clear that the KD needle is richer throughout the entire range.  [Note: Authors of the chart use metric dimensions.]

https://www.classicminidiy.com/technical/needles/#

"Tuning" and "Synchronizing" 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 precise!  Link 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 deal...it seems intuitive that the two carbs need to be doing the same thing at the same time...so after assuring that the linkages act upon the Throttles at the same time (and this can be checked with engine off), the carbs should be dynamically "synched" (an even more impressive term, which simply means they also do the same thing when engine is running!), 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:  http://www.volvoniacs.org/forum/thread.jsp?forum=31&thread=1764&start=0&msRange=15

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! 
 

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SU Tuning Tool Set with "Pointer wires".  Its resting place is right there in the tool box....next 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.  

Synchronizing SUs (much ado about nothing!)

My comments to a thread (https://www.swedespeed.com/threads/tinfoil-gas-cap-67-122s-project-thread.668582/page-4#post-8122313  Post #67 ) about carb synchronization:

" As I noted before, I have used the listening [to the two carbs] technique, but one I find even better, and the one I exclusively use now, is the visual technique of comparing the Dashpot height (gap to Venturi)...and my eyes still work perfectly!   This is in-fact the basis on which those cute little tuning kits with the wire pointers work...you remove the Damping Valves, and insert two little wires into top of Dashpot and let them point at each other and compare...I fail to see what the difference is between that and watching the gap to venturi at the bottom of Dashpot...because there is none!!  

Besides, when Damping Valves are removed in order to install those wire pointers, there can be/will be no damping whatsoever!...and damping is one important carb function I like to compare between carbs (when blipping throttle), but under that test condition, is not even able to be checked!!!  

Yes, the Air Filters need to be removed for this visual comparison of Dashpot height/idle adjustment, but there is little difference with them installed (given they are in relatively clean/low restriction condition)...and yes, it does say in the instructions to use a "Unisyn" for "precision synchronization", but it also clearly states "it is possible to synchronize without flow meter by comparing the height at which each piston [Dashpot] is lifted at idling and at higher RPM." ...that's enough for me...and my practical experience has shown it works wonderfully! "

Excerpt from the Tuning Kit Instructions: 

Happy SU Synchronizing...with on-board equipment...little more!!

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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:  http://www.stevens-wesel.de/werkzeug.htm

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Maintenance:  SU carbs, once set for satisfactory operation, they 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 on the Mixture Adjustment Nut (see below!) also gives better throttle response, but owner/mechanics who insist on needlessly fiddling with their SUs should consider getting a high-maintenance girlfriend, or maybe taking up video games, or knitting, or some other hobbies less harmful to the way their vintage Volvo runs.   

Mixture Adjustment:  

Mixture Adjustment Nut at Green (15mm*).  Jet Locking Nut at Red (18mm*, leave this one alone(!),
unless adjusting and locking jet centering!) * typical sizes, but variations have been observed!

See also:  Mixture Optimization and the Gunson Colortune

Initial Mixture Setting After Jet Replacement, or major service where Jet vertical position may have been moved, should be 2 full turns/12flats on Mixture Adjustment Nut down from Jet Flush at Venturi.  That will enable engine to start so a fine mixture tuning can be performed.

Jet Flush.	Jet "down" 12 flats of Mixture Adjustment Nut.

Link also to separate SW-EM Tech Articles: SU Jet Bearing and Supply Tube Notes  and SU HS6 Jet Supply Line Seal

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:  http://www.aptfast.com

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 "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, with an oil-film, but 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." 

The only quantitative test I've heard of for checking the Dashpot shaft...it effectively also checks the precision clearance between Dashpot OD and Dome ID (those surfaces, which should NEVER be subjected to ANY abrasive!):  

Precision non-contact surfaces...the rate at which air is allowed by these, affects Damping. 
Dome and Dashpot were selected and matched at the factory and their interface must not be changed by mere mortals!

From a FB thread about adjusting Carbs:  "...check that both dampers bob up and down identically when you blip the throttle."

My response: 

"Equal Dashpot excursion when blipping the throttle is the target, but it is not only determined by throttle linkage settings...since it is a dynamic condition, it is also affected by Dashpot movement and Damping, and that is a function of clearances and Damping Valve (Damping oil in both, and resulting action should obviously be the same [or very similar!])...what I'm trying to point out here is that if Dashpot action is not exactly equal when blipping throttle, first thing to check and adjust is throttle linkage [including Idle adjustment], but once those are as close to equal as possible, Damping needs to be checked to be equal, and that means doing the "5-7 Second Test" which checks clearance and precision of Dashpot tube with respect to tube in Dome, to see how close (or different) these are between the two carbs: http://www.sw-em.com/su_carbs.htm#5-7%20Sekunden%20Test ...again, if these are not precisely and absolutely equal, I recommend NOT making yourself crazy about it! Set for best similarity, and seek (moderate) adult beverage based therapy...done! Cheers."

Link to Volvoniacs thread on Damping oil:  http://www.volvoniacs.org/forum/thread.jsp?forum=31&thread=4647&start=0&msRange=15

Link to Brickboard.com Thread: SU dashpot damping effect on mileage?  http://www.brickboard.com/RWD/index.htm?id=709006

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, floating Ring(1) is in contact with Valve Seat.  A small [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 leakage 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 is allowed to fall quite freely.  

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

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My response to thread clarifying what oil in SU carbs does:  http://www.volvoforums.org.uk/showthread.php?p=2183206#post2183206

"In the SU, Damping Oil both dampens the rise of Dashpot AND lubricates its action (I've zero experience with, nor interest in, Strombergs, but I expect since the design is similar in this respect, their oil likely serves similar functions...)

...vacuum in the Dome liberates droplets from the Damping Valve and Reservoir as Dashpot continuously rises and falls, and these droplets get into precision clearance between Dashpot and Dome Tubes, lubricating this joint and its continuous action in this way...and since the relocation of the droplets is driven by vacuum, it also leads to a drop in oil level, as oil is (exceedingly) slowly consumed (through vacuum holes in bottom of Dashpot).

Simplified SU carb showing Damping Oil Reservoir (1) with Damping Valve (2),
lubrication of Dashpot Shaft (3) and path for Consumption (4). 

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The damage shown below is what happens when force of a rising Dashpot acting on the Rod is too much for the plastic threaded Thumbscrew to endure (like from the explosive positive manifold pressure during a backfire).  The viscosity of the oil and extremely rapid rise in pressure pushing up the Dashpot is so sudden and forceful, that Rod can be launched right through the Thumbscrew!  Symptoms after this occurs are lean and possible rough running and lean backfire as there is no Damping action whatsoever. 

Picture credit:  Swedemash.

Cause for this may be too viscous a Damping oil not being able to flow the restricted (bypass) path around the closed valve fast enough, possibly made worse by cold temps or a (lean)Backfire.  Link to related Thread:  http://www.volvoforums.org.uk/showthread.php?t=241673

Excerpt from my response and recommendation:  I recommend you check your Damping oil viscosity (should be 20W, not thicker)...also don't stomp on the accelerator pedal before engine is warmed up, resulting in a lean backfire. Also check your Choke is enriching as it should! (Repair Suggestion) You MIGHT be able to (super)clean the area, file some burs into the rod to give it some "tooth", push it back into its normal position, and epoxy it into place...otherwise, replacement is always an option, but check other things I mention or it might happen again with a replacement...!

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Link to British Volvo Forum Thread "HS6 Slippin?" http://www.volvoforums.org.uk/showthread.php?t=222689

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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)...it'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. 

Here is an interesting link, (https://www.youtube.com/watch?v=zlez_hdoWYk  )...originally about a (Mikuni) snow-mobile carb, but applicable to SU carb bowl valves as an overflow preventative, because as can be seen in the video, the bowl-valve components are absolutely similar to those of the SUs.  The trick is to polish valve body ID with brass polish on cotton swab spun with hand-drill.  I haven't tried this yet, but it seems valid to have the valve bore be as clean as possible...even polished(!) to prevent hang-ups resulting in overflowing...who knows what's hanging around on the valve wall...clever!...I will do this on the next incontinent carb I run into...and show the cotton swab here afterwards...stay tuned!   

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Fuel feed pressure, that the carbs are exposed to, is (indirectly) lowered by spacer under the mechanical fuel pump...it decreases the excursion of the pump-lever by the camshaft lobe, 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.  False Air is defined as air which enters the Intake, sucked in by the vacuum there, by a path other than past the Venturi, where it can be "measured" such that the corresponding correct amount of fuel may be dispensed. 

Minor False Air High mileage SUs typically develop wear-gaps where the brass throttle shafts bear and wear against the light alloy bodies.  This minor wear-gap doesn't noticeably raise the idle, but it still allows enough False Air to be ingested to give problems...  (Refer to Graphic below, 1.), causing a lean mixture at idle, and this is typically compensated for by richening up the Idle mixture...which in-turn leads to an unnecessarily rich mixture throughout the entire operating range.  Worn throttle shafts also cause inconsistent idle and difficulties in tuning in that they don't necessarily return to the same place...lubing the linkages helps, and is certainly recommended, but the proper fix is to rebush the housings to eliminate False-Air, and return the setting to a proper (non-compensating) point.  Other sources of Minor False Air are caused by loose Carbs or Manifolds (2), or (rare, but possible) Manifold Gasket failure (3). 

Because the amount of Minor False Air ingested is a function of inconsistent factors such as throttle shaft location, as well as manifold vacuum, trying to tune an engine while there are sources of  Minor False Air can be a totally frustrating operation... 

Major False Air needs more of an opening to atmosphere for air to enter the intake stream (like a compromised or dislodged PCV or Brake Servo hose 4).  It typically manifests itself in a very high Idle (yet with Dashpot down at the Venturi, a highly unusual condition which should give the troubleshooter an immediate big hint!), but since fuel for a correct mixture is not being dispensed at the Venturi, engine does not make proper power and loading it with the drivetrain collapses the high idle to nothing, or stalls the engine altogether. 

Normal Intake Air travels past Venturi, where Fuel is metered, False Air enters wherever it can and will,
 and since it is not passing the Venturi, fuel is not metered for it, so engine may rev up, but not make any power with the lean mixture. 

False Air past Throttle Overrun Valve:  Not strictly the definition of False Air, because intake air is passing by the Venturi, a Throttle Overrun Valve not sealing correctly can result in a high idle (but with Dashpot raised, so proper amount of fuel is being metered...the action is the same as having an open throttle...because that what it effectively IS!).  The cause of this raised idle is difficult to locate and impossible to correct without first either reseating TOV so that it seals correctly, or disabling it completely.  If engine is idling high and carbs have TOVs, manually raise Dashpots (engine OFF!) and with a light-source, check a TOVs is not dislodged and open.  See separate SW-EM Tech Article on Throttle Overrun Valve. 

I recently found that Tom Bryant of Maine (https://thosbryant.wordpress.com/2014/01/02/su-carburetor-rebuilding-hs-6/comment-page-1/#comment-149  ) has an outstanding article detailing the inspection and rebushing operation which he performs as a service.  I highly recommend the article for anyone considering this repair work themselves (special techniques and tooling are required!), to simply get an idea of what's involved in the procedure, or if one is considering paying him for this work, because the article makes clear all the areas he inspects and makes right, and the tools and techniques he uses...(money well spent IMO...no, he is not my cousin!  See more on this below, in Links: Tom Bryant)  

Cold-starting  Link to Check your Choke tech article

SU Upgrades:

Polished (Exterior Only!) 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 fitted...to 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!...and I'm kinda OK with that...).  On the Yamaha developed engine in the Ford SHO Taurus of a few years ago, a servo-motor system controlled flaps as a function of RPM and changed 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!).

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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 Günther's 68th birthday party.

See also:  Links to Videos below!

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 :   http://www.brickboard.com/RWD/volvo/1009976/444-544/kohler_air_filter_elements_su_cleaners.html
http://www.brickboard.com/RWD/volvo/1262102/120-130/su_hs6_air_filter.html

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...Helicoil or Pem inserts both work nicely.  These make air cleaner removal or installation a one-hand, instead of two-hand-(plus occasional profanity) operation.

Carb flange with Helicoil retrofitted. 

See Separate Tech Article:  SU Carb.  Adding Threaded Inserts for Air Cleaner Securing

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Brickboard Reprints on SUs:

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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:  http://www.brickboard.com/RWD/index.htm?id=760843

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 away...in 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!

Cheers