SU Carb Service
R. Kwas Jan 2008 - 2018 (updated continuously)
Link to full Tech Article on SU Carbs
SU Carb Service - Lite
For those who wouldn’t know a constant velocity, variable Venturi, side-draft SU carb from an orgasmatron, the prospect of cleaning and performing a preventative maintenance, or even just touching them can be pretty intimidating…one wouldn’t want to break or mess up anything...all those complicated mechanical linkages...and add to that, the fact that there are two under the hood…where does one start? To help with this, here is a good, first-time, low risk introduction to getting to know your SUs carbs. While the highlighted Inspection Items included are what I consider to be important items for their correct function, and what to check and watch for, this is only intended as a familiarization, so adjustments are not included. Lubrication is included for reassembly, because it is important, and Adjustments will be covered in the Service - Expanded Version second half.
Before reader gets started with their own hands-on session, viewing the superb two part video by John Twist of University Motors is recommended to understand these little mechanical marvels. While Twist is an MG specialist (we won't hold that against him) the presentation covers both theoretical, and how a particular requirement is practically implemented in the SU design, and applies perfectly to the Volvo configuration of the carbs:
Part 1: https://www.youtube.com/watch?v=7GRAcqDySog
Part 2. https://www.youtube.com/watch?v=60Bj_2cZQnc
Recommended to have on-hand for this procedure: General tools, soft clean rags, spray type carburetor cleaner (with pinpoint applicator snoozle), Automatic Transmission Fluid (ATF), graphite grease (or graphite based anti-seize), preferably in a syringe applicator, a three foot length of 1/4” ID fuelline, plenty of light under the hood.
Disclaimer: This procedure has been prepared with the greatest care, and I stand by it, having performed it many times myself, but it is only a guide to be used in addition to normal, cautious shop practice. I cannot be responsible for your specific actions. You alone are in charge of you knuckles and future! Work Safely!
The following steps are performed with IGN off; battery may be left in place and connected.
Exploded Diagram of an SU Carb as used on Vintage Volvos. Referenced Components are Highlighted.
Dome assembly(11), Domescrews (20), Dashpot (12), dampening valve (15), fiber washer (16), guidetube and spring (14), air filter, idle screw (10), fast idle cam (55) and fast idle screw (49), throttle linkage/plate (43, 44, 46), jet assembly (21), lifting pin (5), venturi (*), clampnuts(*), chokewire stop(*), fuelbowl lid (36), lidscrew (38), bowlvalve assembly (40), fuel float (41), float pin (42),* not shown in exploded diagram
Dissassembly and Static Inspection: Remove air filters. If vehicle is equipped with the earlier two bolt filter type (as shown in exploded diagram), this will require a wrench on the boltheads, and probably a second wrench on the nuts behind the flange on the carb body. The loose nuts like to fall and get lost, so care needs to be taken not to allow this to happen. On later three-bolt types, the flanges were threaded which eliminated the nuts. This allows removal with just one wrench and there are no nuts to fall and get lost.
Having removed the Air Filters, part of the Dashpots of both carbs are visible just inside the throat of carbs, in the all-the-way-down-against-the-bridge (venturi) position. Inspection Item 1: Check Dashpot travel. By raising lightly with a finger, check each Dashpot for free, smooth, damped lifting, and more rapid, undamped, falling, all the way back down to the venturi with an audible clunk. As each Dashpot is raised, the tapered, brass, fuel metering needle will become visible, a bit further into the throat, mounted at the bottom center of the circular Dashpot. Aside from the linkages, and the fuelbowl valve, the movement of the Dashpot is really what makes the simple SU work so well! Because of this, it should be clear, that Dashpot movement is one of the most, if not the most important functions of the carbs, and so should be smooth and unobstructed, as designed and intended.
With a screw driver, remove the three screws holding the Dome of one carb to the carb body. Once all screws have been removed, a gentle twist can help break the Dome free and allow it to be carefully and slowly lifted off – along its axis! The Dashpot should remain down, and be allowed to stay with the carb body. Once the Dome is lifted off, its inner surface should be cleaned with spray carb cleaner and soft rags. Cleanliness and lubrication on reassembly are important for trouble-free operation of these mechanical carbs. Use no abrasives of any kind on any internal part of an SU carb*. Flush and wipe away any abrasive dust or soot on internal components! The unidirectional dampening valve, which allows the free fall of the Dashpot while assuring dampened rise, may be unscrewed from the Dome top, and also flushed and wiped clean. Flush and wipe the internal bore of the central tube. Run a soft rag through the tube. Use no metal, but for instance, a giveaway chop-stick from the oriental restaurant of your choice for this, which cannot scratch the precision machined bore. Place the clean Domes, and their damper valves aside, on a clean surface.
The Dashpot, along with its return spring, is now visible at each carb body. Both may now be carefully lifted off - also along their axis – taking care not to drop the Dashpot back down (without the Dome in place, there’s no centering or damping so metering needle damage can occur if Dashpot was allowed to drop uncontrolled). Handle the Dashpot assembly with great care, as the brass needle on the bottom surface of Dashpot, is critical to the function of the carb, and if bent – even slightly – will need to be replaced, and you have just turned a fun and interesting, SU familiarization session into a bad day, the rest of which will likely be spent on the phone, trying to locate an immediate source for a straight replacement needle!
Pour the dampening oil out of the top dampening tube onto a clean, white paper towel and inspect oil for color and particulate contamination…if ATF or Marvel Mystery Oil (which can be identified by its smell), it will be red, and an indication that one of the recommended oils have been used, although the official damping oil is a typical golden color. Whichever oil comes out should be thin in viscosity and quite clean. Flush and wipe the Dashpot/metering needle assembly clean.
Inspection Item 2: Check metering needle installation into Dashpot. Using a fingernail or small straightedge, verify that the shoulder of the metering needle is flush with the mounting surface of the Dashpot bottom. Inspection Item 3: Check metering needle for contact damage. Visually inspect the sides of metering needle for scratching or other signs of contact with the jet, particularly at the fat end. None should be evident to the unaided eye. Carefully place the cleaned Dashpot assembly and spring aside, with their respective Dome, taking care not to allow the round parts to roll away.
With the filters, Domes and Dashpots of both carbs removed, cleaned and safely grouped by carb (parts would probably fit and allow swapping, but why take chances, and introduce a variable?), throttles plates are now visible even further down the carb throat. Inspection Item 4: Check for carb linkage for synchronization. Slowly actuate the throttle linkage at or near the mount on firewall to simulate normal driver input, while observing the actual movement of both throttle plates...verify both throttles open at precisely the same point as the throttle linkage is actuated! Using the carb cleaner, spray and gently wipe and clean carb throat surfaces of any black soot, and Dome contact surface of contamination. The dirtier the carbs, the more time can be spent cleaning them at this point! Inspection Item 5: Check throttle linkage for wear at the carbs. Wiggle the linkage where it enters the carb body, checking for side to side motion which would occur if shaft or carb body was worn. This is a well known wear area of high mileage carbs, responsible for “false air”. If it weren’t for this wear, SU carbs would probably last forever!
Inspection Item 6: Check Choke function. Verify both carb jets are drawn down from the venturi by about 1/4 inch as a helper slowly activates the choke control on Dashboard, by pulling it out from its rest position to fully pulled position. At the same time, verify the fast idle cams are rotating and contacting the fast idle adjustment screws and thereby opening the throttle plates slightly.
You have now opened the SU carb and have seen into its soul, and observed it’s most intimate secrets. Take notice of the fact that up to this point, you have only inspected and cleaned parts, so have changed (and given a careful reassembly, will change), nothing in the settings of the carbs! Take notice also, that up to this point, very little magic, if any, has occurred!
[Note that missing from the above is the removal, disassembly and Inspection of Fuelbowl Lids and Bowlvalves. Simple as these also are, their reassembly can often be accompanied by overflowing fuelbowls upon restart, which would serve to totally spook some poor beginner into never going anywhere near the his carbs again!...I therefore considered it beyond the scope of this Service - Lite section and have included it in the Service - Expanded Version section!]
Lubing and Reassembly: As the mechanical devices they are, SU carbs need to be lubricated to function well!
Lube linkages: Graphite grease or graphite containing anti-seize is an excellent lube for low movement/high bearing pressure applications because the graphite particles stand up well in the long haul and are not displaced from where they are needed. (LINK: Anti-Seize) Unfortunately, using it sloppily will also make a mess. A syringe pinpoint applicator works well and keeps the mess factor to a minimum (personally, I’d rather have my carbs be well lubricated and have a little mess than clean and bone dry). Lube the ball linkages leading up to the carbs (retaining pin for the joints can be removed and ball levered out of spring loaded socket, cleaned, and greased...once lubed it will only need to be redone in ten years or 100,000miles, whichever comes first!), the throttle shaft entrance through the carb body (as best as possible since its buried behind hardware), and also the jet tubes, to assure smooth operation for a long time with minimum wear of components. Wipe off excess.
Lube Dashpot Lifting Pin: Apply a drop of ATF to the lifting pin mechanism and work the pin a number of times until free. This is strictly a built in service-aid which is never moving during normal service. It is moved only during maintenance operations, to lift the Dashpot a constant amount, so these tend to get locked into place by debris and/or corrosion.
Lube Dashpot: Since the Dashpot spends its service life sliding up and down guided by its shaft sliding in the central guidetube of the Dome, these must be well lubed. Apply a film of ATF to the Dashpot shaft. Replace the Dashpot in carb body, gently allowing the metering needle to slide into the jet as the slot is aligned with the alignment tab in carb body. Fill shaft with ATF to slightly (1/4” ) below the top. Replace the spring onto the Dashpot shaft. Replace the Dome (without damping valve installed) by gently aligning it onto the Dashpot shaft, and sliding it down onto the Dashpot tube until it bottoms onto its mounting surface. Only one circular alignment of the Dome will align all three securing screws. Turn the Dome to find this location...this turning action is actually good in assuring the clean and perfectly flat interface of the two components, and therefore helps the perfect alignment of the Dome, and guided Dashpot within the carb body (and metering needle within the jet).
Replace securing screws and their lockwashers (a dab of anti-seize on the screw threads is a good idea here to prevent galvanic corrosion, as these will be going into the dissimilar aluminum carb. body.)
Once the Dashpot and Dome assembly are secured in place on the carb body, repeat the previous test for smooth Dashpot travel, verifying that Dashpot bottoms out completely on the bridge with a clunk. The Dashpot will follow a finger directly, without the damping valve installed. If good alignment and smooth motion is verified, install damping valve by inserting (brass) valve into Dome and Dashpot tube, pushing it down and into the ATF, finally snugging it handtight only (Beware: Overtightening or tightening with a wrench will surely break the threaded Bakelite part). The fiber washer limits the tightening force and gives some locking action against the vibration present in operation.
Reassembly: This is pretty much the reverse of disassembly as they say, and it shouldn’t be rushed...especially if it is the first time...rushing causes parts to be left over and indigestion!
Operational Check: Once the reassembly is complete, the engine may be normally started. Idle should not be much different that before Disassembly and Inspection, because no adjustments were made, but if a test drive reveals any truly noticeable change after only the inspection and lube parts of this procedure have been performed, then it probably really needed them and could also benefit from the adjustments in the following section...
* My response to suggestion to using "Brasso" polish in a VOC thread: Carburettor Issue: https://www.volvoforums.org.uk/showthread.php?t=281041
"Brasso is a polish and therefore abrasive (if only mildly)...this means it removes material (if only minutely) which results in changing the precision clearance between Dashpot and Dome tubes, and this is to be avoided...I recommend against abrasive of any kind...I do recommend cleaning and flushing (especially the Dashpot and Dome tubes) with a hydro-carbon solvent (spirits, carb-cleaner, etc.), wiping absolutely clean, and lubing pre-installation with ATF."
SU cutaway showing precision, lubricated contact area between (Dashpot and Dome tubes) at Green,
and precision clearance non-contact area between Dashpot outer circumference and Dome Inner circumference) at Red.
End of SU Carb Service – Lite
SU Carb Service - Expanded Version (including Adjustments)
Adjustments: If in the course of performing the previous cleaning and preventative maintenance, items were noticed, as explained in the Inspection Items, which didn’t seem to be as they should, the carbs may require some adjustments. When finished making adjustments, a roadtest is always the best check...
Inspection Item 1: Check Dashpot travel. If for instance, there is contamination between the Dashpot and guidetube, or dirt on the carb surface which the Dome bolts to, or a bent metering needle, or an uncentered jet, movement can be negatively affected or in the worst case, totally jammed. This needs to be corrected, or the carb can’t function as designed.
Adjustment: There is not much to adjust here...clean and lubed, and aligned and unbent components should allow smooth Dashpot travel along its entire range. As final check after any work is performed on a carb, the Dashpot must be able to travel through its entire range of motion smoothly. Link to Service Notes: Centering Jet
Inspection Item 2: Check metering needle installation into Dashpot. If metering needle shoulder is not flush with bottom surface of Dashpot it should be adjusted to that location by loosening setscrew in side of Dashpot which secures it, adjusted, and setscrew retighted. If a brass metering needles is bent and the carb is reassembled, it will likely make contact with the jet as Dashpot falls, jamming Dashpot in a raised position, so extreme care is called for when trying to loosen a stubborn needle...usually the needle can be loosened by hand, but if not and one has to use tools which are easily capable of applying enough force to turn the needle into a pretzel, extreme care must be exercised...I’ve have had good results (no disasters in the form of bent needles!) by loosening holding (set)screw, wrapping a piece of leather around the fat base of needle and very carefully applying just enough twisting force to persuade the needle to loose its grip from the socket. A light film of graphite grease on the setscrew threads and needle base would certainly prevent such difficulties.
Adjustment: If needle is relocated, mixture and idle adjustments will be required after reassembly and startup. See also LINK Mixture Checking.
Inspection Item 3: Check metering needle for contact damage. If contact has occurred for any reason between the brass metering needle and jet, the soft brass will show scratches or worse. This will probably result in taking material away, resulting in enrichening at idle and higher RPMs. No contact and no scratching or changing the needle dimensions in any way is preferred, but if excessive, replacing the needle with an identical one (labeled on side of base with a two letter code, i.e. KD) is about the only option.
Adjustment: If needle is moved or replaced, mixture and idle adjustments will be required after reassembly and startup.
Inspection Item 4: Check for carb linkage for synchronization. If carbs are found to be “out of synch”, that is, one throttle plate opens before the other as throttle linkage is slowly actuated, this will case uneven running at idle and above. This should be adjusted out by loosening the clampnuts which lock throttle shaft to carb throttle linkage, and rotating shaft accordingly. Snug the nuts and recheck.
Adjustment: If linkage position is adjusted to synchronize the throttle action, the idle screws, which have quite possibly have been adjusted to compensate for this (two wrongs making a right), will need to be adjusted to undo this “compensation”.
Inspection Item 5: Check throttle linkage for wear at the carbs. Unfortunately, where the throttle shafts enter the body in an SU, there was no rotational bearing or seal against false air designed in. If these bearing areas were also not at lubricated during their often long service life, the resulting wear will allow unmetered false air to be sucked in past a worn housing. This will cause general problems such as inconsistent idle, and needing to be compensated for, by enriching the mixture.
Preferably, the throttle shafts should be lubed where they penetrate the carb body. This will slow wear in the long term, and prevent problems. The trouble is, that the precise place where the lubrication needs to be is blocked and not very accessible from the outside without disassembly, so this actually having been done in the life of the carb is rare, with predictable results. The shafts are at least partially accessible (from the inside!) while the Dashpots are removed. I recommend graphite grease or graphite containing anti-seize for this (again for the advantages of non-displacement and particulates). A syringe with a large gauge needle (for medical purposes anyway) of about 0.50” diameter allows pinpoint application from both sides, with the carb vacuum serving to draw the lube into the bearing area from the outside.
Lubing throttle penetration through carb body only, will not necessitate any adjustments.
Inspection Item 6: Check choke function. The choke linkage is a somewhat complicated pivot linkage activated by the pull-cable coming from under the dashboard...rust in the cable itself, a sluggish pivot linkage at the carb or a dry jet tube moving within its guidetube all contribute drag which can be minimized by lubricating with graphite grease. Using a small amount will keep the mess limited, but makes a huge improvement both in action of the mechanisms, and wear prevention.
The inner cable should be connected to the carb linkage such that when fully pulled at the dashboard control, about 1/4” of lowering of the jet results as the movement is transferred by the linkage. Loosening the Phillips screws of the wire stops will allow this adjustment.
Lubing the choke cable is also required periodically and once again, the advantages of particulate graphite can make this service interval a decade! Here is a copy of the recommended procedure from the previously posted in Appendix 1 of the Check your Choke Tech Article.
Adjustment: After choke actuation has been changed, the fast idle screws will need to be adjusted.
Inspection of Fuelbowl Lids and Bowlvalves [Work carefully and with clean rags only – dirt or contamination in fuelbowls is BAD!!] After first blasting externally with carb cleaner, and wiping away dirt around both fuelbowl lids, remove three lidscrews, and lift both fuelbowl lids (leaving connected together by the short section of fuelline), gently turning both up-side-down while lifting them up and away from the fuelbowls. If gasket sealer has been used on the bowl gaskets in a (false) effort to try and stop fuel leakage due to sticking bowlvalves and overflowing from the vent, gaskets may separate, with part of the gasket staying with lid and part staying with bowl. Carefully pick the one they are stuck to the most, and separate the gasket from the other, so that gasket is saved and can be reused...if gasket is torn apart, a new gasket will have to be fitted. A gasket must be in place or leakage will surely result!
Placeholder for Bowlvalve Picture.
Disconnect the fuel supply hose from the front lid, catching any spilling fuel in a rag. Holding lids up side down, compare the positions of the two floats. These determine the bowl fuel level at which they close, and should be similar. Level of fuel left in the fuelbowl after removing lids with their floats should also be in quite similar. If fuel levels are quite different when bowls are removed, it can be assumed that they are also different when lids are in place. Since this is an important prerequisite for proper operation of the SU carb, this should be check and possibly adjusted. See: Check and Adjustment of Fuelbowl Valves
Looking into the fuelbowls with a light, one should see clean, clear fuel with no sediment settled out on the bottom. A small amount of sediment is not unusual, but if there is a lot of sediment evident in the sump of the bowl, this certainly makes the point for fitting a post-fuelpump filter...after this reaches a certain level, it would impede the fuel flow to the jet.
Clean out bowlsump by soaking fuel out with a rag, then flushing with carb cleaner, wiping and flushing alternately. The length of fuel line can also be used to blow the bowl sediment into a rag with a strong exhale! Finally flush the jet supply tube by inserting snoozle into jet at the venturi (Dashpot will have to be removed for this), and blasting any sediment back toward the fuelbowl and into a rag.
Check and Adjustment of Fuelbowl Valves The support arm of the float is made of lightweight metal which can be gently bent to give the specified dimensions. MORE TO COME ! XXXXXXXXXXXXXXXX
Check of Fuelbowl Valve Seat Valve is actuated by action of float. Remove the pin on which the float pivots. Pins often have a flattened area which means they can only be installed from one direction, removed from the other. Once the pin is removed the float can also be removed. As the float is removed, valvepost can fall from the valvebody, so care should be exercised not to loose this part. MORE TO COME ! XXXXXXXXXXXXXX
End of SU Carb Service - Adjustments
Additional Resources: The previous information and procedure should help a vintage Volvo owner to get familiar (or maybe even comfortable) with their SU carbs. If in the course of this familiarization additional questions arise, help is (almost immediately) available by posting to the LINK: Brickboard Forum.
Here is also a list of further resources I compiled from the hands-on guys at the brickboard (thanks guys for contributing your info and comments) (Link to thread: http://www.brickboard.com/RWD/index.htm?id=1254539 ) for replacement parts, rebuilding kits, rebuilding services (I am not associated with these resources in any way, other than being aware of them, and the contact info is presented for convenience and in no particular order...maybe alphabetical, maybe in the order I got them :
Resource, Company or Individual's Name:
Parts and Services Available:
Site or Contact info:
Burlen Fuel Systems Ltd
Phone: 00 44 (0)1722 412 500
Fax: 00 44 (0)1722 334 221
JOE CURTO 718)762-7878
22-09 126TH ST FAX 718)762-6287
COLLEGE POINT, NY 11356
Dave S: "He is THE
man, and you can tell him I said so."
Island Automotion LTD
Royze Mfg. Co.
O.K. says: I have used their kits and have been quite happy with the price/quailty of their products.
"Of course, if you need air filters you can always try http://su-filters.com !"
Tom Bryant's Blog with his EXTENSIVE AND DETAILED
EXPLANATION of an SU rebuild:
Contact Tom Bryant at: email@example.com
Matt B. says: "...does it all.
I've only ordered parts from them... never service. But I was more than
impressed with the parts and service."
...trying to determine linear distance along metering needle, for each flat of adjustment of the jet adjustment nut.
LD/flat = 1 / (TPI x 6)
Formula: D = P / 6
Where: D is linear
P is thread pitch
6 is the number of flats a Hex nut has
Question: What is thread pitch of jet nut?
Answer, thanks to Brickboard newcomer Mark (Toolguy): 0.0062" / Flat in other words, a full turn of six Flats, is 0.037" or a bit more than one third of a measurement increment up the needle. A Fun-Fact for the next Volvo Trivia night!
Link to Thread: http://www.brickboard.com/RWD/index.htm?id=1259135
External material sources are attributed. Otherwise, this article is Copyright © 2016. Ronald Kwas. The terms Volvo, University Motors, are used for reference only. I have no affiliation with any of these companies other than to try to keep their products working for me, help other enthusiasts do the same, and also present my highly opinionated results of the use of their products here, or to recommend viewing their highly informative presentations. The information presented comes from my own experience and carefully considered opinion, and can be used (or not!), or ridiculed and laughed at, at the readers discretion. As with any recipe, your results may vary, and you are, and will always be, in charge of your own knuckles!
You are welcome to use the information here in good health, and for your own non-commercial purposes, but if you reprint or otherwise republish this article, you must give credit to the author or link back to the SwEm site as the source. If you don’t, you’re just a lazy, scum sucking plagiarist, and the Boston Globe wants you! As always, if you can supply corrections, or additional objective information or experience, I will always consider it, and consider working it into the next revision of this article...along with likely the odd metaphor and probably wise-a** comment.