FAQs about Carburetors

  • Racing carb distributors.
  • Performance difference between SU and Strom .
  • Zenith carburetor tools.

    Racing carb distributors.

    Date: Thu, 16 Jan 92 10:31:09 EST
    From: wiegman (Herman L. N. Wiegman)
    To: John_E_Werner.Wbst311@xerox.com, wiegman@orion
    Subject: Racing Carbs



    Performance Innovations
    Steve Nelson/Dave LeVine
    (818) 764-6768 (CA)
    (800) 338-8190

    Mikuni american corp.
    8910 Mikuni Ave
    Northridge, CA 91324-3496
    (818) 885-1242
    (213) 873-2101

    there are many full line distributors (closest)

    Fox Distributing, Inc.
    504 East St. Charles Road
    Carol Stream, Illinois 60188
    (708) 690-9700


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    From: Chrome <ford@me.rochester.edu>
    Date: Mon, 16 Mar 92 12:23:49 -0500
    To: volvo-net@me.rochester.edu
    Subject: 144 dual carburettors
    Cc: ford@me.rochester.edu

    >2.) Last Thursday I bought a 1968 144 Volvo for a $100.00 dlls. I bought

    > just out of curiosity as I never had an older Volvo. It has the smaller B20 engine. It is a"peppy" car and looked OK. I have a number of questions to ask just to make the car run better.

    > a) the car has 2 carburators. How do one optimally adjust them?

    If your carbs are SU's, adjustment usually is a necessary thing. (like mixture adust on K-jets without lambda). Granted, the first time should take you an afternoon, or two sometimes, but it is rewarding work if the carbs are in good shape to begin with. After the initial adjust, they seem to require about 20-30 minutes attention about once a month. First, make sure they are full of ATF. Always. Then you have several things to think about. One, you want the carbs to be doing about equal work, and you want them at about equal mixture. The SU's are easy to determine in both cases, not half because you can actually see what's going on inside while the car is running (try this on a quadrajunk, if you dare stick your face that close to one...)

    Take off the air cleaners and fire up the car. Look inside the carbs and see if the pistons move exactly (or about) the same amount when you run the engine at different constant speeds. If so, they are balanced. If not, play with the linkage until they do so. If you blip the throttle, and they move different speeds but end up in the same place when you reach a constant rpm, your piston dampers are worn and unless you have some money, you shouldn't worry about it.

    Ok, that done, shut off the car. Raise the pistons until you can see inside the carbs. On the bottom of the venturi you will see the hole for the jet, and maybe the jet. The adjust screw is on the side of the carb (this is where Strombergs and SU's differ). Screw the screw until the jet is on the same plane as the bottom of the venturi (both jets should now be in the same place, in their respective carbs). Screw both down one and a half turns. That is your basic setting, and you can now start the car. Now is the hard part. You must now adjust each one separately until the car runs well, but you can't do one much more than the other or you will loose your balance.

    So move the screw on one carb slightly one way or the other. Note how much you moved it, if the car runs worse, move it back, if it runs better, move the other in the same direction. Be VERY patient with this part, because the better you do this, the better the car will run. If you loose your place, my recommendation is to start with the basic setting again (this is where it runs into two afternoons, cause its better to give up and come back to it sometimes). BTW, when I talk about direction, that means the direction of the jet, no necessarily the direction of the screw. Note which way you turn the screw to move the jet up or down on each carb.

    Sound easy? No problem. Good luck.

    BTW, do your ignition and valve adjustments FIRST. The car will never run well if your ign. timing is off. I found that about 20 degrees of advance was nice on mine, but it depends on what kind of gas you plan on running. Also, a part usually worth replacing is the timing gear - its not expensive, although it is kind of a bitch to replace, but it's fiber, so it is not everlasting. Also, when you do that, either find yourself a steel spacer (behind the timing gear - will not torque down properly without it), or turn the original one over, so that the least worn side shows. The steel ones came on the later models. When you replace your gear, replace the timing gear cover with one machined to accept a neoprene seal. These usually cut down the leaks. (This is not a terribly important update, and sometimes, if there is a nick in the crank where it contacts the seal, or if it is worn eccessively, it won't seal well anyway)


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    From: southern@neit.cgd.ucar.edu (Lawrence Buja)
    Subject: Tuning SU carbs
    To: volvo-net@me.rochester.edu (Volvo mailing list)
    Date: Mon, 16 Mar 92 10:41:54 MDT

    }If your carbs are SU's, adjustment usually is a necessary thing.

    } ...nice SU tuning article by roman saved to archive....

    This is a FAQ with us british-cars folks and one of the xperts put together a step-by-step procedure for tuning SU carbs. I've appended this below. Someone else put together a PC program for selecting needle sizes:

    >From: deeds@sed170.hac.com ( Dean Deeds)

    >In a nutshell, the programs will help you sort through the SU needle charts to find a needle of a particular profile (e.g., a

    >specified amount richer than your current needle within a given range), and do other similar manipulations.

    >The package should now be available in compressed, tared, uuencoded form by ftp from directory /pub/sol on our host machine

    >(hoosier.utah.edu, a/k/a autox.team.net).

    >The Haystack program on Hoosier is now stored as a self-extracting exe file. This was done to simplify loading onto PC's. Just FTP the

    >file suneedle.exe from HOOSIER.UTAH.EDU to your PC (binary mode) and run suneedle. The Haystack programs and manuals will be

    >extracted and placed in the current directory. This file should also be available from the file server at server@ingr.com; send a

    >message with a subject line


    >The Haystack manual is now in plain ascii text, instead of MS Word.

    >Thanks to Roger Garnett for enabling me to do this, and to Michael Burdick for making the original version available zipped. Some

    >people have reported difficulty in reading the manual, even with Word, so you may find it useful to get this new version with the

    >ASCII text manual. (Or just email me if that's all you need.)

    /\ Lawrence Buja Climate and Global Dynamics Division

    \_][ southern@ncar.ucar.edu National Center for Atmospheric Research


    From: garnett@theory.TN.CORNELL.EDU (Roger Garnett)
    Date: Tue, 9 Apr 1991 13:01:59 EDT
    Subject: SU Tuning, updated

    On Apr 8, 3:23pm, Daniel J. Dasaro wrote:

    } Can I get this car in tune with no more than a Unisyn?

    You bet! You don't even need a Uni-Syn, just a piece of vacumn line and your ear will do the trick. Which means, that it's about time for a posting of how to twiddle SU's. Hmm- it's been a year since I updated this, and I've been promising to add Type HIF info. So, here it is, the new, SU-it-yourself proceedure.


    Tuning Your S.U. Carbs

    -Roger Garnett Rev. 4/9/91

    Well, it's not really that hard to set up SU's, just different. Of course it always gets more interesting when you have more than one...There is a very good Haynes SU carb manual available, reccommended reading. The basic syncing process also applies to Zenith-Stromberg's, but the adjustment mechanisms are different. Here is a laymans guide to adjusting SU's (long):

    step 1- Tune up the rest of the engine- REALLY! clean or replace, and set the points, set the timing, plugs, valve lash, and remove the air filters. (have new ones ready) All of these things can affect the setting of the carbs, which should be done LAST, (if at all). The carbs rarely need to be adjusted, once set. Also replace/install the gas filter. Of course, it helps if the carbs are in good mechanical condition as well. But you can consider a rebuild once you have gotten things working first!

    step 2- clean the carbs! use gum-out or similar stuff, clean all external linkages, shafts, and stuff.

    step 3- Remove the float bowl covers, clean the float bowls, remove old sediment, and check/adjust the float setting. (turn the cover upside down, and get a *1/8" in drill bit, set the drill bit accross the cover, the float tab should just touch the bit.) Make sure the needle is moving and seating properly. This is just like *most* floats. Replace the cover.

    * This is for HS4 SU's- (1/8-3/16") if you are dealing with 1",

    H's, HS2's HS6's, HIF's, etc.- check the spec for your carb.

    Note: You can check for matching float settings, after setting the mixture, by removing the pistons, and peering down at the jets. The fuel level should be about the same on both carbs, a little below the top surface of the jet. (After car has been run only)

    step 3b- Go get a pint of ale, or something close, and set it nearby.

    step 4- Remove the piston covers. CAREFULLY remove the piston, DO NOT BEND THE NEEDLE. Set the piston down on a clean wadded rag to prevent rolling. Clean the inside of the carb. Check operation of the throttle. Check the throttle shaft slop- this is the most common place for wear on an SU, and is often where air/vacumn leaks occur. The bushings and shafts can be replaced, but it requires some machining. A small amount of leakage can be tolerated, the car just won't idle as evenly. Clean the piston. Stare in awe at the odd carborator design, simple and effective, (constant velocity). Dump the old oil out of the damper if you haven't already spilled it. clean. Reassemble, check piston movement, raise it, then let go, it should fall freely. If not, check assembly again, make sure the piston isn't binding against the carb body, it should ride only on the damper shaft. Do not strech the spring. When all is operating properly, fill the damper with Marvel Mystery Oil for light damping, or use motor oil for heavier damping. (I use MMO) If you get "flutter" on, acceleration, you might try the heavier oil.

    step 5- Start the car and warm it up, then turn off/disconnect/otherwise disable the choke mechenism. (Loosen the nuts on the clamps so that the choke stuff isn't doing anything) This will get set later. (Later Zenith-Strombergs have a thermostatic choke, not a cable.)

    step 6- Check coarse throttle adjustments- make sure the throttle cable pulls on both carbs equally, and returns completly when released. This is adjusted by loosing the set screws on the throttle shaft and matching the two sides. You can also adjust the cable length at this time, using the cable set screw/retainer at the end of the cable. You can check the float adjustments now by removing the piston & cover, and looking at the fuel level in the needle seats. Both carbs should be about even, a little below the top surface of the jet. If not, readjust one or both floats to match the level.

    step 7- Syncronize the throttles- if you have a uni-syn, here's your chance to use it, (or other air flow guage), if not use a tube and listen to the airflow. The Uni-Syn is much easier to use, and can result in better balance. Alternatly adjust the idle screw on each carb, attempt to set the idle as low as possible (~800-1000 RPM). Adjust until the airflow is *close* to the same at each carb. The engine may now be running rough, just keep the idle speed high enough to keep running. Give the throttle a quick snap to make sure everything is settled, then check sync again. Periodicly snap the throttle to make sure everything is seated. Large differences in where you can adjust the two carbs may indicate air/vacumn leaks, or other problems, such as a bad valve)

    Magic Time- Relax, and shake your voodoo rattle...

    step 8- Adjust the mixture- this is done with the spring-loaded hex fitting under the carb, where the fuel supply tube enters from the float. Turning the fitting raises and lowers the needle seat. Pick a carb, and turn the fitting 3 flats (1/2 turn), first in one direction, then back 3, then 3 in the other direction. Note where the engine runs better, idle speed should increase. Turn to the best setting. Repeat this proceedure until you get the best operation you can, (higest idle speed), keeping track of flats turned will help you remember where you were. If you get lost, turn all of the way in, then back out 12 flats and start again. Periodicaly snap the throttle and push up on the fitting to make sure everything is seated.

    Note: Type HIF carbs (With integrated float bowl) no longer have the hex nut to adjust the mixture. Instead, there is a screw to twiddle, on front of the front carb, and behind the rear. The screw is connected to the needle seat through a temperature compensated gizmo, which is said to make the carbs more stable. Adjustment can be done in much the same way, by counting turns/flats of your screwdriver. There is less adjustment range than with the the basic models.

    When you think you're close, stop, uncramp your fingers, breath deep, and do the same to the other carb. Then retune the first carb, and then the second again. This serves to match the mixture of the 2 carbs, and prepare you for the beer sitting over there in the sun. (why do you think the British drink warm beer?)

    step 9- repeat step 7, setting the idle speed as low as possible, and re-syncing the idles. Now go back and readjust the mixtures. After a couple of iterations, the engine should be running smoothly (controlled by mixture) and at a low idle. Repeat as necessary. Set the final idle to 800-1000 RPM, depending on the condition of the rest of the engine.

    step 10- DRIVE! Take a quick spin up the lane, you need the reward of driving a properly tuned car.

    step 11- Adjusting the choke- I won't get into the temperature compensation in the type HIF, or the Thermostatic choke in the later strombergs. Check the manual for more info. The choke is supposed to do two things; the first half of travel moves a cam on each carb which opens the throttle, for warm up. The second half pulls down on the needle seat to enrichen the mixture, for starting.

    Start with the choke in the off position (knob in). Adjust the so that the cam only starts moving the throttle after you start pulling out on the cable (adjust with shafts and adjusting screws). Try to get both carbs adjusted the same, so that both screws begin to hit the cam at the same time. This is not real critical, but you can use your Uni-Syn to match air-flow on both sides, with the choke partly engaged.

    After the cable is about halfway out, it should start engaging the lever which pulls down on the needle seats. Adjust the linkages so both carbs are acted on equally. You can do this by adjusting for even running of the engine. Of course, for a warm engine, the richness of this mixture will cause some roughness. Make sure the needle seats return freely when you release the choke.

    step 12- Drink that warm beer (only one, no DWI now...) it will taste great at this point!, go wash up, and go for a ride.

    The first time through carb adjustments can be confusing, once you've done it, all of the stuff in the manuals makes sense. Go back and read them again- As always, I reccommend Bentleys, which is a repro of the original factory manuals, and then Haynes, and throw out the Chiltons. (orginal factory manuals are to be read in a clean enviroment, repros are for smearing grease all over, except, if that's all you got, use it!) Haynes has an excellent manual just for SU carbs, it covers operation, theory, rebuild of all models, and has needle charts for hundreds of car/engine/carb setups. They also have a manual for Zenith-Strombergs, which, while similar, are a whole 'nother beast.


    Anybody who publishes this is required under penality of leaky throttle shafts to give credits, and send or bring a copy to me at the below address!

    Roger Garnett


    /___ _ \ | South Lansing Centre For

    /| || \ \ | Wayward Sports Cars

    | |___|| _ | | 39 Ridge Rd. Lansing, NY, 14882

    | | \ | | | | Rte. 34B, near the intersection of Rte. 34.

    \| \ |__/ / |

    \________/ | SAFETY FAST!

    Date: Fri, 20 Mar 1992 08:30 -0800 (PST)
    From: PRAX@slacvm.slac.stanford.edu
    Subject: NEW SU's
    To: volvo-net@me.rochester.edu

    I know this wont be of use to the newer volvo owners on the net, but for those of us with 60's vintage.....I have found a source of NEW SU HS4 carburetors for the sum of $349.95 (plus shipping,ect.) a pair.

    The place is Victoria British LTD. Box 14991, Lenexa, Kansas 66285-4991 phone 1-800-255-0088. Their catalogue also has a good collection of SU and stomberg carb parts. They are a MGB/MGC parts house, nice catalogue!

    Joe Praxel

    From: John_E_Werner.Wbst311@xerox.com
    Subject: Forwarding: SU carbs
    To: wiegman@orion
    Reply-To: John_E_Werner.Wbst311@xerox.com


    First, check that the chokes are hooked up to both SUs. When you pull the choke all the way (i.e. for cold starts), the jet assemblies on both SUs should lower making the mixture much richer.

    Second, clean the carbs with carb cleaner (i.e. gumout).

    Third, check to see that the dash pots are full. If not, refill them. I use 3in1 oil for the SUs and ATF for the strombergs. I have also heard some people say that for high performance applications you should leave them empty to get better throttle response.

    Fourth, check the ignition system. Time spent fiddling with the carbs will do little to solve fouled plugs.

    Haynes has a very good manual on the SU carbs. If the car makes it up to the Volvo-Net Meet, I can take a shot at tuning it there.

    BTW: It is quite possible that the SUs will are shot. and need a complete rebuild. Check the needles to make sure they are still round. Also check the jet seats. They should be round. If either of these is gone, replace them.

    Another thing to check is the area where the throttle shafts go into the carburator body. The carburator body is Al, but the shafts are steel. There are no bearings there, and over time/use the holes be come oblong, making it impossible to tune the carbs. If the throttle shaft has play, that's the problem. I have heard of a couple of places that will bush these areas with bronze bushings, but I don't have any information. I do remember that the cost was a bit high for something I could do myself, given access to a machine shop.

    This is a pretty good description of tuning SUs. It is basically what I do, but there are some minor differences.

    Some Notes:

    1. Don't play with the throttle syncing unless it has been already screwed with. It is real pain to resync them. Idle balancing is quite different and is very easy to do.

    2. For the SUs Volvo used, there is an adjustment nut on jet assembly. Turn the nut one way and the seat raises, making the mixture leaner. Turn it the other, and the jet lowers making it richer.

    3. A trick I use in adjusting SUs is to raise the dash pot while the car is idling. If the car idles faster, the mixture is rich (raising the dash pot leans the mixture). If the car slows down, the mixture is lean. (This may be backwards, it should be obvious when you try it.) To raise it too far, about 1/8 to 1/4 inch should do. Over raising it can cause the car to run lean and nasty things can happen then. On the bottom of the dash pot cover is a little rod you can push up to raise the dash pots.

    Good Luck,

    -- John

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    Performance difference between SU and Strom.

    Date: Mon, 23 Aug 1993 06:33:28 PDT
    From: werner.wbst311@xerox.com
    Subject: Re: Carburetors (SU vs. Strom)
    To: useins@mcl.mcl.ucsb.edu
    Cc: swedishbricks@me.rochester.edu
    Reply-To: werner.wbst311@xerox.com

    >I ask for net wisdom, who can explain the difference between how the SU and the Zenith Stromberg's will perform on a B20b,

    >1970. Does anyone have a definate opinion on which is better?


    The difference in performace between the SUs and the Stroms depends a lot on which particular models you have. In the US, volvo used dual SU HS-6 carbs up to about 1970, the switched to the Strombergs (I forget the model). If everything is in perfect condition, the carbs that were set up by the factory for the car (i.e. the stroms) will work the best.

    Now, things rarely are in perfect condition. Enter: _The_Real_World_. This is something that most engineers try very hard to ignore. Sometimes they can get away with it, but sometimes not. In the real world, the carburation adjustments that work in Sweden don't work in Texas. Just the temperature difference will cause problems. (Cooler, denser air requires requires the carburaters to be adjusted richer then Warm air.)

    My experience has been that given a good set of SU HS-6 carbs in perfect mechanical order, they will be much better in the real world then the Strombergs. Why? Because you can adjust the mixture on the SUs to compensate for the motor and atmospheric conditions. (Someone around here as an article on adjusting these beasts.) My experience has had the best luck with the SUs. I have never been able to get my Strombergs to start when cold. This is on a '70 145 with a B20 motor, and eventually on a the same '70 145 with a 2.1 liter, IPD cammed, re-worked head, B20. (I figure I am getting about 140 to 150 bhp out now, it would be around 165 if I went to Webbers or FI.)

    The trick is finding carbs in good condition. Here are the things to check:

    Both SUs and Strombergs:

    - The condition of the seat for the needle (in the main body). If the hole is oblong, it needs to be replaced. The condition of the needle. Look for any signs of wear. Replace if bent or worn. (* Seat and needle wear mean replacing both *)

    - The condition of the holes/bushings where the throttle shafts go through the body of the carb. If the seal is not air-tight, you will never get the mixture set. On the SUs, you can have a new, brass bushing put in there. Originally, there were no bushings between the steel throttle shaft and aluminum carb body.

    - The condition of float bowl filling valves.

    - The condition of the floats - They may become porus and start to take on fuel, which really does not help with making sure your float bowls don't overflow. (BTW: What large, very hot object is underneath the float bowls on a B20 motor?)


    - The condition of the rubber diaphram in dash pots. (There is bad joke of questionable moral value here that a mechanic once told me. I will leave it out so as not to create a stir. But, I did hint at it.)

    - A miriad of assorted 'extras' tacked on to carb to make it work in a more 'environmentally friendly' way.

    BTW: A big tip I have found on both SUs and Strombergs is the using of a fuel pressure regulator. With a brand new, high flow fuel pump and old floats, you can get enough fuel pressure to force the valves in the float bowls open. By putting a pressure regulator in, you can help prevent this.

    Good Luck


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    Date: Mon, 23 Aug 1993 08:17:57 PDT
    From: werner.wbst311@xerox.com
    Subject: Re: Carburetors (SU vs. Strom) + 122 Trick
    To: useins@mcl.mcl.ucsb.edu
    Cc: swedishbricks@me.rochester.edu
    Reply-To: werner.wbst311@xerox.com

    Oops, forgot something....

    The biggest problem I have run into on the Carburated B20 motors in the 140s (and later B18s in the 120s - 68 and on) is the intake/exhaust manifold unit. The units used on these cars had an integral air heater/exchanger and an extra set of butterfly valves. When idling, the air went through this section, was warmed up, and helped pull some of the engine breathing. Under full throttle, attachments on the throttle rods opened the buterfly valves in the intake manifold and created a normal (short) path into the engine. (And you though Ford's dual length intake manifold on the SHO was a radically neat idea :-)

    The problem is that the air box tends to get gunky, and leaks for around the shafts securing the butterfly valves in the intake part of the manifold.

    The best suggestion I have for this is to change the common intake/exhaust manifold over to seperate intake / exhaust manifolds. You will need to find an intake manifold from an eary 122 (pre 67), or a 123GT. This unit will be aluminum. Then, you want to find a dual exhaust manifold from a 123GT, or find a common unit that has been cut apart. I reffer to it as a dual exhaust manifold because it has 2 outlets, not one like on the earlier 122s. I have been told that FI exhaust manifolds also work (they are essentially the same thing).

    You may also want to consider getting a set of headers instead of looking for an exhuast manifold. JC Whitney had some of unknown quality for $75. The best used price I have seen for the exhaust manifold is $100. (BTW: The factory race prep manuals say use the exhaust manifold with 2 pipes of 6" that join into a 2.25" pipe. It does not recommend headers for stage III and above motors.)

    (PS: One quick trip on a 122 for more power is to replace the single down pipe / exhaust manifold exhaust with a later 2 down pipe / exhaust manifold unit. The later 122s (68 and on) had the dual head pipes with either a common or split exhaust / intake manifold setup. Get the split setup and the later head pipe.)


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    Zenith carburetor tools.

    Date: Mon, 23 Aug 93 22:21:01 EDT
    From: " Michael Rensing" <mrensing@irus.rri.uwo.ca>
    To: swedishbricks@me.rochester.edu
    Subject: Zenith carb tools

    Well, I have been messing with the Zenith 175CD carb on my B21A engine for a while. The main problem has been that once I figured out the adjustments, I was facing a continuous loss of damping oil from the damping piston. Thanks to Scott (who may not be on the net right now) I have a copy of the manual pages relevant to the carb. The only possible fault which leads to oil loss according to the troubleshooting section is leakage from the o-ring on the metering needle adjustment screw. I have been avoiding this repair, since we only have the one car, and I have not had the confidence to try and find out whether I can get it back together again.

    Now, thanks to a former 'phibe (still owns two Volvos, but withdrew from the net) I have a spare carb!! I have put his carb in, and it appears to run better, but my old carb was professionally rebuilt last year, so I'd like to solve the final problems out, and re-install it....

    The problem is that special tools are required to remove, replace, and adjust the metering needle (and the seat). I know how to make a simple doo-dad to adjust the mixture, but pounding on the brass parts with something like that does not seem wise. I could spend a day or so in the machine shop making some of these tools, but before I invest the time, does anyone know how easy it is to get these parts, where to get them, and how much they cost??

    The tool numbers (from the Volvo manual) are:

    5195 (for installing and adjusting the metering jet)

    2962 (a drift for removing the jet)

    2897 (a press from above the jet)

    2895 (a press from below)

    Any suggestions about how I could replace these tools with an adequate cheap alternative would also be appreciated. Since I plan to keep this car for a while, I figure I'd better master the carb, or forever be at the mercy of $65(Canadian)=$90(U.S.) per hour mechanics who can't do the job right anyway...

    Direct replies would probably make sense; if there's enough input, or the results seem to be of general interest, I'll post a summary.


    Michael (and Miles the blue 242)

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