Some History: The “S.U.” in S.U. carburetors stands for Skinners Union. At the turn of the last century, there was a shoe company in England called Lilley and Skinner. The Skinner side of the company was one George H. Skinner, a genius of sorts, and in 1905 he was granted a patent on the SU carburetor. He soon teamed with his brother, Thomas Carlyle Skinner to produce a working model. Their small workshop was located in London. Using their knowledge of the shoe industry, it’s not surprising to learn that the first carburetors used leather bellows. As production increased the business moved to a larger facility, still located in London. WW I and the recession that followed slowed production, but the company managed to survive. Indeed, several quality car manufacturers of the time such as Bentley, Invicta and Napier used SU carbs on their engines. Morris Motors purchased the S.U. Carburetter Company in 1926 and the operation was transferred to Birmingham alongside the Morris Commercial Cars, Ltd. Factory. Morris Motors formed the S.U. Carburetter Company, Ltd. in 1936 after total acquisition. With the onset of WW II, the company manufactured carbs for Spitfires and Hurricanes of Battle of Britain fame. In 1976, the company became a part of a division of Service and Parts for British Leyland called SU/Butec, which eventually failed causing SU to lose its name and become Austin Rover Fuel Systems. Fortunately for us, a company called Burlen Fuel Systems, Ltd. rescued the SU logo and it is this company that presently manufactures all SU carbs prior to the HIF44E (horizontal integral float) and SU electrical and mechanical fuel pumps. Burlen’s web site is www.Burlen.co.uk.
Preface: Before turning any nuts and bolts, let’s review the basic design of the SU carb. The SU carb is a semi-downdraft, variable venturi carburetor—semi-downdraft because the carb tilts toward the engine and variable venturi because the size of the venturi varies. The end result is a constant vacuum across the jet bridge. The SU carb is also sometimes referred to as a variable choke, constant vacuum, or constant velocity carburetor. Almost all other carbs are fixed venturi carburetors requiring extra bells and whistles—choke valves, idle and acceleration pumps, ad infinitum—and all of which produce a variable vacuum across the throat of the carb. The SU carb is a model of simplicity. Its design, engineering, efficiency, economy and utility are nothing short of ingenious. Once adjusted, it seldom needs fiddling with. A wide variety of variable-jet carburetors are manufactured by SU. They differ only in the method of jet assembly and the means of feeding fuel from the float chamber to the jet. The H-type is fairly widely used and is produced in a range of throttle body sizes.
In the SU carburetor, the piston is guided by a bush fitted in the suction chamber and is a close fit, which is necessary to prevent excessive air leaking past the piston. The piston has a hollow stem filled with oil and some carbs have a plunger (damper). The sliding piston controls the area of the venturi throat and the position of the piston is determined by the degree of throttle opening. If the throttle is almost closed, as when the engine is idling, the flow of air through the venturi drops. The weight of the piston (plus, in some cases, a spring) causes the piston to fall, leaving only a small gap for the passage of air.
When the throttle is opened by depressing the accelerator pedal, the swifter passage of air through the venturi increases the partial vacuum above the piston. This causes the piston to rise and further increase the flow of air into the engine.
A tapered needle attached to the piston and passing into the fuel jet controls the flow of fuel. As the piston rises, the needle rises also, allowing more fuel to be drawn from the jet. The main air passage is connected to the suction chamber by a passage in the piston, so that air can be drawn from the chamber. The position of the jet and the shape of the needle ensure the correct proportion of fuel and air. The internal combustion engine runs on a mixture of air and fuel equal to roughly 15 parts air to 1 part gasoline, by weight. The actual mixture may vary between 17:1 on the weak side to 13:1 on the rich side.
On some SU carbs, enrichment of the mixture when accelerating is provided by a damper, which slows the rate of rise of the piston when the throttle is opened. This increases the partial vacuum at the fuel jet and so provides a temporary enrichment.
Since the air pressure in the venturi remains reasonably constant at any given engine speed, there is no need to provide a separate fuel circuit for idling, as in the fixed-jet carburetor. The fuel is fed into the air stream at the point of maximum velocity, ensuring efficient atomization of the fuel.
The float chamber in an SU is separated from the rest of the carburetor and is attached by a single bolt, either rigidly or with a flexible rubber mounting.
Fuel is fed to the jet through the base of the float bowl or by a flexible pipe, depending on the model SU carburetor installed. The position of the jet can be altered by sliding it in a long bearing pressed into the carburetor body. Adjustments are made by means of a nut or a remote screw and a linkage.
For cold starting, pulling out the choke control on the car’s instrument panel lowers the jet assembly about 3/8 in. This ensures a larger fuel discharge area and gives the required rich mixture.
Tuning the SU Carb First Things First....Preparation:
The Float Chambers: In order for the carburetor to meter out fuel accurately, the fuel level in the jet must be kept more or less constant. The fuel level in the float chamber (or bowl) controls this. The fuel level in the fuel chamber is in turn controlled by a float-operated needle valve or, alternatively, a preferred Grose jet. This works much like the shut-off valve in a toilet tank. When the fuel level drops, the float drops with it, allowing the valve to open. When the engine is running, the valve is constantly opening and closing to keep the fuel level approximately constant.
- Clear or clean off a spacious area on your workbench. Various bits and pieces of the induction system and the carbs are going to be removed and it’s important that they be kept clean and sorted.
- Fix some good lighting on the carbs. You’re going to be delving into, around and under the carbs and you need to be able to see what you’re doing.
- Nobody likes to work on a dirty engine, especially dirty carburetors. Spray some carburetor cleaner wherever it’s needed, but have a spray bottle filled with water standing by. Why? Because carb cleaner can cause diecast aluminum to “bloom”, ruining the finish. Spray on the cleaner, work the hard stuff with a brush, then rinse immediately using a water-filled spray bottle.
- Make sure you have a drip pan under the engine. This will catch any liquid drippings and loose parts that slip through your fingers. Don’t put the carb cleaner away just yet.
- Remove the air cleaner(s) and any air cleaner ducting. While they’re off, this might be a good time to get out the aluminum polish, give them a polish and any parts a new coat of paint, if needed. “Blue Magic” is an excellent aluminum polish and can be purchased at most auto stores.
- Start the engine and warm it up. When warm, adjust the throttle to 2500 RPM and spray carb cleaner into both carb’s intakes. This will remove any gummy material that may be lurking on the insides. When finished, be sure and turn off the ignition.
- Disconnect the linkage between the jets, letting it hang from the aft jet. Disconnect the mixture control cable from the bottom of the aft jet linkage and unhook the tension springs from the jet levers.
Fuel Level: The fuel level need not be exact, as normal jet adjustment will compensate for minor variations. However, the standard measurement for all HS carbs is 3/8 inch below the top surface of the jet bridge, and it should not deviate too much from that. Some of you may have noticed the 3/8-inch specification as listed in your workshop or operators manual instead of the 7/16-inch specified by the S.U. Carburetter Company. No one knows why MG chose to deviate from the standard SU measurement, but it doesn’t really matter. Either dimension will provide a fuel level in the jet that is within acceptable range with no difference in performance.
The Float: Using a piece of wire with a small hook bent in one end, fish the float out of the chamber. Dry off the outside, then shake vigorously. If it rattles, or if there the sound of liquid swishing about, the float should be replaced. Also, if there was a lot of grit in the bottom of the bowl, some of it may have gotten into the small passageway which transfers fuel from the bowl to the main body of the carburetor. Remove the bowl by undoing the bolt that attaches it to the bottom of the carburetor. Don’t lose the sealing washers! Rinse out the chamber thoroughly and attach it back to the carburetor. When reattaching, make sure the bowl is at right angles to the carb body (looking straight down). Don’t forget: the three washers that go under the arm of the bowl are comprised of two fiber and one brass washer. The brass washer goes in between the two fiber ones. A larger fiber washer goes between the bowl arm and the body of the carburator.
- Remove the large banjo bolt that attaches the fuel line to the lid of the float bowl. Be sure and save the two washers, spring and wire mesh filter from the bolt hole. Then, remove the hold-down bolt that passes through the top of the lid (more washers to save), push the overflow pipe aside, and lift the lid. You may have to rotate the lid one direction or the other to get enough space to lift the lid off the bowl. Clean the filter if it needs it.
- Turn the lid upside down to see the float lever and, under it, the needle valve. Remove the pivot pin, lift off the lever, and lift out the needle. If its conical tip is grooved, install a new needle and seat. If you’re not sure whether the old needle is usable, insert the needle back into the seat and blow through the fuel line opening in the lid holding the needle against the seat with light finger pressure. If no air leaks through, then the old assembly is probably okay to use.
- Examine the pivot pin and install a new one if it is badly grooved where the float lever bears on it. Prior to 1955, the pin was a slip fit into the lugs on the lid, but most pins made since then are knurled on one end to provide a press fit. Either type is suitable, as long as it is not worn.
- Examine the float lever. If there is any obvious damage, replace it with a new one. Both arms of the fork should be equally curved, and the portion of the lever between the fork and the pivot pin should be perfectly straight. The area where the lever bears on the needle valve may be shiny, but it must not be deeply grooved. When satisfied, reassemble the parts back into the float lid.
- With the lid assembly still upside down, insert the shank of a 7/16 inch drill bit (or bolt) between the lid and the inside curve of the forked end of the lever. The lever should rest on the test rod and on the needle valve at the same time without depressing the spring-loaded pin in the nylon-bodied needles of later design.
- If it doesn’t, bend the lever carefully at the point where the curved fork joins the straight section, being careful to see that the straight section remains straight. Also, make sure both prongs of the curved fork rest equally on the test rod.
The Suction Chambers and Pistons: The suction chamber assembly is the heart of the SU carburetor and is the major design difference between the SU and most other types. It is the rise and fall of the piston under the influence of vacuum in the chamber that changes the size of the venturi and moves the needle in and out of the jet to tailor the fuel/air mixture to the engine’s needs. If something goes wrong with this assembly, the carburetor won’t work, so it should be cleaned and inspected every time the carbs are tuned, or at least once a year. The pistons are not interchangeable from one suction chamber to another, so when working on dual carb setups, work on one carb at a time.
- Reassemble everything making sure you put the float back in and that the fiber seal for the lid is replaced. Make sure all sealing washers, including the spring and filter are reinserted in the fuel inlet. Use a little anti-seize compound or grease on the banjo bolt and lid bolt threads to keep them from freezing in place. Tighten the bolts firmly, but don’t overdo it, as the threads in the diecast lid are easy to damage.
- Testing The Fuel Level: With everything put back together and the float bowl lids reinstalled, check for leaks, then remove the suction chamber dome and piston from the carburetor body. Turn on the ignition switch to activate the fuel pump. Pull down the jet to its lowest level and peer down into the top of the carburetor and onto the jet bridge. You should see the fuel level above the top of the jet and below the jet bridge itself. Ideally, the level should be 3/8" below the jet bridge, but this is hard to measure. With the jet in the lowest position, any fuel level above the jet head and below the jet bridge is satisfactory. If this condition is not met, then you must turn off the ignition and remove the fuel chamber lid and reset the fuel lever as necessary. This may involve a slight deviation from the 7/16" specification called for between the lid and the fuel lever, but it’s critical that the fuel in the jet bearing be brought to the correct level as stated above.
Having got this far, you now have to start all over and repeat the whole procedure with the second carb. The mixing of needles, springs and dampers from one carb to another is not critical, but under no circumstances should you switch the piston from one carb to the suction chamber of another. Pistons and chambers are assembled into matched sets by selective fit to ensure the correct clearance between them. However, it’s okay to com-pletely switch the complete chamber/piston assembly from one carb to another.
- Disassembly and Cleaning:
- To remove the suction chamber, first unscrew the cap at the very top. The cap may or may not have a rod and plunger assembly (damper) attached to it. Unscrew the two or three screws that secure the suction chamber to the main casting of the carburetor. If yours is a two-screw model, mark the chamber and the carb body so the chamber can be returned to the same position later on. Lift the chamber straight up without rocking it to avoid damaging the needle. As you lift, look underneath to see if there is a large coil spring between the suction chamber and the piston. If so, don’t let it fly away! Now lift the piston out of the carburetor body, again being careful of the needle. If you found a spring in the assembly, you may also find a steel or bronze thrust washer down inside the piston where the spring rests. Don’t lose it.
- Examine the inside of the chamber and the outside diameter of the piston. Both must be spotlessly clean. If not, wipe them off with a rag dipped in gasoline. If the dirt seems to be baked on, use some of your carburetor cleaner to free it up, but use it sparingly and rinse it off quickly. As stated earlier, some types of cleaner will make the diecast aluminum “bloom” slightly if left on too long. This isn’t important on the outside of the carburetor, but it can close up the critical clearance between the piston and the suction chamber on the inside.
- When the parts are clean and dry, put a drop or two of oil on the steel piston rod and insert it into the chamber. Don’t put oil on anything else! Move the piston out of the chamber slowly while spinning it to distribute the oil over the rod and its bore. While doing this, listen carefully for any scraping sounds that indicate that the outer edge of the piston is rubbing on the chamber wall. If you do hear scraping sounds, try lining up the piston inside the chamber in its normal operating position, as determined by the keyway in the side of the piston and the original orientation of the chamber to the body. If the piston slides in and out in this position without scraping, then all is well. Usually, the problem will be a nick or burr on the surface of either the piston or the chamber, which must be worked down flush with the surrounding metal using a super fine file or a scraper.
- The Needle:
- Insert the piston into the chamber and spin it, observing the tip of the needle. If it wobbles, it is bent and should be replaced. If it seems to be straight, examine it for shiny marks on the side. If there are any, it means the needle has been scraping the side of the jet, usually due to an incorrectly centered jet assembly. This calls for a new needle. Ideally, you should also replace the jet, since the rubbing will have enlarged its opening.
- Remove the needle by loosening the setscrew in the side of the piston near the bottom. If the needle is stuck, you can grasp it with pliers, but only at the very tip. Pull it out with a twisting motion, being careful not to bend it.
- You should see numbers and/or letters stamped on the shank of the needle where it fits into the piston. These indicate the size of the needle, and you should confirm yours is the correct size. In every case, unless you race your car or are driving it an altitude of more than 6,000 feet, you should be using a needle marked ES.
- Assuming the needle is okay, reinsert it into the piston so its shoulder is flush with the face of the piston. To prevent future sticking, it helps to put a slight smear of anti-seize compound on the shank of the needle before inserting it, also on the threads of the setscrew. Tighten the screw firmly once the needle is in the correct position.
- Reassemble by lowering the piston into the carburetor body, being careful not to bend the needle or nick the outer edge of the piston. Install the spring and thrust washer, if your model requires them (TD MkII, TF, TF 1500). If one end of the spring is smaller than the other, the smaller end should go toward the piston and a thrust washer should be used. If both ends are the same, as is the case with recently manufactured springs, then it doesn’t matter which way the spring is inserted and no thrust washer is required. If you have a TD Mk II/TF/TF 1500 and your car doesn’t have springs, order some. These cars will not run well without them due to an excessively weak mixture. The springs are color-coded, so make sure you specify the correct color for your car. The TD Mk II uses red-colored springs, and the TF/TF 1500 uses blue-colored springs.
- The Damper: The purpose of the damper is to slow down the rise of the piston when the throttle is opened suddenly. The resulting high vacuum over the jet enriches the mixture momentarily, serving the same purpose as the acceleration pump found on most “normal” carburetors. Dampers improve acceleration from low speeds, so you may want to retrofit them to your carbs if you don’t already have them. Damperless carbs are fitted with a plain brass cap at the top of the suction chamber.
- Dampers were used on all TDs and most TCs and most TFs, but not on TAs, TBs and early TCs.
- Examine the cap to see if it has a small (1/16") vent hole in it, then examine the suction chamber to see if it has a 3/16" vent hole inside the small top section below the threads for the cap. You must have one or the other, but not both. The TF carburetor is the so-called “dustproof” type with a vent hole in the chamber neck, and dustproof carbs may have been used as replacements on earlier models. Dust-proof carbs must have non-vented caps. If you find yourself with the wrong type, drill a 1/16" hole in the cap or plug up the existing hole, depending on which is required or order new parts.
- Fill the hollow piston rod to within 1/2" of the top with SAE 20 motor oil. Insert the damper and screw the cap down firmly. These caps tend to loosen due to vibration and the action of the dampers, so don’t be too gentle. Unvented caps must have a sealing washer under them, but check to see if it’s really there, as they are easily lost. Washers are not required on vented caps, but it’s a good idea to use them anyway.
Fine Testing and Tuning: Some manuals tell you to listen to the sound of the exhaust while the engine idles. Forget it. The preferred method, which is considerably more accurate, is to lift the piston a specified amount and observe the effects of this on idling speed.
- Centering The Jets: After the suction assembly has been cleaned and refitted to the carburetor, you must make sure that the jet is centered in relation to the needle. The entire length of the needle must be able to enter the jet without touching the sides of the jet opening. If it does touch, the needle and jet will both wear at the point of contact. The resulting enlargement of the jet opening and reduction in needle diameter will diminish the carburetor’s ability to meter out fuel accurately. The mounting of the jet bearing assembly in the bottom of the carburetor is designed in such a way as to allow enough lateral movement for centering purposes. Once the correct position is found, the assembly is locked in place by a large nut.
- Having previously delinked the jet lever and unhooked the tension spring, now remove the pin that attaches the lever to the jet head and swing the lever out of the way. Mark the side of the jet facing away from the engine so it can be returned to the same position, then grasp the jet head and pull the jet straight down out of the carburetor. Unscrew the jet-adjusting nut, remove the locking spring, and screw the nut back as far as it will go.
- Inspect the outside diameter of the jet. It should be smooth, with no signs of grooves or uneven diameter. If the opening at the top is obviously oblong instead of round, replace the jet. The probability is that if the opening is oblong, the needle needs replacing, too.
- The standard jet for all T-types has a .090 in. opening. You will occasionally find that some misguided previous owner has mistakenly installed a larger jet (usually .100 in.). Jets are marked with a “9” on the jet head, identifying a .090 in. jet, or with a “1” to identify a .100 in. jet. If you can find no such marking on your jet, use a 3/32" drill as a crude gauge. It should be impossible to insert the shank of the drill into the jet. If it will fit in, then the jet is either very worn or the wrong size. In either case, get a new one.
- Smear a very light coating of petroleum jelly on the outside of the jet, and then insert it back into the carburetor. Push it up until the jet head abuts the adjusting nut. Make sure the side you marked earlier is facing the right direction if you are installing the old jet. If you are installing a new jet, rotate it until the jet head is correctly positioned to accept the jet lever. In either case, keep the jet in that position throughout the rest of the tuning process. The opening in the top of the jet is not always concentric with the body of the jet. If, after centering it, you rotate the jet 180°, you may find that it is no longer correctly centered on the needle.
- Reach into the mouth of the carburetor, lift the piston a bit, and let it drop. If the air cleaner is still in place, you can still lift the piston. The TF carbs have lifting pins in the flange under the suction chamber. Simply push the pin up as far as it will go, then let go. Earlier carburetors do not have lifting pins, but do have vent holes in approximately the same position. Insert a stiff wire into the vent hole located under the base of the carburetor to lift the piston.
- When you let go of the piston, it should drop against the jet bridge with a metallic click. If you hear a click, the jet is centered properly and need not be fiddled with. If you don’t hear the click, then the needle is rubbing on the jet and preventing the piston from dropping freely to the jet bridge. This jet needs to be re-centered.
- There are two methods for centering jets, either one of which does a satisfactory job.
- Most MG suppliers sell a simple jet-centering tool that consists of a round bar with a smaller diameter bar on one end. The tip of the small bar is the same size as the opening in the jet bushing—which is adjustable for centering the jet.
- Remove the suction chamber and take out the piston. Reattach the chamber to the carburetor body being careful not to tighten down the side screws too tight which might warp the chamber. Loosen the large nut locking the jet bearing assembly. With the cap off the chamber head, insert the jet-centering tool into the top of the suction chamber and lower it down onto the jet bridge. The small end of the centering tool should enter the jet assembly. If it doesn’t, grasp the nut locking the assembly and wiggle the jet bearing (the threaded piece onto which the jet adjusting nut screws) until the centering tool drops into the assembly. Tighten the nut. The jet should now be centered.
- The second method involves slacking off the large jet locking nut until it is just possible to rotate the bottom of the jet bearing. Insert a thin screwdriver or similar implement into the top of the suction chamber and push down gently on the piston rod. At the same time, wiggle the jet assembly gently to help it move, while keeping pressure against the jet head to prevent the jet from dropping. By pushing down on the piston and up on the jet, you will push the thickest part of the needle into the jet opening, thus forcing the jet to assume a position concentric with the needle. Now tighten the jet locking nut in its new position.
- Lift the piston again and let it drop to see if you get the necessary soft click, still holding the jet tight against the adjusting nut. If not, loosen the locking nut and try again. If you are unsuccessful, withdraw the jet, remove the adjusting nut and reinsert the jet. With the adjusting nut removed, you will be able to push the jet higher than before, making the centering action more positive. When you think you have it right, test your work by listening to the click with the jet in the fully up position and again with the jet fully down. If the click has a sharper sound when the jet is down, you have to try again.
- Now remove the jet again, unscrew the jet adjusting nut, replace the spring, and replace the nut. Screw the nut up as far as it will go, then back off one full turn, or six flats. This is a reasonable starting point for the final jet adjustment. Insert the jet again and push it up tight against the adjusting nut.
- Synchronization: One of the major goals of a tune-up is to ensure that all cylinders are doing approximately the same amount of work. If the engine has two carburetors, this cannot be achieved unless both carbs are doing equal work. The throttles must be set to operate in unison so that the same amount of air is drawn through both carbs. This is called synchronization. It’s also necessary to ensure that both carbs mix the same amount of fuel with the incoming air. This is called mixture adjustment. Mixture strength is determined in part by the amount of air passing over the jet opening, and this airflow is determined by the throttle setting, so it should be obvious that the throttles must be synchronized before the mixture can be adjusted. For some unknown reason, several of the tuning manuals deal with synchronization and mixture adjustment in reverse order. The carbs must be synchronized first, regardless of what your favorite manual might seem to imply.
- Hose method: This method involves actual measurement of the airflow through both the carburetors while the engine in running. Most SU carb manuals suggest that you use a piece of tubing as a crude stethoscope. I don’t recommend this method as it assumes your hearing is good enough to discern small differences in volume and that you haven’t suffered a hearing loss by exposure to jack hammers or the neighbors’ kid’s boom boxes.
- Uni-Syn method: This instrument, which can be purchased cheaply from most MG parts suppliers, fits over the intake end of the carburetor and measures the vacuum at that point. When the readings are identical for both carbs, then both are drawing the same volume of are and are dynamically balanced.
- Loosen the throttle connecting spindle clamp and turn the throttle adjusting screws in or out as necessary to make corrections which may be required. When you are satisfied that the airflow is identical through both carburetors, retighten the clamp bolt(s) on the throttle connecting spindle. Finally, adjust the idling speed to between 700 rpm and 800 rpm by turning both throttle adjusting screws in or out exactly the same amount. Once the throttles are synchronized, any change in the setting of one adjusting screw must be duplicated exactly in the other screw.
- Mixture Adjustment: Now that the carbs are clean inside and out, the float levels are adjusted to specs, the jets are centered, and the throttles are synchronized, you are ready to adjust the mixture. This is the part of the SU tuning procedure which seems to baffle so many owners and which has contributed greatly to the SU carburetor’s undeserved bad reputation. A large part of the problem may be the way the procedure is described in most workshop manuals. However, be warned that the procedures described in the manual are essentially correct, whether or not you understand it as written. If you analyze the procedure carefully, you will find that it really isn’t much different from adjusting the idling mixture on a “normal” fixed-venture carburetor. The major difference is that on most other carbs you turn a screw to change the mixture, while on the SU you turn a nut. As stated in Part I of the tech session, the SU has one very big advantage over other types in that it provides a means for testing the adjustment to make sure it is correct. There’s no easy way to do this on most other carbs.
- Preparation: Mixture adjustment may be done with the air cleaners on or off, according to your preference, but there are advantages to leaving them on as you will see.
- Having centered the jets, screw the jet adjusting nuts to their topmost position, then lower them one full turn (six flats). Make sure the jet heads are tight up against the adjusting nuts. This is a good preliminary setting for the jets and ensures that both jets on dual-carb engines start off in the same position.
- By turning the adjusting nut, move the jet up or down until the fastest idling speed is achieved. The initial setting of six flats is usually too lean, so begin by turning the adjusting nuts down one flat at a time to enrich the mixture. The engine will speed up as you enrich the mixture, but will eventually reach a point where it begins to slow down again due to an overly rich mixture. When it does, turn the nuts back up again until the highest idling speed is reached.
- The mixture should now be approximately correct for the speed at which the engine is running. However, that speed will now be somewhat higher, so turn the throttle adjusting screws out a bit to obtain the ideal 700 rpm – 800 rpm range. The mixture may be a little too rich now for the idling speed, so raise the adjusting nuts a bit. You may have to go back and forth between the jet nuts and throttle screws several times, but eventually you’ll reach a point where the idling speed does not exceed the recommended range when you adjust the jet nuts.
- Things to Remember: On dual-carb models its imperative that you move both throttle screws the same amount when you adjust the idling speed, and that you move both jet nuts the same number of flats when you adjust the mixture. Normally, the jets will move up or down as you adjust the nuts due to the tension of the jet lever spring, but if not then you will have to help it along with finger pressure—pushing the jet up against the jet bearing.
Finishing Up: When you are satisfied that the mixture is set correctly, install the jet lever return springs and any other miscellaneous parts you removed in the course of the tuning procedure. On dual-carb models, leave the rod that connects the two jet levers together disconnected for the time being. If you had the air cleaners off while you tuned, put them back on now. Most air cleaners restrict air flow at least slightly, so the mixture may be too rich. This can be determined by lifting the pistons as already described, then adjusting the nuts accordingly. It will usually be necessary to raise the nuts a flat or two after installing the air cleaners. This isn’t necessary if you tuned the carbs with the air cleaners in place.
- Using a knife blade or piece of wire through the lower vent hole (air cleaners installed), lift the piston of one carburetor about 1/32". This need not be a precise measurement, but be aware that it amounts to only a slight nudge upwards. Lifting the pin increases the size of the venturi and, with the throttle still in the closed position, the actual airflow through the engine remains unchanged. The resulting decreased vacuum at the jet opening weakens the mixture slightly.
- If you have adjusted the jet position correctly, the engine should speed up for a moment, then settle back to only slightly above the original idling speed.
- If the jet setting is too low, giving a rich mixture, the idling speed will increase noticeably and stay there without dropping off.
- If the setting is too lean, the idling speed will drop off and the engine may even stall.
- As you lift the piston beyond the 1/32" spec to 1/4", thereby weakening the mixture even more, the engine should start to slow down and by the time you reach 1/4", the engine should stall. If it begins to slow down or stalls after only 1/32", then the mixture is too lean. If it continues to run at 1/4", then the mixture is too rich.
- Remember that when adjusting two carb models, the reaction to lifting the piston on one carb is not as dramatic as on a one carb model because only two cylinders are affected. Also, because of the balancing effect of the manifold, a change in the jet position on one carb may necessitate a change on the other carb, even though the second carb may have tested out okay the first time you tried it. This may require going back and forth between carbs until both test properly. You may come to a point where both carbs test just right with equal turns of the jet adjusting nut, but this is not likely to be the case. You may have to make minor adjustments on one carb to compensate for the adjustments made to the other carb.
- The objective is to get exactly the same reaction from both carbs when you lift the pistons individually. The key to success is to work slowly, moving the adjusting nuts equal amounts and testing after every adjustment until at least one of the carbs tests okay, then make whatever minor adjustments are necessary to bring the other carb up to snuff.
Choke and Fast Idle Adjustment: The TB and TC carburetors have two separate controls to aid in cold starting: the choke or “mixture control” which provides an enriched mixture when the dashboard knob is pulled, and a hand throttle or “slow running control” which increases the idling speed when its dashboard knob is pulled. The TD and TF have only one control knob combining both functions.
Inability to Achieve Slow Idle: Usually caused by air leaking into the induction system somewhere between the throttle butterflies and the intake valves. Check the throttle spindles for looseness, the ends of the balance tube, and all gaskets for leaks. Any source of air leaking into the system without being controlled by the throttle will make it impossible to adjust the carbs properly. Check for leaks by using a handheld butane torch (unlit, of course) and directing the nozzle in and around the gaskets and spindles. A change in RPM indicates a leak.
- Choke: The connecting rod which links the two jet levers together must be adjusted so that both jets are lowered simultaneously the same distance. First, make sure both jet heads are tight up against their adjusting nuts, then adjust the length of the connecting rod so the connecting pin can be inserted without altering the position of the lever. Pull back on the rear jet lever and release it slowly to see if the return springs are able to pull both jet heads tight up against the adjusting nuts. A drop of oil on each of the pins (three per carb) often helps, and should be applied in any event to prevent wear.
- Reconnect the choke cable to its jet lever, leaving enough slack to allow the knob to be pulled out about 1/8" before the jet begins to move. If the cable is connected without any slack, it can prevent the jet levers (and therefore the jets) from returning to their normal running positions when the knob is pushed in.
- Fast Idle: To adjust the slow running control, turn the adjusting screw until there is about .016 in. clearance between the screw tip and the rocking lever. Make sure you turn the right screw or you will upset the synchronization on dual-carb models. On the TA, TB and TC, the screw you want is the rearmost one on the front carb. On the TD and TF it’s also the screw closest to you as you face the side of the engine, again on the front carburetor. The purpose of the small gap is to make sure the fast idle mechanism can’t hold the throttle slightly open even when the knob is in the full off position.
Inability to Achieve Lean Enough Mixture: Suspect either a incorrectly positioned needle, incorrectly adjusted float lever, a leaky float valve, or a leaky upper jet cork gland washer.
Jet Nut Position Not Equal on Both Carbs: Although you began the mixture adjusting procedure with both jet nuts six flats down from their topmost position, by the time the mixture is correct you will probably find that the final positions are not the same for both nuts. This is perfectly okay. However, as a rule of thumb, all is okay as long as the difference amounts to no more than two full turns (twelve flats). If greater than that, suspect and incorrect float level, incorrect needle setting, or a leaky jet as described earlier. Otherwise, the only cure is to rebuild the carburetors.
Sticking Jet: If, while you were adjusting the mixture you found that the return spring was unable to keep the jet head tight up against the adjusting nut, then something is causing that jet to stick. This is often cured by pulling the jet lever back as far as it will go and smearing a dab of petroleum jelly on the shaft of the jet, then working the lever back and forth. If there is a burr or blemish on the outside of the shaft, the best bet is to replace it.
Poor Cold Starting: Make sure the ignition equipment is in good order before you start fiddling with the carburetors. At least 99 percent of all cold starting problems are caused by the ignition system, not by the induction system, so make sure the points and plugs are adjusted correctly and that you have a good spark.
Leaks Around the Carbs: When the carbs have been rebuilt and reinstalled, the bane of the rebuilder is leaks that occur where the banjo bolts and the bolts that attach the float bowls to the carbs are located. Because of the hazy line between getting it just right or over-tightening and stripping the threads, stopping these leaks is more an art than science. Over the years, I’ve developed a technique that almost always works.
To begin with, when reinstalling the carbs, studiously avoid overly tightening the banjo bolts that connect the fuel lines to the float bowls and the bolts that attach the float bowls to the carbs. Your objective is the middle ground between stripping any threads and stopping leaks. That said, and assuming you’ve properly renewed all the associated washers and they’re in the correct order, and that you haven’t overly tightened the bolts, turn on the ignition switch and check for leaks. Run your finger under the banjos and under and around the float attaching bolts. Usually, you’ll find leaks. Not to worry. Just don’t retighten the bolts right away unless the leak is really bad. Let the fuel pump run for a few seconds to thoroughly soak the fiber washers with fuel. After a few seconds, turn off the ignition and using a ˝ Whitworth or 9/16 BS wrench, tighten the banjo bolts (if they’re leaking) only a slight amount. If needed, do the same for the lower float bowl bolts using a 3/8 Whitworth or 7/16 BS wrench. Turn on the fuel pump again and recheck for leaks. If any are found, retighten again, but only a little, repeating the process until you think the leaks have stopped. What you’re doing is tightening in increments. Quit for the day and come back the next day and repeat the checks. Almost always, the next day will reveal yet more leaks, albeit minor ones; but remember, the objective here is to completely stop all leaks without stripping threads. Repeat the process carried out the day before, again being careful not to apply too much torque on the wrench. Itty-bitty, incremental turns of the wrench is what you seek. When you’ve done this for two, three or four days—or whatever time it takes, come back and check the lines in a week. They should be leak free.
A certain combination of errors or omissions from the tuning procedure can lead you down a path so frustrating that you will want to swear off of SU carburetors forever. The tale of woe begins when you either neglect to synchronize the throttles or don’t do it correctly, so that one throttle is open quite a lot more than the other. It’s possible under these conditions to adjust the jet nuts in such a way that the idle will be fairly smooth, but only by setting the jet in a very lean position on the first carb (the one with the wider throttle setting). The resulting smooth idle will fool you into thinking these settings are correct, when in fact they are very wrong.
Then, when you lift the piston to test the first carburetor, the engine speed will not change even though the mixture supplied by that carb is in fact extremely lean. Again, you are fooled into thinking the mixture is correct. In reality, lifting the piston does not weaken the already excessively lean mixture enough to have any effect on the engine. On the other hand, when you lift the piston on the second carb, the engine will slow down dramatically, perhaps even stalling, which leads you to believe the mixture is set too lean even though it is in fact very righ. Your natural reaction is to try to enrich the apparently weak mixture by lowering the jet even more, making things worse rather than better. The farther down you go, the worse the situation becomes, leading you to lower the jet even more to correct what tests out as a weak mixture. Seem impossible? Try it some afternoon when you have nothing better to do.
The way to avoid this particular path to insanity is never to attempt mixture adjustment without first confirming that the throttles are synchronized.
MG Carburetor Designations:
TB/TCTD/TF (1939-56): HS-2
MGA (1956-62): H-4 (H-6 for Twin Cam)
MGB (1963-71): HS-4
MGB (1972-74): HIF-4
MGB (1975-80): Zenth-Stromberg 1-3/4"
The T-Type Restoration Handbook, 1993, The New England MG T Register, with various articles by F. E. Old.
The AA Book of the Car, 1970, British Automobile Association
The Brown Books for TA/TB/TC/TD/TF
Weber Carburetor Manual, 1995, Legg, Peers, Maddox and Haynes, Haynes Publishing Group.
The MG Workshop Manual, 1971, W. E. Blower, Robert Bentley, Inc. (The Blowers Manual).
The Sacred Octagon, April, 2000, NEMGTR, “S. U. Fuel Pumps And a Little History” by Donald M. Lawson #5687
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