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Carburetor Tuning (Part 2 of 3)

By F. E. Old

Originally published in the Austin Healey Magazine, February 1984

The Suction Chambers & Pistons

The suction chamber assembly (fig. 5) is the heart of the S.U. carburetor, and is the major design difference between the S.U. 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 I recommend that owners of dual-carb models work on one at a time.

Disassembly & Cleaning

To remove the suction chamber, first unscrew the cap at its very top. The cap may or may not have a rod and plunger assembly (the damper) attached to it. If it does, be careful not to bend the rod. 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. This isn’t necessary on three-screw models, since the screw holes will line up only when the chamber is in the proper position. 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 (fig. 1 #8) 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 thrust washer down inside the piston where the spring rests. Don’t lose it.

Now examine the inside of the chamber and the outside diameter of the piston (Fig6). Both must be spotlessly clean. If not, wipe them off with a rag dampened 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. Some types of cleaner will make the diecast aluminum “bloom” slightly if left on for too long. This isn’t important on the outside of a carburetor, but can close up the critical clearance between the piston and the suction chamber.

When the parts are clean and dry, put a drop or two of oil on the steel piston rod (fig. 6) and insert it into the suction chamber. Don’t oil anything else! Move the piston in and out of the chamber slowly while spinning it, to distribute the oil evenly over the rod and its bore. While you do this, listen carefully for 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 carb body. If the piston slides straight in and out in this position without scraping, then all is well. If not, you must look for the spots where interference occurs and correct them. 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 with a super-fine file or a scraper. Work only on the faulty area.

Wholesale rubbing with emory cloth or sandpaper over the entire surface will upset the clearance between piston and chamber, which will adversely effect the operation of the carburetor. On models with only two hold-down screws, try rotating the chamber 180° in relation to the piston, and test again. This may eliminate the scraping, precluding the need for work with scraper and file. Two-screw chambers will fit on the carb body in either position.

The Needle

When this is taken care of, again insert the piston into the chamber and spin it. This time watch the tip of the needle. If it wobbles as the piston spins, it is bent and should be replaced.

If it seems to be straight, inspect it for shiny marks on one side. If there are any, this means that the needle has been scraping on the bore of the jet, usually due to an incorrectly entered jet assembly. This also calls for a new needle, since the scraping may have altered its diameter, upsetting its ability to meter fuel accurately. Ideally, you should also replace the jet, since the rubbing will have enlarged its opening, cut we’ll cover that later.

Now remove the needle by loosening the set-screw in the side of the piston near the bottom (fig7). If the needle is stuck, you can grasp it with pliers, but only at the very tip (the last 1/8”). Pull straight 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 (fig 7) where it fits into the piston. These indicate the size of the needle, and you should confirm that yours is the correct size for you car, as shown in table 1.


M.G. Model

S.U. Type

Spec. No.




two HV3

AUC 374




two H2

AUC 429




two H2

AUC 549




two H4

AUC 578




two H4

AUC 728




one H2

AUC 456




two H2

AUC 480



If you find a non-standard needle, obtain the correct type unless you know there is a good reason for using a different type in your car. S.U. and M.G. provide recommendations for alternate weaker and richer needles for special conditions, but they are not normally needed.

The richer needles are useful only for racing applications or when the car is driven without the air cleaner in place, and the weaker or leaner needles are required only if the majority of your driving is done 5000 feet or more above sea level. If you find yourself in either of these situations, see the shop manual for recommendations. Otherwise stick with the standard size for your car.

Now insert the needle into the piston so its shoulder is flush with the face of the piston, as shown in (fig. 8). Some older needles have a tapered or rounded shoulder as shown in the left-hand example in that illustration, and this type is difficult to position correctly. All needles made in recent years have square shoulders and are easy to position in the piston. A straightedge held across the face of the piston for the square shoulder of the needle to butt against will preclude the possibility of error. To prevent future sticking, it helps to put a very light smear of anti-seize compound or grease on the shank of the needle before inserting it in the piston, and also on the threads of the setscrew. Tighten the screw firmly once the needle is in the correct position.


Lower 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 (see table 1). If one end of the spring has a smaller diameter than the other, then the smaller end should go towards the piston and a thrust washer should be used. If both ends are the same diameter, as is the case on most recently manufactured springs, then it doesn’t matter which way the spring is inserted and no thrust washer is required. If, according to table 1, your car should have springs on the pistons but does not, then order some. The car will not run well without them due to an excessively weak mixture. Springs are color coded to indicate their strength, as shown in the table, so be sure to order the right ones.

Put the suction chamber over the piston, being careful to align your index marks if it is a two-screw type. The chamber must be a good fit onto the carburetor body to prevent air leakage. No gasket or sealant is used, so make sure the mating surfaces are impeccably clean. Tighten the hold-down screws firmly, but don’t get carried away. Over tightening can warp the chamber and cause the piston to rub.

The Damper

Now turn your attention to the cap and damper assembly. The purpose of the damper is to slowdown the rise of the piston when the throttle is opened suddenly. The resulting high vacuum over the jet enriches the mixture momentarily, serving much the same purpose as the acceleration pump found in most “normal” carburetors.

Dampers were used on all TDs, all Ys and most TCs and most TFs, but not on TAs, TBs, early TCs and early TFs. The dampers improve acceleration from low speeds, so you may want to retrofit them to your carbs id you don’t already have them. Damperless carbs are equipped with a plain brass cap at the top of the suction chamber.

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 just below the threads for the cap. You must have one or the other, but not both. The carburetors used on most T and Y Types have no vent hole inside the chamber neck, and these must be fitted with vented caps. The TF carburetor is the so-called “dustproof” type with the vent hole in the chamber neck, and dustproof carbs may have been used as replacements on earlier models. Dustproof carbs must have non-vented caps. If you find yourself with the wrong type of cap, drill a 1/16” hole in the cap or plug up the existing hole, depending on which is required, or order new parts.

Now fill the hollow piston rod to within ½” of the top with SAE 20 motor oil (fig. 9). Insert the damper and screw down the cap firmly. These caps tend to loosen due to vibration and the action of the campers, so don’t be too gentle. Un-vented caps must have a sealing washer under them, but check to see that it is really there, as they are easily lost. Washers are not required on vented caps, but it’s a good idea to use them anyway.

If your car has two carburetors, as do all but the Y and YB, you must now repeat the whole procedure on the suction chamber assembly from the second carb. The mixing of needles, springs and dampers from one carb to the other is not critical, but under no circumstances should you switch the piston from one carb to the suction chamber of the other. Pistons and chambers are assembled into matched sets by selective fit to ensure the correct clearance between them. Don’t mix them up. However, it is perfectly okay to switch the complete chamber/piston assembly from one carb to the other.

Centering The Jets

After the suction assembly has been cleaned and refitted to the carburetor, you must make sure 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 of the needle diameter will diminish the carburetor’s ability to meter out fuel accurately, and in really bad cases the friction between needle and jet can cause the piston to get stuck in one position. Neither condition is desirable. The mounting of the jet assembly in the bottom of the carburetor is designed n such a way as to allow enough lateral movement for centering purposes. Once the correct position is found, the assembly is licked into place by a large nut.

Preparation & Inspection

On dual-carb models, disconnect the linkage between the two jet levers by removing the clevis pin from one of its forked ends. The rod may be left hanging from the other jet lever. Disconnect the choke cable from the rear jet lever. Unhook the tension spring from the jet lever, remove the clevis pin that attaches the lever to the jet head, and swing the lever out of the way. Mark the side of the jet head 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 on as far as it will go (fig10).

Now inspect the jet. Its outside diameter should be smooth, with no sign of grooves or uneven diameter. If such defects are present, the jet should be replaced. If the opening at the top of the jet is obviously oblong instead of round, this too is reason for replacement. Think back to your earlier examination of the needle. If it was shiny on one side, indicating that it had been rubbing on the jet, then assume the jet is worn and replace it and the needle.

The standard jet for all T and Y Types has a .090 in opening, but you will occasionally find that some misguided previous owner has mistakenly installed a larger jet (usually .100 in.) Jets are sometimes 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 markings on your jet, use a 3/32” drill as a crude gauge. It should be impossible to insert the shank of the drill into a .090 in. jet (don’t force it). If the drill will fit into the jet, then the jet is either very worn or the wrong size in either case, get a new one.

Put a very light smear of petroleum jelly on the outside of the jet, then insert it back into the carburetor. Push it up until the jet head abuts against the adjusting nut. Make sure the side you marked earlier is facing the right direction if you are reusing the old jet. If you are installing a new jet, just 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 procedure. The opening in the top of the jet is not always exactly 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.


If you still have the air cleaner ducting off, reach into the mouth of the carburetor, lift the piston a bit, and let it drop. If the air cleaner is in place, you can still lift the piston. The TF carbs have Lifting pins in the flange under the suction chamber, as shown in figure 11. Simply push the pin up as far as it will go, then let go. Earlier carburetors do not have lifting pins, but they do have ventholes in approximately the same position. Insert a nail or stiff wire into the vent hole to lift the piston.

No matter how you go about lifting the piston (fig. 12), when you let it go it should drop against the jet bridge with a metallic click. Some pistons have spring-laded bumper pins in their undersides to soften the impact of the piston hitting the jet bridge, but you still hear a soft click. If you hear a click, then the jet is centered correctly 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.

Slacken off the large jet locking nut until it is just possible to rotate the bottom of the jet bearing(the threaded piece onto which the jet adjusting nut screws) by finger pressure. Insert a thin screwdriver or similar implement into the top of the suction chamber and push down gently on the piston rod (fig 13). At the same time, wiggle the jet assembly gently to help it move, while keeping some 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 portion of the needle into the jet opening, thus forcing the jet to assume a position concentric with the needle. (fig. 14) Now tighten the jet locking nut to lock the jet 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 after several tries, withdraw the jet, remove the adjusting nut, and reinsert the jet.

With the adjusting nut removed, you will be able to push the jet up higher than before. This makes the centering action more positive. When you think you finally 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. Repeat the whole procedure on the other carburetor if yours is a dual-carb model.

An explanation of the term “flats” might be in order here, since we have just used it and will use it more often as we proceed. The jet adjusting nut is six-sided, so we can say that it has six flats. If we begin with one flat facing us, then turn the nut 1/6 of a turn so the nest flat faces us, we will have turned the nut one flat. Thus one flat equals a sixth of a turn, three flats equals half a turn, six flats is a full turn, and so on. We will also speak in terms of so many flats up or so many flats down, not clockwise or counter-clockwise, in or out, or anything else equally fuzzy. Up means up towards the carburetor and down means down away from the carburetor. You can'’ go wrong unless the car is upside down, in which case tuning the carbs won’t do a bit of good.

You should go through this entire centering procedure if this is the first time you have given the carbs a thorough going over or if you are fitting new needles or jets, and then perhaps once a year after this if the car is used often. In between, it will be sufficient simply to raise the piston and listen for that all-important metallic click without going through all the bother of disconnecting the jet levers, removing the springs, and so forth.


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 the same amount of 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 of the carbs, and will be dealt with in this section. It is also necessary to ensure that both carbs mix the same amount of fuel with the incoming air. This is called mixture adjustment, and will be dealt with in the next section. Mixture strength is determined in part by the amount of air passing over the jet opening, and this airflow is controlled by the throttle setting, so it should be obvious that the throttles must be synchronized before the mixture can be adjusted. For reasons known only to their authors, 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. Naturally, synchronization is unnecessary on single-carb engines, so Y and YB owners may skip this section and move right along to mixture adjustment.

Some tuning manuals recommend a very simple synchronization procedure that consists basically of starting with the throttles in the fully closed position and turning both adjusting screws down equal amounts. This ensures that both throttle butterfly valves are rotated the same number of degrees away from their full closed position, but it does not guarantee that the flow of air past the butterflies will be equal even though that is the ultimate objective of synchronization. Even when a throttle butterfly is fully closed, there is always a small gap between its outside diameter and the inside diameter of the throttle bore. Thus a small amount of air can get past the butterfly even when it is closed. Unfortunately, the size of that gap (and therefore the airflow through the gap) is never identical on any two carburetors. For example, one carb might allow airflow of 5 cfm (cubic feet per minute) past the closed butterfly, while the other may allow20 cfm. If we then rotate both butterflies the same amount, say 5° from fully closed, the second carb will still flow more air than the first even though both butterflies were opened exactly the same amount. These carbs might be synchronized statically (engine at rest), but they are certainly not synchronized dynamically (in relation to actual airflow with the engine running).

Static synchronization can be used to obtain a preliminary setting if you are installing carbs and are tuning them from scratch, or if some previous tuner has really botched up the adjustment. The procedure is simple, so I’ll describe it just in case you need it. However, if you are tuning a car which has been running reasonable well all along, you can assume the throttles are already synchronized reasonable well. If so, skip static synchronization and go on to the dynamic synchronization procedure that I will explain in a moment.

Static Method

Begin by loosening one clamp bolt on one of the flexible couplings on the short spindle that connects the two throttles (fig 15). You should now be able to open and close the throttle on one carb without affecting the other throttle. Back the fast idle or slow running control screw (fig 15-a) on the front carb all the way out so it won’t prevent the throttle from closing completely. Now unscrew the throttle adjusting screw (fig 15) on one carb out until it no longer touches the abutment on the carburetor body. Then screw it back in until it will just barely hold a piece of paper between its tip and the abutment. Finally, turn the screw in one additional full turn. Do the same on the other carburetor. Both throttle butterflies are now open approximately the same amount. If you were to retighten the spindle clamp both butterflies would then open and close in unison, and would be statically synchronized. But, as I explained earlier, it is unlikely that equal amounts of air will flow past both butterflies. This must now be confirmed by dynamic testing.

Dynamic Method

As implied above, this method involves actual measurement of the airflow through both carburetors while the engine is running. Most manuals suggest that you use a piece of tubing as a crude stethoscope.

With the engine running, hold one end of the tubing to your ear and the other end at the intake of one carburetor (fig 16). You should hear a hissing sound caused by the air rushing past the end of the tube. Now move the tube to the other carb, being sure to hold it in the same position relative to the opening. If both carburetors are drawing the same amount of air, there should be no difference in the loudness or pitch of the hissing you hear through the tube as you move from one carb to the other. Unfortunately, many people find it difficult to discern small differences in volume, so this method may not work for you. Add to this the fact that you will also hear an assortment of burbles, pops, sputters and other indecipherable sound, all which make it difficult to gauge the loudness of the hissing accurately. I recommend that you use this method of testing only if you are unable to do better.

There are several devices available which make the job easier and more accurate. The PSW tool kit includes a means of measuring the rise and fall of the pistons in the suction chambers. If both pistons are at the same level at any given engine speed, then both carburetors are drawing the same amount of air and are dynamically synchronized. The Uni-Syn and similar gauges fit over the intake end of the carburetor and measure vacuum at that point. When the readings are identical for both carbs, then both are drawing the same volume of air and are dynamically synchronized.

Regardless of which of these tools you use, it’s hard to go wrong if you follow the manufacturer’s instructions carefully. Loosen the throttle connecting spindle clamp bolt as described earlier and turn the throttle adjusting screws in or out as necessary to make any corrections which may be requires. When you are satisfied that the airflow is identical through both carburetors, retighten the clamp bolt 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 at 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 S.U. tuning procedure which seems to baffle so many owners and which has contributed greatly to the S.U. carburetor’s undeserved bad reputation in this country.

A large part of the problem may be the way the procedure is described in most workshop manuals. However, be warned that the procedure described in the manual is essentially correct, whether or not you understand it as written. Regardless of what you may have heard (usually second or third hand), there is no simple tuning secret discovered by a little old mechanic in Moosedip, Alaska, and whispered on his deathbed to an ancient trapper friend who disappeared into the tundra never to be seen again (or any of several variations on that same theme, some of which are even more absurd). There is nothing wrong with the method described in the manual, but there is a great deal wrong with the way it is described. I’ll try to do better.

If you analyze the procedure carefully, you will find that it isn’t really all that much different from adjusting the idling mixture on a “normal” fixed-venturi carburetor. The major difference is that on most other carbs you turn a screw to change the mixture, while on a S.U. you turn a nut. The S.U. has one very big advantage over other types in that it provides the means for testing the adjustment to make sure it is correct. There’s no easy way to do this on most other carbs.


Mixture adjustment may be accomplished with the air cleaners on or off, according to your preference, but there are advantages to leaving them on as you will learn later. Disconnect the choke cable from the rear jet lever, if you haven’t done so already, otherwise a tight cable can prevent the jet head from butting against the jet-adjusting nut. On dual-carb models also disconnect the linkage between the two jet levers by removing the pin from one of its forked ends.

If you didn’t do so after centering 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 right up tight against the adjusting nuts. This is a good preliminary setting for the jets, and ensures that both jets on dual-carbs engines start off in the same position.

Now start the engine again and let it run until thoroughly warmed up. Adjust the idling speed if necessary to bring it into the 700 rpm to 800 rpm range. Remember on dual-carb models to turn both throttle-adjusting screws equally. (Final article in the series appears in next issue)

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