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Carburetor Tuning (Part 1 of 3)

By F.E. Old

Originally published in the Austin-Healey Magazine, January 1984

“Back to Basics”, for those of you who are fairly new to the Register and TSO, is a series of articles designed to help the would-be back yard mechanic cope with basic automotive maintenance procedures. Unlike most of the M.G. workshop manuals currently available, which assume that the reader is familiar with automotive fundamentals, we have tried in this series to assume nothing. Our aim has been to provide instructions that the rock bottom beginner can follow without too much difficulty.

We began way back in the June 1975 issue with a discussion of the basic tools needed for auto maintenance work. In August 1975, part two dealt with useful manuals and a comparison of British and American automotive terminology. Part three, in April 1976, covered chassis lubrication. In August 1976, part five discussed the theory behind engine tuning and explained how to perform a compression test. In April 1977, part six took the reader through the procedure for adjusting valve lash or rocker arm clearance. Number seven, in August 1977, described the workings of the ignition system and explained how to clean and gap spark plugs. Part eight, in December 1977, explained how to service the distributor. In April 1978, part nine reviewed what had gone before, for readers just joining in, and mentioned a few things that had been omitted from earlier installments. In December 1978, part ten dealt with carburetion theory and described the S.U. carburetor. Now about two and a half years later, #11 is finally finished and ready for the printers. The delay was a mainly due to the fact that I found it very difficult to write a reasonably foolproof how-to article on the subject of S.U. carburetor tuning. What follows is my fifth or sixth attempt. It’s far from perfect, but I don’t want to delay publication any longer.

Incidentally, please don’t ask me for copies of earlier installments, as I am not able to supply photocopies. Individual back issues of TSO may be ordered from the Register Librarian.

See the Register Regalia page of TSO for details.


Most of the tools you’ll need should already be in your M.G. toolbox: wrenches in Whitworth and British Association sizes, screwdrivers, and pliers. To do a topnotch job you’ll also need a PSW tool kit for the S.U. carburetor, which is sold as and “S.U. Tool Kit” by most of the T Series parts suppliers and may also be found at many foreign car parts stores. You might also want to obtain a Uni-Syn or similar synchronizing tool. You can get by without the PSW kit and the Uni-Syn, but they will certainly make life easier for you.

For cleaning the carbs you will need several clean, lint-free rags, a stiff brush, and some carburetor cleaning fluid. The most convenient type of cleaning fluid comes in aerosol cans, but you can also get it in bulk form if you prefer. Denatured alcohol can be used in a pinch, but it is not as effective.


The average back-yard mechanic seems to have a carburetor fetish. When the engine isn’t running right, out comes the screwdriver to adjust the carburetors. When he does what he naively thinks is a tune-up, the first things he attacks are the carburetors. When he has nothing better todo, he fiddles with the carburetors. This is wrong! Most causes of poor running are caused by malfunctions in other areas of the engine, not by the carburetors.

The carbs should be the last items checked in a trouble shooting sequence unless you are experienced enough to know that they are the only possible cause of the engine’s strange behavior.

Similarly, the carburetors should be the last items attended to in the course of a tune-up.

Unless the compression checks out okay and rocker arm clearance, spark plug gap, breaker point gap and ignition timing are all correct, it will be useless to adjust the carbs.

You may think you have adjusted them correctly, only to find you must do it all over again after you’ve adjusted the plugs, points and so on.

With the exception of the Y and YB, the cars we are dealing with in this series all have two carburetors. Dual-carburetor setups are especially sensitive to engine condition, and can be difficult if not impossible to tune properly if the rest of the tune-up sequence is not taken care of first. Even then, you will find carburetor tuning very difficult if in the course of the earlier tune-up procedures you discover that the engine is suffering badly from age. If, for example, during your compression test you find that compression in one cylinder is considerably lower than that of the others, then it will be very difficult to adjust the carburetor serving that cylinder. Or, if the automati cadvance mechanism in the distributor is so badly worn that it can no longer fire the plugs consistently at the right time, then carb tuning will be difficult and the resulting setting will quite probably not be correct over the entire rpm range.

These factors are important on single-carburetor models, but are not so critical. When all four cylinders draw through one carburetor, the effects on the carburetor of imperfections in one cylinder aren’t as strong. Let’s consider one of the worst possible examples: an engine which produces a compression gauge reading of nearly zero on one cylinder (due probably to a burnt valve). That cylinder will draw very little air. On a sing-carb engine this means about a 25% loss of air flow through the carb, which is bad enough, but on a dual-carb model it means a 50% loss of airflow through the carburetor serving that cylinder. That carburetor will be impossible to tune properly, so don’t even bother to try until the cause of the compression loss is found and corrected. Obviously, this is an exaggerated example, but it serves to illustrate my point. Don’t attempt to adjust the carburetor until you have tested the compression and found it satisfactory, gapped the spark plugs, inspected and adjusted the distributor, and set the valve lash to the recommended clearance.


The most important cleaning will be done to individual parts as you disassemble and adjust the carburetor, but before you begin you should clean the outside of the carb as thoroughly as possible. This serves two purposes. First and most important, it precludes the possibility of dirt and grime on the outside of the assembly from getting inside. You won’t be doing a great deal of disassembly, since this is a tune-up rather that a complete overhaul, but the parts you will be removing must be kept spotlessly clean. Secondly, it’s no fun to work on anything that is covered with countless years’ accumulation of gum, grit, oil and miscellaneous crud.

This is where the spray can of carburetor cleaner comes into play. Following the instructions on the can, spray the stuff all over the outside of the carb. Carb cleaner is pretty potent stuff, so be careful to keep it off your skin and off the car’s paint. Allow the cleaner to soak into the grime for a short time, then use a stiff brush to loosen stubborn spots. If the instructions on the can say to rinse with water after cleaning, then do so, as some types of cleaner will eat into the aluminum diecastings of the carburetor if left for too long. In any case, it’s a good idea to have a water supply handy for rinsing overspray off you and the paintwork.

When the outside of the carb is clean, remove the air cleaners (TF) or air cleaner ducts (all others). With the engine running at 2500 rpm or higher, spray the carb cleaner into as many corners of the carb’s innards as you can reach. This will clean most of the gum, and carbon off the throttle bore, throttle plate, and lower part of the piston. Don’t use up all the cleaner quite yet, though. You may need it later.

Now we’re ready to start exploring the inner working of the mysterious S.U. carburetor.

The instructions that follow apply to the type H carbs used on the TB and later cars, and in the main to the type HV carbs used on the TA. We occasionally see the more modern type HS carbs fitted as replacements, easily identifiable by the fact that fuel is carried from the float chamber to the carb via a piece of tubing. While the same general tune-up principles apply to the type HS unit, detail differences exist which make the purchase of an S.U. tuning manual a good idea.


In order for the carburetor to meter out fuel accurately the fuel level in the jet (fig. 1 #5) must be kept more or less constant. This is controlled by the fuel level in the float chamber (fig 2) (or float bowl, as it is usually called in this country). The fuel level in the float chamber is in turn controlled by a float-operated needle valve (fig 2 #90 IN THE CHAMBER LID. This works much like the shut-off valve in a toilet tank. When the fuel reaches the correct level in the chamber, the float (fig 2 #10) closes the valve and stops the flow of fuel into the chamber. When the fuel level drops, the float drops with it, allowing the valve to open. When the engine is running, the needle valve is constantly opening and closing to keep the fuel level approximately constant.


The fuel level in the jet need not be exact, as normal jet adjustments will compensate for minor variations. However, the standard measurement for all type HV, H, and HS carbs is 3/8inch (9.5mm) below the top surface of the jet bridge, and it should not deviate too much from that. The jet bridge is the flat surface on the bottom of the carburetor throat, against which the piston rests and through which the jet bore is drilled. Even with the suction chamber and piston (fig 1, #3& 4) removed and the jet in its fully lowered position, it is nearly impossible to measure the fuel level accurately at this point. Therefore, we substitute a purely mechanical measurement of the position of the float lever when the float valve is closed. This measurement is made between the lever and the chamber lid, as shown in figure four. For the type T2 chambers (2 ¼” diameter) used on all T and Y type carburetors, the measurement should be 7/16 inch (11.1mm). If this measurement is correct, then the float needle will close when the fuel level in the chamber and in the jet are correct.

In order to make this measurement you must first remove the chamber lid. Remove the large banjo bolt which attaches the fuel line to the lid, (fig 2) remove the fuel line(save the fiber washers), and remove the spring-loaded wire mesh filter from the bolt hole. Then remove the hold-down bolt which passes through the top of the lid(more washers to save), push the overflow pipe aside, and lift off the lid. Turn the lid upside down and you will see the float lever (fig 2 #11) and, under it, the needle (fig 2#9) valve.

Remove the pivot pin, lift off the lever, and lift out the needle. If its conical tip is grooved, you should install a new needle and seat. If you’re not sure whether or not the old assembly is usable, insert the needle back into its seat and blow through the fuel line opening in the lid while holding the needle against the seat with light finger pressure. If no air leaks through the closed valve, 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. The old type won’t fall out, but is more difficult to remove. Use a pin punch, nail, or stiff wire smaller in diameter that the pin to push it out from the end opposite the knurling. I like to reduce the diameter of the knurling with emery cloth or a fine file to make the whole disassembly and inspection process easier.

Also 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 ears should be perfectly straight. The area where the lever bears on the needle valve may be shiny, but must not be deeply grooved. When you are satisfied that the needle valve, float lever and pivot pin are in satisfactory condition, assemble them back into the float chamber lid.

With the lid assembly still upside down, insert a 7/16” diameter rod between the lid and the inside curve of the forked end of the lever, as shown in figure three. A rod of the correct diameter is provided in the PSW tool kit, but a 7/16” dowel or a longish 7/16” bolt will do just as well. The lever should rest on the test rod and on the needle valve at the same time. 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 (see fig3). Also make sure both prongs of the curved fork rest equally on the test rod. It’s very easy to twist the lever slightly out of kilter when you bend it.

The original type of valve needle was made of solid stainless steel, and with this type it’s hard to make a mistake in the adjustment. However, in recent years this type has bee superseded by a nylon-bodied needle with a spring-loaded pin inserted into the end that touches the float lever. The spring loading lessens the closing impact of the needle and prolongs its life, but it necessitates more care in adjustment. Do not push the lever down against the spring pressure in an attempt to make the fork rest on the test rod. The measurement must be made with the pin fully extended, supporting only the weight of the lever.

In service the setting will change gradually due to the wearing of the needle and the pivot pin, but this wear takes place very slowly. Given the low annual mileage traveled by most T & Y types these days, once the float level is correctly set, you should never have to set it again. Still, it should be checked each time the carburetors are tuned.

Those of you who have factory workshops manuals (that should include all of you) may have noticed that in some of these manuals the use of a 3/8-in. test rod is specified instead of the 7/16-in. rod specified by the S.U. Carburetor Company. I don't know why M.G. chose to deviate from the standard S.U. measurement, but in practice it doesn’t really matter. Both dimension will provide a fuel level in the jet that is within the acceptable range, and there will normally be no difference in performance.


Use a piece of wire with a small hook bent into one end to fish the float, (fig 2 #10) out of the chamber. Dry off the outside of the float, then shake it vigorously. If it rattles, or if there is no sound at all, then the float is okay. The rattle is probably just a small blob of loose solder, and is nothing to worry about. If however, you hear the swish of liquid inside the float, then the float leaks.

To find the leak, submerge the float in a pan of very hot water. As the air heats up and expands it will be forced out of the hole or holes, and you will see bubbles in the water.

Remove the float from the water, mark the holes, then put it back in the water and let it float.

Keep the water very hot. This will speed the evaporation of the gasoline, and eventually your float will be empty. In really bad cases it may be necessary to boil the float all night, but under no circumstances should you try to hasten the evaporation by heating it with an open flame.

The resulting rapid evaporation can cause the float to explode, ruining a potentially repairable float and peppering your tender bod with shrapnel.

Once the gasoline is driven out, the holes can be sealed with solder. Use a hot soldering iron, not a torch. However, this should be considered only a temporary repair. Chances are good that there are other thin spots waiting to break through, so you had best order a replacement float in the near future.


Now examine the inside of the float chamber. Any grit inside the chamber indicates filtering problems, so check the condition of the wire mesh filters in the fuel pump and in the inlet union on the float chamber lid. Clean the filters if necessary, or install new ones if they are damaged or missing. Also consider cleaning out your fuel tank. If grit is present in the chamber, then it has probably also gotten into the small passageway which transfers fuel from the chamber to the main body of the carburetor. Remove the chamber 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.


Now the float chambers and lids may be reassembled. Drop the floats back into their chambers; it will be rather embarrassing later on if you leave them out. When you put on the lids, make sure all sealing washers are returned to their original positions. Use 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.

On all T and Y types the carburetors are mounted in an “emi-downdraught” position, which means that they slope slightly downhill towards the engine. However, the mounting arms on the float chambers are arranged in such a way that the chambers are level, and this is the only position in which they will work properly. If your chambers are tilted just like the carbs, then someone has installed chambers meant for use on horizontally mounted carburetors.

Replace them with the correct type or you will have trouble with sticking floats, sticking needles, and fluctuating fuel level. Even with the correct chambers fitted, they will be exactly level only when the mounting arms are perpendicular to the carburetor body as viewed from above. This is easily adjusted by loosening the mounting bolt (fig. 2 #12) under the carburetor body, rotating the chamber to the desired position and re-tightening the bolt. (To be continued in next issue)

Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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