Carburetor Tuning (Part 3 of 3)

By F.E. Old

Originally published in the Austin-Healey Magazine, March 1984


Adjustment


The jets must now be moved either up or down by turning their adjusting nuts until the fastest possible idling speed is achieved without altering the setting of the throttle screws (fig. 17). The initial setting of six flats down will usually provide too lean a mixture, so begin by turning the jet adjusting nuts down one flat at a time to enrich the mixture. On dual-carb models turn both nuts exactly the same amount. The engine should gradually speed up as you enrich the mixture. When it does, turn the nuts back up again until the highest idling speed is reached again.


In the rare case where the initial setting (6 flats down) is too rich, you will hear the idling speed start to drop off immediately as you lower the jet nuts. In this case, screw the nuts back up evenly to weaken the mixture. The engine will speed up as you approach the correct setting, but continue to screw the nuts up until the speed start to drop off due to a weak mixture.

Now proceed as in the preceding paragraph. Not all tuners will agree on this, but I find that it is usually easier to start off with the mixture a bit lean, then work from there towards the correct mixture.


The mixture should now be approximately correct for the speed at which the engine is running. However, that speed will now be somewhat higher than the ideal 700–800 rpm, so turn the throttle adjusting screws out a bit to bring the idle back down. The mixture may now be slightly too rich for the reduced idling speed, so raise the jet adjusting nuts a bit. As before, the aim is to adjust the jets for the highest possible idling speed at the new throttle setting. You may have to go back and forth between jet nuts and throttle screw several times, but in the end you will reach a point where the idling speed does not exceed the recommended range when you readjust the jet nuts.


This process may seem a bit tedious, but I recommend it if this is your first attempt at S.U. tuning because it forces you to become familiar with the functions of the throttle screws and jet nuts and their interrelationship. The experienced tuner may use a shortcut, which is simply to start off by adjusting the throttle screws until the engine is idling as slowly as possible without stalling. This should be somewhere around 500 rpm or so. Then, as the jet nuts are adjusted for the correct mixture, the idling speed will not climb so far above the 700-800 rpm range and not so much fiddling will be necessary.


As you go through the above procedure, keep several things in mind. On dual-carb models it is 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. It is also important to understand that it is the position of the jet relative to the tapered needle that determines the position of the jet. The jet must move up and down with the nut as you turn it, otherwise you will turn the nut all day without having any effect on the mixture. Normally the tension of the jet lever spring will keep the jet head tight up against the nut, but if not then you will have to help it along with your finger pressure. Refer to the last section of this article for several possible cures. If the springs are not able to pull the jets up against the adjusting ad you tune the carbs, then neither will they so when you push the choke knob back in after a cold start. This means trouble.


Often it is possible to turn the jet adjusting nuts with your fingers. If not, use the jet wrench supplied with the PSW tool kit or available separately from T Series and foreign parts suppliers. You may find it awkward to use the wrench with pancake-type air cleaners in place. If so, remove them. You may also find it helpful to remove the jet lever springs so you’ll have more room to swing the wrench. If you remove the springs, you must use your fingers to hold the jets hard up against the adjusting nuts. It helps to have three hands, but few of us are so equipped.


If you were adjusting the mixture on a “normal” fixed-venturi carburetor, your job would be done at this point. In fact, even on the S.U. the mixture should be very close to correct if you have followed instructions carefully. However, the S.U.’s unique variable-venturi construction provides the means for testing your adjustment and fine-tuning it to a degree not possible on most other carburetors. This is particularly handy for owner of dual-carb models, as it allows you to ensure that both carbs are providing the same mixture strength.


Testing & Fine Tuning


Some manuals tell you to listen carefully to the sound of the exhaust while the engine idles. If the sound is uneven in a sort of non-rhythmical or “splashy” pattern, then the mixture is too lean and should be enriched by screwing the jet adjusting nuts down a bit. If the sound is uneven in arhythmical pattern, and if the exhaust pipe throws out black smoke, then the mixture is too rich and the jet nuts need to be raised. Personally, I feel much the same about this as I do about trying to synchronize the throttles by holding a piece of tubing to the ear. It’s too easy to misinterpret all the strange sounds you might hear. Also, on dual-carb models the exhaust can sound pretty good even though one carb is running lean and the other is running rich, thanks to the small degree of balancing which takes place between the two halves of the intake manifold.


The preferred method, which is considerately more accurate, is to lift the piston a specified amount and observe the effect of this on the idling speed. The lifting of the piston is carried out exactly as it was in the section dealing with jet centering. However, unless you have the air-cleaners off and can actually observe the movement of the piston, you have to be very careful. Probe upwards with the lifting pin, nail or whatever until you feel it actually come into contact with the bottom of the piston, then push farther to actually lift the piston. Learn to differentiate between pin (or nail) movement and piston movement, because the two are not necessarily the same. This is particularly important if your carbs have lifting pins, because you have to take up a lot of free playin the pin before the piston actually starts to move.


Now we will have to deal with single-carb and dual-carb models individually, because the rest of the testing and fine-tuning procedure varies slightly depending on which setup you have.


On single-carb models, use your lifting pin, nail knife blade or whatever to lift the piston about 1/32 of an inch (0.8mm). This need not be by precise measurement, but be aware that it amounts to only a slight nudge upwards. Lifting the piston increases the size of the venturi. The throttle is still in the closed position, so the actual airflow through the venturi is 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, but should almost immediately settle back to only very 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 (jet too high), then the idling speed will drop off andthe engine may stall. Fig. 18 depicts these reactions graphically. Adjust the jet up or down asindicated by the results of the test. Move the nut one flat at a time, testing again after each change,until you get the correct response when you raise the piston.


A variation on this theme is possible, and can be used to confirm your finding if you are unsure of yourself. Lift the piston very slowly to a height of ¼”. By the time you reach 1/32” the engine should have reacted as described above for a correct setting: a slight increase in speed followed by settling back almost to the original idling speed. As you continue to lift the piston, thereby weakening the mixture even more, the engine should begin to slow down. By the time you have lifted the piston the full ¼”, 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 ¼”, then the mixture is too rich.


For dual-carb models the procedure is essentially the same, but the reaction to lifting the piston on one carb of the pair is not as dramatic because only two cylinders are affected. Also, because of the balancing effect of the manifold, a change in jet position on one carb may necessitate a change on the other carb as well, even though the second carb may have tested out okay the first time you tried it.


It doesn’t matter which carb you begin with, but for the sake of labeling convenience we’ll call them “first” and ”second”. Lift the first carb’s piston 1/32” as described earlier and listen for a change in idling speed. If the setting is correct, the RPMs will either remain unchanged or will rise very slightly before dropping back to normal. If too rich, the engine will speed up and stay speeded up. If too lean, the speed will drop off and the engine will run very rough, although it probably won’t die unless the mixture is extremely weak. Again, refer to figure eighteen. Before you make any adjustments, go on to the second carburetor and test it.


If both carbs react in the same way, with both testing out either too rich or too lean, then adjust both jet nuts one flat at a time in the same direction until the engine reacts correctly to your lifting of the piston. If one carb test out too rich while the other is too lean, adjust the nuts one flat at a time in opposite directions. Remember that you must move the nut and jet up to weaken the mixture, down to enrich it. In either case, test after each adjustment.


By turning both jet nuts equal amounts you may be lucky enough to reach a point where both carbs are just right. If so, stop. Don’t fiddle with them any more. However, it is more likely that you will come to a point where one carb test okay but the other is still a little bit off. In this case, leave the good one alone for the time being and continue adjusting and testing the other one. When it finally tests okay, go back to the first carb and test it again. You may have to make a minor adjustment to compensate for the adjustments made to the other carb.


In any event, the goal 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 juts equal amounts and testing after every adjustment until at least one of the carbs test okay, then make whatever minor adjustments are necessary to bring the other carb up to snuff. If you try to adjust one carb all at once, then go to the other one, you will probably waste a lot of time going back and forth correcting the setting of the first carb to compensate for changes made on the second, which were made to compensate for changes made on the first, and so on

ad infinitum.


Closing 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 disconnected for the time being. If you had the air cleaner off while you tuned put them back on now. Most air cleaners restrict airflow at least slightly, so the mixture may now be a bit too rich. This can be determined by lifting the pistons as already described, then adjusting the jet 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 cleaner in place.


Choke & Fast Idle Adjustment


TA, 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 the hand throttle or “slow running control” which increases the idling speed when its dashboard knob isturn3d. The TD, TF and Y Types have only one control knob combining both functions. The jet control lever and fast idle rocking lever (front carb only on dual setups) are interconnected in such a way that when the choke knob is pulled the mixture strength and idling speed are increased simultaneously. The adjustment of both types is essentially identical.


Choke


On dual-carb models the connecting rod which links the two jet levers must be adjusted so that both jets are lowered simultaneously. First make sure both jet heads are tight up against their adjusting nuts, then adjust the length of the connecting rod so the clevis pin can be inserted without altering the position of the lever. Pull back 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. If not, figure out why. A drop of oil on each of the clevis pins (three per carb) often helps, and should be applied in any event to prevent wear. For obvious reasons, there is no jet lever connecting rod to adjust on single-carb models, but you should still lubricate the clevis pins and make sure the jet head returns tight against the adjusting nut when the jet lever is released. The choke cable is designed to pull the jet levers to the choked position not to push them back to the normal running position. It’s up to the return springs to do that.


Now 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 lever begins to move. If the cable is connected without any slack, it can prevent the jet levers (and therefore the jests) from returning to their normal running positions when the knob is pushed in. Operate the choke knob several times to make sure everything works smoothly. If you find that the choke knob slips out of the detents that hold it in intermediate positions, loosen the cable connection at the jet lever, twist the stranded wire inner cable one turn clockwise and retighten the grub screw. The resulting torsional tension on the cable will tend to keep the knob locked into the detents. Make sure you still have adequate slack in the cable after you've done this.


Fast Idle



To adjust the slow running control on early models or the fast idle linkage on later cars, turn the adjusting screw until there is about.016 in. (.4mm) 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 or the idling speed on single-carb models. On the TA, TB and TC the screw you want is the rearmost one on the front carb. On the TD, TF and YT it’s the screw closest to you as you face the side of the engine, again on the front carburetor (fig 19). On the Y and YB it’s also the screw closest to you, but on the only carb. The purpose of the small gap is to ensure that the fast idle mechanism can’t hold the throttle slightly open even when the knob is in full off position.


The adjusting isn’t critical on early models with a separate slow running control knob. The gap should be no less that .016 in. for the reason just stated, but it may be a great deal more than .016in. if you don’t mind having to twiddle the control knob a lot before anything happens. If you find that you have too much gap even with the adjusting screw turned down hard, then turn it back outa bit and take enough slack out of the slow running control cable to give you a useful range of adjustment at the screw. Conversely, if you can’t get a wide enough gap even with the screw all the way out, then add a bit of slack to the cable. On later models with a single knob controlling both functions, the gap should not vary very much from .016 in. it the mixture and fast idle functions are to be properly coordinated. If your car has been retrofitted with carburetors manufactured much after the T series era, you may find that the fast idle is controlled by a snail-shaped cam rather than by the older rocking lever. On this type the gap between the adjusting screw tip and the cam should still be .016 in., but in this case a slightly wider gap will do no harm.


Common Problems


There are a number of problems that can crop up as you tune your carburetors, some of which wil lmake accurate tuning difficult or ever impossible. While it is beyond the scope of the “Back to Basic” series to instruct you in a complete carburetor overhaul, I feel I should at least point out some of the more common problems and their remedies.


Inability To Achieve Slow Idle


You may find it impossible to slow the idling speed down to the recommended 700-800 rpm range no matter how far you back off the throttle adjusting screws. Air leaking into the induction system somewhere between the throttle butterflies and the valves usually causes this. Grasp the ends of the throttle spindles where they protrude from the carburetors and try to wiggle them around. If they seem to be loose in their bores, then you have found at least part of the problem. New spindles should be installed, and in bad cases it may even be necessary to install bushing in the carburetor body to bring the spindle bores back to standard. The correct (new) clearance between spindle and bore is only .0025 in. Wear occurs rapidly if the spindles aren’t oiled frequently enough. It helps to oil them while the engine is idling, so the oil will be sucked into the spindle bores by manifold vacuum.


Defective gaskets between the manifold and the carburetor or between the manifold and the cylinder head are another common source of air leakage. To test for this, brush or squirt oil all around the joint sealed by the gasket. When (or if) the oil fills the leaking area of the gasket, the idling speed will slow down for a moment. A much less messy method is to direct the flow of gas from an unlighted propane torch all around the joint. When you find the leaky spot, the gas will be sucked in by manifold vacuum and the idling speed will increase.


In rare instances one of the core plugs at the ends of the balance tube on the manifold may come loose, allowing air to be sucked into the system. Leaks at the plugs can usually be sealed with silicone sealant or hardening gasket cement, but the only permanent cure is to install a new plug.


Regardless of the source, any air leaking into the system without being controlled by the throttle will make it impossible to adjust the carbs correctly. To compensate for the extra air, you will have to adjust the jet nuts down an abnormal distance to get a smooth idle. Unfortunately, the effect of manifold vacuum on those leaks at different engine speeds and under different load conditions almost guarantees that the mixture won’t be right at any speed except idle. The consequences of this can be far more devastating than the simple annoyance o not being able to get the engine to idle slowly.


If you find any leaks, don’t try to cure them by tightening the manifold nuts or carburetor flange bolts unless they are obviously loose. Over-tightening can strip or break the manifold clamp studs, which are none too strong, and will almost certainly warp the carburetor mounting flanges. The only sure cure for a leaking gasket is a new gasket.


It sometimes happens when a new throttle spindle is installed that the throttle stop arm is pinned on in the wrong position. Normally there is plenty of clearance between the arm and the abutment on the carburetor body, but if the arm is incorrectly positioned there may be no clearance with the result that the throttle cannot close for enough to achieve slow idle. The only proper cure for this is to fit another new spindle, this time being more careful (or more knowledgeable) about the positioning of the throttle arm. Not only does this arm serve to hold the idle speed adjusting screw, but the end opposite the screw also serves as a stop (against the bottom of the same abutment)when the throttle is fully open. The arm should be positioned on the spindle sot the throttle butterfly is perfectly aligned with the throttle bore (in other words, wide open) when the end of the arm opposite the adjusting screw is hard up against the underside of the abutment.


If the throttle arm is the cause of your problems you can still use the engine without fear of damaging it. You may not be able to achieve a slow idle, but at least in this case all the airflow is controlled by the throttle. The mixture setting will remain satisfactory throughout the whole range of throttle openings, unlike the situation caused by leaky gaskets.


Inability To Achieve Lean Enough Mixture


If you discover that it is impossible to achieve a lean enough mixture even with the jet nut screwed up as far as it will go, suspect either an incorrectly positioned needle, incorrectly adjusted float level, a leaky float valve, or a leaky upper jet cork gland washer. The first three items have already been discussed, but go through those procedures again if you are unsure of yourself.



Jet leakage is not likely to affect both carburetors equally on a dual-carb engine, but it can happen. If the lower spring-loaded gland washer leaks, the errant fuel drips out around the bottom of the jet assembly. This wastes fuel and is a potential fire hazard, but has no effect of the mixture except in unusually bad cases. If, on the other hand, there is leakage past the upper spring-loaded cork washer or past the copper washer that fits between the upper half of the jet bearing and the carb body, then the excess fuel is sucked into the air stream to enrich the mixture. This is most pronounced at idling speeds when vacuum at the jet opening is highest, and can make it impossible to achieve a lean enough mixture. Even if you are able to position the jet nut high enough, the mixture at higher speeds will then be too lean, with serious consequences in severe cases. If fuel drips from the bottom of the jet assembly (mere surface dampness may be disregarded), then the top washer is probably also bad. Both should be replaced. See figure 20 for an exploded view of the jet assembly, which may be removed from the carburetor by unscrewing the large jet locking nut. Be sure to re-center the jet before you tighten the locking nut again.


If the carburetors are impossible to set lean enough, and if none of the faults mentioned above are in evidence, then it is permissible to change the position of the needle in the piston. As explained earlier, the standard position is with the squared should of the shank flush with the surface of the piston. If you can’t get a lean enough mixture, set the needle (both of them on dual-carb models) so the should protrudes 1/32” beyond the face of the piston. This enables you to lower the jet-adjusting nut by about the same amount, which will usually provide a satisfactory range of adjustment.


Jet Nut Position Not Equal On Both Carbs


Although we 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. For example, one may be two turns down from the top and the other three turns down. This is perfectly okay. Even with new carburetors there can be a difference of as much as one turn (67 flats) between the two nuts, due usually to slight differences in the needle position or internal tolerances. On older carbs the factor comes into play, and the variation in nut position may be even greater. As a rule of thumb, all is okay as long as the difference amounts to no more that two full turns (12 flats). If the final setting results in a difference greater than that, then suspect incorrect float level, incorrect needle setting, or a leaky jet as described earlier. If none of these conditions exist, it is probable just a matter of extreme old age or extreme neglect, and 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 often can be cured by pulling the jet lever back as far as it will go, smearing a dab of petroleum jelly on the shaft of the jet, then working the lever back and forth to distribute the grease evenly. If that doesn’t do the trick, unhook the jet lever and withdraw the jet. If there is a burr or similar blemish on the shaft of the jet, work it down with a very fine file, crocus cloth, or 600-grit sandpaper. Don’t remove a lot of material from the jet, or the seals will not longer be able to do their job. Lubricate the jet as described above and all should be well. It’s also possible that some misguided former owner has substituted weaker return springs, or that the original springs have lost their tension.


Poor Cold Starting


If your M.G. is difficult to start in cold weather, make sure the ignition equipment is in good order before you start fiddling with the carburetors. At least 99% 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 fat spark. If the ignition system is okay, then examine the jets and jet linkage. The rich mixture required to start a cold engine is provided for on the S.U. carburetor by the moveable jet. When the choke knob is pulled, the lever lowers the jet in relation to the tapered needle. This creates a larger jet opening and thus a richer mixture. When the jet lever is pulled back to the fullest extent of its travel, the jet head should drop away from its adjusting nut between 5/16” and 3/8”. If it doesn’t, you problem may be that the carburetor isn’t able to supply a rich enough mixture at full choke. This usually occurs when the jet nut is set quite a way down from it s topmost position, say four or five full turns. The “fully choked” position of the jet lever is controlled by a lug on the lever that hits the pressed steel link on which the lever pivots. The jet head hitting the jet-adjusting nut controls the “unchoked” position. Thus, the lower the setting of the jet nut, the less the available jet movement and the weaker the mixture in the fully choked mode.


If, when you adjust the mixture, you find that the final position of the jet nut is quite far below its topmost position, then measure the available jet travel as described above. If it is less than 5/16”,you may increase it by repositioning the needle as much as 1/16” deeper into the piston. This allows the jet nut to be raised an equal amount, which in turn increases the available jet travel. On dual-carb models it is best to set both needles the same way even if one doesn’t seem to need it. One needle set with the shoulder flush with the piston and the other recessed by 1/16” or so will not have any effect on the tunability of the carbs, but this can conceal other faults (jet seal leakage, improper float level, etc.) which might later be brought to light by large differences in the heights of the adjusting nuts.


If recessing the needle into the piston still doesn’t provide the required amount of jet travel, screw the jet adjusting nut as far up as it will go. Now the total available travel should be about 7/16”. If not, you may as a last resort file a small amount off the lug on the jet lever where it contacts the pressed steel link. This will allow the lever to be pulled farther back, but don’t get carried away with this cure or the jet may be pulled too far out of its bearings in the full choke position.


Owners of very early TAs and earlier models should note that jet assemblies manufactured prior to 1937 provided a maximum jet travel on only 5/16” with the jet nut screwed all the way up. With the jet nut in the normal running position, the available jet travel for cold starting is quite small on these early models.


Testing Abnormalities


On dual-carb models a certain combination of errors in or omissions from the tuning procedure can lead you down a path so frustrating that you will want to swear off S.U. 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 is 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) and in a very rich position of the second carb (the one with the small 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 lead you to believe the mixture is set too lean even though it is in fact very rich. Your natural reaction is to try to enrich the apparently weak mixture by lowering the jet ever more, making things worse instead of better. The farther down you turn the nut, the worse the situation becomes, leading you to lower the jet even more to correct what still test out as a weak mixture. Seem impossible? Try it some afternoon when you have nothing better to do.


The obvious way to avoid this particular path to insanity is never to attempt mixture adjustment without first confirming that the throttles are synchronized. Never rely on the static method of synchronization by itself, as it isn’t accurate enough. Dynamic synchronization is the only accurate method, even if you only measuring gauge is a piece of rubber hose. The use of a Uni-Syn or similar vacuum gauge designed for the purpose is highly recommended.


Shortcuts


I hope that by following the instructions laid out earlier the rock bottom beginner will be able to tune his much-neglected carburetors. Once you are familiar with the procedure, and once the carbs are properly tuned, subsequent tune-ups should go considerably faster. As long as you resist the temptation to fiddle with them between tune-ups, they should perform satisfactorily for a long time. At the next tune-up they should require only cleaning, testing, and possible very minor adjustment. The procedure should go something like this:


CLEANING: Always clean the exterior of the carbs as described earlier, if for no other reason than to keep your hands from getting too grimy and transferring some of that grime to the carb’s delicate innards.


FLOAT CHAMBERS: Blow through the fuel inlet as described to test the seating of the valve, then use your test rod to confirm the float level setting. It shouldn’t need adjustment; anything between 7/16” and 3/8” is acceptable. Shake the float to test for leaks, and clean out the chamber.


SUCTION CHAMBERS: Clean and test for free piston movement as described earlier. Inspect the needle for wear as described, but doesn’t remove it unless it needs to be replaced. If you did your first tune-up right, you should know what size it is and should have positioned it correctly.


JET CENTERING: Unless you left the jet locking screws loose during your previous tune-up, or unless you have just had to replace the needle and/or jet, the jet should still be properly centered. Test it just to make sure. Screw the jet nuts all the way up, but count the number of flats so you can return them to their original positions. If you hear that telltale click as the piston drops, screw the nuts back down the correct number of flats and go on to the next step. If not, center the jets as described earlier.


SYNCHRONIZATION: Unless you have fiddled with the throttle screws since your last tune-up, the throttles should still be synchronized. Test with your rubber hose, Uni-Syn, or whatever. If okay, go on to the next step. If not, loosen the spindle clamp and twiddle the adjusting screws until the throttles are again synchronize. In any event, they should still be close enough that you can omit the routine with the piece of paper and go straight to the rubber hose, Uni-Syn. Set the idling speed to 700 – 800 rpm.


MIXTURE: The mixture should still be close to perfect unless you fiddled between tune-ups or unless you had to adjust the float level, jet centering or synchronization in one of the steps above. Test it by lifting the pistons, making sure the jet heads are tight against the adjusting nuts. If okay, go on to the next step. If not, disconnect the choke cable and jet lever connecting rod and proceed as described under “Mixture Adjustment: Testing and Fine Tuning”. If you have trouble getting it right, go through the complete mixture adjustment procedure as if you were starting from scratch.


CHOKE & FAST IDLE: These settings won’t need attention unless you altered the throttle screw settings or had to adjust the mixture. Check the choke cable slack, the fit of the jet lever connecting rod, and the fast idle screw gap, but don’t bother to change them unless necessary.


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