If you have already worked through our guide to setting up SU carburettors, your car should now idle cleanly, pick up smoothly, and run without the flat spots or hunting that plague a poorly adjusted SU installation. Good. That is the foundation. This article is about what comes next: taking a correctly set-up SU system and optimising it for the way the engine actually breathes, the way the car is actually used, and the modifications, if any, that have been made to it. The tools involved are different. A little more judgement is required. And the results, when it all comes together, are satisfying in a way that no amount of modern fuel injection mapping can quite replicate.
Why two identical engines can need different settings
This is the point at which many owners become frustrated, and it is worth understanding from the outset. A 1968 Speed and Sports Publications tuning guide for SU carburettors, a remarkably thorough period document that repays careful reading even now, makes this point directly and without apology: even small variations in engine specification can radically alter the carburation requirements of an otherwise identical unit. Two engines of the same type and displacement, fitted with the same carburettors, can legitimately require different needles, different piston springs, or different air intake arrangements to achieve optimum results.
The variables include the shape of the combustion chambers and ports, whether any grinding or polishing has been carried out, the camshaft specification, whether a performance camshaft has been fitted, valve sizes, compression ratio, and the precise characteristics of the exhaust system. Any of these, individually or in combination, alter the engine’s breathing characteristics and therefore its fuelling requirements. The factory needle fitted at the time of manufacture was chosen for a standard engine producing standard power in standard tune. Modify anything significant and the original needle specification may no longer be correct. This is not a fault of the SU system. It is simply the consequence of the engine being different from what the original specification assumed.
The practical implication is that this article cannot tell you which needle to fit to your specific car in its specific state of tune. What it can do is explain the principles that govern needle selection, the testing methodology that reveals what adjustment is needed, and the systematic approach that turns a process of apparently bewildering complexity into something manageable. Which, again, is rather more useful than a list of numbers.
The needle: what it actually does and why it matters
The jet needle is the heart of the SU carburettor’s fuelling system. It is a tapered pin suspended from the piston, sitting inside the jet, and its diameter at any given point determines how much fuel can pass through the jet at that piston height. As the piston rises in response to engine demand, the needle rises with it, exposing progressively more of its taper to the jet. The diameter of the needle at each point along its length therefore governs the mixture strength at the corresponding engine condition.
The 1968 tuning guide referenced above uses the CE needle as a worked example, and the analysis is illuminating. The needle has thirteen measurable positions along its length, and the guide associates specific positions with specific driving conditions. The uppermost positions govern idle and datum. The middle positions govern pick-up, part-throttle driving, and cruising at moderate road speeds. The lower positions govern high-speed full-throttle conditions. On smaller carburettors, the lowest two or three positions may not come into play in metering at all, because the piston never rises high enough to expose them.
Understanding this positional relationship is the key to targeted adjustment. If the engine runs well at idle and at moderate speeds but feels weak at full throttle at high revs, the problem is in the lower section of the needle. If it hesitates on pick-up from low speed in top gear but runs cleanly at higher speeds, the issue is in the middle section. And if the idle is rough but everything above it is fine, the datum dimensions at the top of the needle require attention. The needle is not a single component: it is several different mixture controls occupying the same piece of tapered metal.
When making needle changes, the 1968 guide offers a piece of advice worth committing to memory: when moving from a weak needle to a richer one, change by approximately two thousandths of an inch at a time. When moving from rich to weak, change by no more than one thousandth. The reason is asymmetric: running too weak is potentially damaging, so the conservative approach when leaning the mixture is appropriate caution rather than excessive timidity.
Diagnosing which part of the rev range needs attention
Before changing any needle, it is necessary to establish precisely where in the rev range the problem lies. Changing needles on guesswork is an expensive and time-consuming exercise that rarely produces good results. The diagnostic process involves specific on-road tests, and the period tuning guide describes these with admirable precision.
The acceleration test
The first test targets the pick-up and mid-range. With the engine fully warm, accelerate from approximately 20mph to 50mph in top gear. A clean, progressive surge with no hesitation or flat spot indicates the middle needle dimensions are correct. If there is hesitation, a stumble, or a sensation of pulling back, partially enrichen the mixture temporarily by pulling the choke out slightly. If this improves matters, the third through sixth needle positions require a richer profile. The middle section of the needle is too lean. The 1968 guide is specific about this correlation, and it holds good for the vast majority of SU installations on classic British cars.
The steady speed test
The second test targets part-throttle cruising. Drive at steady speeds of 30, 40, and 50mph in top gear and observe whether the engine runs smoothly or exhibits a slight rhythmic surging, sometimes described as a see-sawing sensation. Mild surging at steady throttle openings in this speed range indicates the cruising mixture is slightly incorrect. Richening the mixture slightly should cure it. If the surging appears at constant speed but disappears when the throttle is varied, the middle needle dimensions are again implicated.
The high-speed test
The third test is the most important from a mechanical safety standpoint. The 1968 guide makes this clear, and the reasoning is sound: running lean at high speed and full throttle generates excessive combustion temperatures that can damage pistons, valves, and valve seats with remarkable efficiency. Check that the engine pulls cleanly and strongly at high speed under full throttle conditions. Any sense of flattening out, running out of breath, or losing power at the top of the rev range warrants investigation of the lower needle dimensions before any sustained high-speed driving is undertaken.
Reading spark plugs and exhaust smoke
Spark plug readings complement the on-road tests. A plug that is white, pale grey, or shows a powdery deposit is running lean. A plug that is black, sooty, or oily is running rich. The correct colour is a medium tan or light brown on the insulator nose and around the electrode. Our spark plug reading guide covers this in detail and is worth reading alongside this article, because the plug tells the story of the mixture in the upper rev range where the engine is working hardest.
Exhaust smoke provides a complementary diagnostic. The 1968 tuning guide includes a section on this that remains entirely accurate. A too-weak mixture produces an irregular exhaust note, a splashy misfire, and colourless exhaust. A correct mixture produces a regular and even exhaust note. A too-rich mixture produces a regular or rhythmical misfire and noticeably blackish exhaust smoke. The exhaust note alone, listened to carefully at steady throttle on a trailing overrun, reveals a great deal about mixture strength in the cruising range.
Piston springs: the colour code system
The piston spring sits above the carburettor piston and controls how readily the piston rises in response to the partial vacuum created by the engine’s intake stroke. It is not merely a physical restraint on piston movement: it fundamentally affects the mixture strength throughout the entire operating range. A heavier spring resists piston movement more, which keeps the piston lower at any given engine speed and vacuum level, which enrichens the mixture across the board. A lighter spring allows the piston to rise more readily, weakening the mixture throughout the range.
The colour coding for springs on carburettors up to and including 1¾ inch bore is well established and appears in the period literature. Blue is the lightest, at 2½ ounces. Red is the medium spring at 4½ ounces and is generally the correct starting point for most standard or mildly tuned engines. Yellow is heavier at 8 ounces, and green at 12 ounces is the heaviest available for standard carburettor sizes. The 1968 tuning guide recommends beginning with the red spring in cases of uncertainty, because it represents the middle of the practical range and is appropriate for the majority of engine specifications.
The correct spring for a given application is the one that allows the piston to reach its maximum travel at the point in the rpm range where the engine develops maximum power. A spring that is too heavy prevents the piston from rising fully and starves the engine of mixture at high revs. A spring that is too light allows the piston to rise too readily at low engine loads, weakening the mixture at cruising speeds and causing the flat, unresponsive behaviour sometimes misdiagnosed as a needle problem.
If the carburettor has been fitted to an engine that is significantly larger or more powerful than the standard application, the blue spring may be appropriate because the stronger vacuum generated by the larger engine will raise the piston more aggressively than a standard unit. Conversely, if the carburettor is slightly oversized for the engine, as can happen when upgrading from standard specification, the yellow spring may be required to prevent the piston rising excessively at low loads. The 1968 guide notes this relationship explicitly and advises consulting the carburettor specification for a comparable engine as a starting point.
One practical note: changing the piston spring affects the mixture throughout the entire rev range simultaneously, unlike changing the needle which can be targeted at specific conditions. A spring change is therefore a blunt instrument compared to needle selection. It is most useful when the mixture is broadly too weak or too rich across the full range, rather than when the problem is confined to a specific part of the rev range.
Air intake: the effect of cleaners and ram pipes
The air intake arrangement has a direct effect on the mixture strength delivered by the SU carburettor, and it is an area that the 1968 tuning guide addresses with some precision. The basic principle is straightforward: any restriction to the air entering the carburettor reduces the velocity and volume of the incoming charge, which affects the partial vacuum that raises the piston and meters the fuel. Remove a restriction and the mixture weakens. Add one and the mixture richens.
The practical consequence is that removing the air cleaner from an SU carburettor weakens the mixture, and the effect is most pronounced at the upper end of the rev range where air velocity and volume are highest. A car that ran correctly with a specific needle and an oil bath air cleaner will run lean if the cleaner is removed, because the oil bath type imposes more restriction than the open intake. Paper element cleaners impose less restriction and therefore have a smaller weakening effect if removed. The period guide notes this distinction specifically. If you have changed the air cleaner type or removed it entirely as part of a performance modification, the needle specification will almost certainly need revisiting.
Ram pipes fitted to the intake flange of the carburettor are a different matter. Short trumpet-shaped pipes of between two and three inches in length, fitted in place of the standard air cleaner connection, can produce a modest power increase in the mid-range by improving the induction efficiency of the carburettor at the intake flange. The effect is generally a slight weakening of the mixture at mid-speed, and the 1968 guide notes that there is no simple formula for ram pipe length: the effect varies with the specific installation and requires road testing to evaluate. For a road car used in normal conditions, the improvement is typically marginal. For a competition engine used at sustained high revs, ram pipes are rather more worthwhile.
Tuning twin carburettors: the additional considerations
MGB, GT6, Triumph Vitesse, and a number of other classic British cars run twin SU carburettors, and tuning these for optimum performance involves an additional layer of complexity beyond what applies to a single unit. The two carburettors must be correctly balanced with one another before any needle or spring optimisation is attempted. Attempting to refine individual needle profiles on an unbalanced pair is like trying to adjust a piano while one of the strings is missing. The effort will be wasted.
The 1968 guide devotes a complete section to twin carburettor tuning and the procedure it describes is substantially the same approach still used today. The throttle openings of both carburettors must be equal at idle, verified by comparing the intensity of the intake hiss at each carburettor. When the hiss from both is identical, the throttles are balanced. The mixture controls must then be adjusted equally and simultaneously, so that both jets move at the same time. Mixture strength is checked on the front carburettor using the lifting pin method, then verified on the rear. Because the two carburettors are interconnected and interdependent, any adjustment to one affects the other, and the guide is explicit that after adjusting either carburettor the other must be rechecked. This iterative process is the reason twin carburettor tuning takes considerably longer than it theoretically should.
For twin SU owners, the SU carburettor setup guide covers the synchronisation procedure in detail. Once synchronisation is confirmed, the needle and spring optimisation described in this article applies to both carburettors simultaneously rather than individually.
The consumption check: separating performance from economy
Once the needle selection has produced an engine that accelerates cleanly, cruises smoothly, and pulls strongly at high revs, the period tuning guide recommends a final series of consumption tests to determine whether any further refinement is possible. The principle is straightforward: once performance is satisfactory, it may be possible to lean the mixture slightly at specific points in the rev range and recover some fuel economy without noticeably compromising performance.
The test involves sustained driving at fixed speeds of 30, 40, and 50mph, measuring fuel consumption at each. If consumption is noticeably worse at a particular speed, the needle dimensions governing that speed range may be slightly richer than necessary for optimum efficiency. A needle with slightly leaner dimensions at that section can recover economy while maintaining adequate mixture strength. However, and this cannot be emphasised strongly enough, always confirm that performance at high speed remains satisfactory after any leaning adjustment. Economy gains achieved by running weak at full throttle are not economy gains at all: they are engine damage presented as financial prudence.
Setting realistic expectations
SU carburettor optimisation is one of the more rewarding exercises available to the classic car enthusiast who is prepared to approach it methodically. It also requires patience, systematic testing, and a willingness to accept that the perfect needle setting on a road car may involve a degree of compromise between idle, mid-range, and top-end performance that cannot be entirely eliminated with a single needle profile. The 1968 tuning guide is candid about this and notes that without a test bed and the ability to conduct controlled assessments, road testing is the practical approach, and road testing is by definition approximate.
What it will not do is transform an otherwise poorly prepared engine. The SU carburettor meters fuel with great precision when correctly set up. But it cannot compensate for worn valve guides, incorrect valve clearances, a tired camshaft, or ignition timing that is adrift. Before attributing poor performance to carburation, it is worth ensuring that the rest of the engine is in good order. Our valve clearance guide and our pre-season safety check cover the mechanical foundations that carburettor optimisation depends on. Get those right first, and the SU system will reward you with a level of responsiveness and driveability that makes the investment of time entirely worthwhile.
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