If the SU carburettor is the British classic car world’s most celebrated variable-choke unit, the Zenith-Stromberg is its slightly less glamorous sibling: broadly similar in operating principle, different in construction, and fitted to a significant proportion of the cars that crossed the Atlantic from the late 1960s onward. The Stromberg arrived because American emissions regulations were becoming increasingly hostile to the SU, and Zenith’s engineers produced a solution that met the requirements while retaining the constant depression principle that made variable-choke carburettors so effective. The result is a carburettor that works beautifully when properly maintained and drives its owners to distraction when it is not. This guide will put you firmly in the first category. Our SU carburettor guide covers the HS and HIF units; this one is for the Stromberg and the owners who have one, whether by choice or by the accident of having bought a US-specification TR6 and discovered the fact only after the paperwork was signed.
Which cars have Strombergs
The Zenith-Stromberg CD series was fitted to a range of British classics, predominantly in US-market specification but not exclusively. The most common configurations are:
- Triumph TR6 (US specification): twin 175 CD-2 carburettors, replacing the Lucas mechanical fuel injection of the UK car and producing, as a consequence, approximately 46 fewer brake horsepower. American TR6 owners have had four decades to come to terms with this.
- Triumph Spitfire and GT6: single 150 CD on various market specifications
- Triumph Stag: twin 175 CD on carburettor-equipped versions
- Triumph 2000 and 2500 saloons: various CD specifications
- MGB and MG Midget (later US specification): single 175 CD, replacing the earlier twin SU arrangement that the Americans were not allowed to have any more
- Rover 2000 and 2200: CD units on some specifications
Stromberg carburettors are identified by a number and letter code. The number (125, 150, 175) indicates the choke diameter in fractions of an inch. The letters indicate the type: CD, CDS, and CD2S are the pre-emission versions found on most classic British cars and the ones this guide primarily addresses. The suffix E (CDSE, CD2SE) denotes an emission-controlled version with additional equipment, restricted adjustments, and a generally more sullen disposition. All types follow the same fundamental tuning procedure, though the emission versions will test your patience in ways the standard units do not.
How the Stromberg works: the important differences from the SU
Like the SU, the Stromberg is a constant depression carburettor. A variable-choke venturi, an air valve (the Stromberg’s name for its piston) that rises and falls to maintain constant pressure across the jet, and a tapered metering needle controlling fuel delivery across the throttle range. The operating elegance is identical to the SU and stems from the same insight: that a self-regulating system produces better results across a wider range of conditions than a fixed-choke carburettor trying to cover different operating conditions with multiple fixed jets.
The critical constructional difference is the diaphragm. Where the SU uses a solid piston sliding in a machined bore, the Stromberg air valve is surrounded by a thin rubber diaphragm that seals the suction chamber from the atmosphere. This is the component that defines Stromberg ownership. The diaphragm must flex freely and seal perfectly at all times. A pinhole, a tear, or a diaphragm that has hardened with age will produce symptoms ranging from mild rough running to fuel spitting from the vent pipe and a complete inability to idle. Every Stromberg diagnosis starts with the diaphragm. Every single one. Not the needle. Not the jet. The diaphragm. Write it on your hand if that helps.
The second important difference from the SU is the mixture adjustment mechanism. On the SU, mixture is adjusted by moving the jet relative to the needle. On the Stromberg the jet is fixed and the needle moves within the air valve via an adjuster accessible from the top of the dashpot. Raising the needle richens the mixture; lowering it weakens it. The effect is identical to the SU but the method is different enough to catch out anyone approaching their first Stromberg with confident SU habits and insufficient preparation.
The one tool you actually need
Adjusting the Stromberg needle requires an Allen key inserted through the top of the dashpot. The problem is that turning the Allen key without restraining the diaphragm hub imparts a twisting force on the diaphragm that can tear it. This is not a remote theoretical possibility. It happens with sufficient regularity that Zenith designed a specific adjustment tool to prevent it: a tube that engages with a notch in the diaphragm hub to hold it stationary while the Allen key turns the needle adjuster inside. Without this tool you are relying on the diaphragm’s goodwill, and diaphragms on classic British cars have had a complicated few decades and are not feeling particularly generous.
The tool costs around £10 to £15 from Moss Europe, Rimmer Bros, or SC Parts and pays for itself immediately by not costing you a new diaphragm and three hours of unexplained frustration. Buy it before you need it. The alternative, if the tool is genuinely not to hand, is to remove the dashpot cover completely, lift the air valve out, make the needle adjustment while holding the assembly safely in your hand, and reassemble. Slower, more steps, perfectly legitimate, zero diaphragm risk. What is not legitimate is inserting an Allen key without the tool and hoping that this time it will be fine. It will not be fine. The diaphragm will demonstrate this at a time and location of its own choosing.
You will also need: a small coin or wide flat screwdriver for the jet adjusting screw, a tachometer for idle speed, SAE 20 or engine oil for the dashpot damper, a timing light (ignition timing should be confirmed before carburettor tuning, not after, as one directly affects the other), and genuine patience about warming the engine fully before touching anything. All carburettor adjustments on a Stromberg are made at full operating temperature. Adjustments made on a cold engine are accurate for a cold engine and meaningless for a warm one, which is the only condition the car actually spends most of its time in.
Step one: inspect the diaphragm
Remove the air cleaner. Undo the dashpot cover screws: typically four or six small screws set in aluminium, which means they are the kind of screws that strip if approached with anything more forceful than polite intent. Use a good-fitting screwdriver and apply firmness rather than enthusiasm. Lift the cover carefully and inspect the diaphragm under a bright light, holding it up to check for pinholes as well as visible tears. Check the edges where the diaphragm seats in its groove, the area around the two moulded tabs, and the centre where the air valve rod passes through. Any damage at all means replacement before proceeding.
A lesser-known issue worth knowing: if the diaphragm has been in contact with moisture or cleaned with anything damp, it may have swelled slightly and will not seat correctly in its groove until it has dried at room temperature. A diaphragm that looks intact but fits loosely or will not settle in its recess needs an hour to dry, not more persuasion. And never use solvent-based cleaners, strong carburettor cleaner, or trichloroethylene anywhere near a Stromberg diaphragm. Paraffin on a clean cotton rag is the approved method, applied gently. The diaphragm was not designed to resist aggressive chemistry and will make this clear if you test the theory.
Replacement diaphragms are not expensive and are available from all the major British car parts suppliers. Fitting a new one takes ten minutes and transforms the tuning process from an exercise in mounting frustration into a procedure that actually produces consistent results. Keep a spare in the glovebox if you use the car regularly. A diaphragm that fails forty miles from home on a Saturday afternoon produces a very specific and entirely preventable kind of afternoon.
Fitting a replacement diaphragm: the details that matter
The diaphragm has two moulded tabs, one on the inner diameter and one on the outer. The inner tab engages with a slot in the air valve. The outer tab engages with a slot in the carburettor body. Both tabs must be correctly located before the cover goes back. An incorrectly oriented diaphragm will either prevent the air valve from moving freely or will tear within minutes of the first engine start. Take the time to confirm both tabs are properly seated before tightening a single screw.
There are also two small holes in the bottom rim of the air valve piston. These face toward the engine, not away from it. If the piston has been removed for any reason and you are not certain of its orientation, the holes go toward the intake manifold side. Getting this wrong produces a carburettor that does not function correctly in ways that are genuinely difficult to diagnose without knowing this specific detail exists, and it is not the kind of thing covered in most workshop manuals with sufficient emphasis.
When refitting the dashpot cover, tighten the screws in a cross pattern to ensure even seating of the diaphragm bead all the way around. Overtightening strips the threads in aluminium. Undertightening leaves an air leak at the cover joint that produces lean running identical in symptoms to several other causes. Snug and even is the target.
Step two: check the dashpot damper
The dashpot damper slows the rise of the air valve under sudden throttle opening, providing a momentary enrichment that prevents the flat spot otherwise caused by the needle lifting too quickly out of the jet. Without a working damper the engine hesitates on acceleration in a way that no mixture adjustment will cure, because the problem is the rate of valve travel rather than the mixture setting itself. Many a carburettor has been unnecessarily rebuilt because the damper was empty and nobody checked it first.
Remove the damper rod from the top of the dashpot and check the oil level in the hollow tube. It should be within 6mm of the top. Top up with SAE 20 or engine oil. Zenith specified their own product but any clean engine oil of similar weight works perfectly well.
A lesser-known refinement: the viscosity of the damper oil affects throttle response character. Heavier oil produces more pronounced damping and a slightly fuller response at the bottom of the throttle range. Lighter oil gives a quicker, crisper response. Most owners use straight engine oil and are perfectly satisfied. If the car hesitates on very rapid throttle blips despite correct mixture and a full damper, experimenting with a slightly heavier oil is worth trying before suspecting anything more fundamental.
Step three: check the float chamber and fuel level
This step is skipped by almost everyone and causes problems for a disproportionate number of people. The float chamber maintains a constant fuel level that underpins correct mixture delivery across the entire operating range. Too high and the carburettor runs rich regardless of needle position. Too low and it runs lean under load regardless of how carefully everything else has been set. Both conditions can be tuned around to a degree at idle using the jet screw, but the mixture will be wrong across the rest of the throttle range and no amount of needle adjustment will compensate for an incorrectly set float level.
To check: remove the float chamber from the base of the carburettor, invert it, and check the position of the floats relative to the chamber face. The correct dimension varies by model and should be confirmed in the workshop manual for your specific car, but on most CD series Strombergs the floats should sit level with or just slightly proud of the chamber face when inverted. Adjust by carefully bending the tang on the float arm that contacts the needle valve, working in very small increments and rechecking after each adjustment.
While the float chamber is removed, inspect the float needle valve. This small spring-loaded valve controls fuel entry to the chamber and is a known weak point on all Strombergs. A valve that sticks open allows the chamber to overfill and produces rich running that the jet screw cannot correct. A valve that sticks closed starves the engine under load. A popular and genuinely effective upgrade is the Grose jet: a ball-valve replacement for the standard needle valve that seals more positively, is less prone to sticking, and is resistant to the debris that occasionally clogs the original needle valve seat. Available from American TR6 and MGB specialists for around £8 to £12 per carburettor, the Grose jet is one of those improvements that receives universal approval in the TR6 community on the basis that nobody who has fitted one has ever needed to think about it again, which is the highest recommendation a carburettor component can receive.
Step four: check jet centralisation
Before adjusting mixture, confirm that the jet orifice is correctly centred beneath the needle. With the air cleaner removed, lift the air valve by hand and allow it to fall. If the jet is correctly centred the valve falls with a sharp, decisive click as the needle enters the orifice cleanly. A soft thud, a muffled drop, or an absent click indicates the needle is fouling the jet walls. No mixture adjustment will produce correct results until this is resolved. Attempting to tune a Stromberg with an off-centre jet is like trying to tune a piano with a hammer: the motions are broadly correct but the outcome is not what anyone wanted.
To re-centre: remove the damper rod and insert a pencil firmly down the hollow tube to hold the jet bush in position. Slacken the jet assembly half a turn to free the orifice bush, then retighten. Recheck by lifting and dropping the air valve. Repeat until the click is sharp and consistent.
Step five: confirm ignition timing before touching the carburettors
Check the ignition timing before adjusting the carburettor. A correctly set carburettor on incorrectly timed ignition will need re-setting when the timing is eventually corrected. Establishing the correct timing first means the carburettor setup is done once rather than twice. Our ignition timing guide covers the full procedure.
With timing confirmed, warm the engine to full operating temperature. Leave the air cleaner fitted throughout the tuning process. John Twist of University Motors, one of the most respected MGB and Stromberg specialists in the world, noted that the Stromberg was calibrated to work with its original air cleaner in place and does not perform or tune correctly without it. An aftermarket filter of different dimensions alters the airflow characteristics and produces a carburettor that is correctly set for the filterless condition and wrong for the normal running condition. Fit the original or a correctly specified replacement, and leave it in place while you work.
Step six: set the idle speed and adjust the mixture
With the engine at operating temperature, set the idle speed to 850 to 950 rpm using the throttle stop screw before adjusting mixture. On twin carburettor installations, adjust only the primary (master) carburettor’s throttle stop screw at this stage.
The jet adjusting screw
The jet adjusting screw is the brass fitting set centrally in the base of the carburettor body. A small coin gives better feel than a screwdriver and reduces the chance of the tool slipping. Viewed from below, turning clockwise weakens the mixture. Anti-clockwise richens it. Make adjustments of one eighth of a turn at a time, wait for the engine speed to stabilise, and assess before making the next change. Patience here produces accurate results. Haste produces a carburettor that has been adjusted several times in quick succession and is now in an unknown state relative to where it started, which is its own special kind of problem.
The lifting pin test
The lifting pin test assesses mixture strength at idle. With the engine at operating temperature, locate the lifting pin beneath the edge of the dashpot (not all Stromberg types have this; if absent use a long slender screwdriver or stiff wire to lift the air valve approximately 1mm, no more). Observe the engine speed:
- Speed rises and stays up: mixture is too lean. Richen by turning the jet screw anti-clockwise one eighth of a turn and retest.
- Speed drops immediately: mixture is too rich. Weaken by turning the jet screw clockwise one eighth of a turn and retest.
- Speed rises slightly then settles back to normal: mixture is correct.
Work toward the correct result in small increments. One eighth of a turn, wait, assess, repeat. The engine will tell you what it needs if you give it time to respond after each adjustment.
The needle adjuster
With the jet screw correctly set at idle, the needle adjuster provides finer control across the mid-range. Using the proper Stromberg tool engaged with the diaphragm hub notch, turn clockwise to lower the needle (weaker) or anti-clockwise to raise it (richer). The total range is approximately three turns. There is a hard stop at the full rich end. There is no stop at the lean end, which means turning too far anti-clockwise disengages the needle from the adjuster and drops it into the jet. The engine will then run as though several important decisions were made without adequate consultation. The cure is removing the dashpot cover, recovering the needle, and resetting to one and a half turns from the full rich stop. For a standard engine in good condition this is also the correct starting position.
A useful visual check: with the dashpot cover removed and the air valve lifted clear, the bottom of the needle shank should be approximately flush with the bottom face of the air valve at the one-and-a-half-turn position. This gives a physical reference point that is easier to confirm than counting small turns of an Allen key in a confined space with the engine running.
After any needle adjustment, recheck idle speed and repeat the lifting pin test. The two adjustments interact and a significant needle change will alter the idle mixture enough to require a small compensating tweak to the jet screw.
Balancing twin carburettors
Twin Stromberg installations on the TR6, Stag, and others require balancing as well as individual mixture tuning. Both carburettors feed different cylinders, and if they are not flowing equal volumes of air the engine will idle unevenly and perform inconsistently regardless of how carefully each unit was set up on its own. Balancing cannot be substituted with a better mixture setting. It is a separate step requiring a specific tool, and skipping it on a twin carburettor car is the equivalent of tuning two instruments separately and wondering why the duet sounds peculiar.
The tool needed is a carburettor airflow meter. The Gunson Carbalancer is the traditional choice: a device held against each carburettor air intake in turn that displays airflow on a dial. The Uni-Syn is a popular American alternative, widely used by TR6 owners. The Morgan Carbtune is the more refined option for the owner who takes these things seriously, using manometers rather than a mechanical gauge for greater precision. All work on the same principle of measuring and comparing the airflow through each carburettor.
A ColourTune spark plug is a useful separate tool for confirming mixture quality on each cylinder by observing combustion colour through a transparent plug body. It is not an airflow meter and cannot be used to balance carburettors against each other. The distinction matters because the two tools are occasionally confused, and using a ColourTune where a flow meter is needed produces no useful balancing information and the mild bewilderment of having done something technically correct for the wrong purpose.
Balancing procedure: with both carburettors individually tuned and the engine at operating temperature, hold the flow meter against each carburettor air intake and compare readings. They should be equal. If they are not, adjust the throttle stop screw on the secondary carburettor until its airflow matches the primary. Recheck overall idle speed and adjust the primary throttle stop screw if necessary to restore the correct rpm. Then recheck balance. The two affect each other slightly and may need one iteration to settle.
Once balanced at idle, blip the throttle and allow the engine to return to idle. It should settle quickly and evenly. A car that idles smoothly but stumbles on the return from a blip often has carburettors that are balanced at idle but diverging on the overrun, which points to unequal damper oil levels or differing needle settings between the two units. Check the damper oil is at the same level in both before pursuing anything more complicated.
After balancing, repeat the lifting pin test on each carburettor individually. Throttle adjustments made during balancing can alter idle mixture slightly, and confirming both units are still correctly set after the balancing process ensures the whole setup is consistent rather than correct in isolation.
The fast idle, choke, and seasonal adjustment
The fast idle stop screw provides increased idle speed when the choke is engaged. With the choke fully pushed home (the normal warm running position), there should be a gap of approximately 1mm between the fast idle screw head and the choke cam. If this gap is absent, the fast idle screw is holding the throttle slightly open at all times, producing a persistently elevated idle speed that no amount of throttle stop screw adjustment will fully cure. This is worth checking on any car where the idle speed seems reluctant to come down to the correct figure despite apparently correct throttle stop setting.
Some Strombergs have a two-position fast idle screw with a spring under its head. This is a seasonal adjustment provided by Zenith: with the spring compressed the fast idle is reduced for summer use; with the spring released the full fast idle is available for cold winter starting. This feature is rarely discussed in mainstream workshop guides and can cause puzzling cold-start behaviour on a car that has been correctly set up for summer and then asked to start on a cold January morning with the spring in the wrong position. If cold starting is inconsistently poor despite everything else being correct, check whether this adjustment exists on your carburettor and what position it is in.
The later emission-equipped versions use a water-heated automatic choke that opens as coolant temperature rises. This is either a sensible piece of engineering or a source of ongoing entertainment depending on the condition of the water connections and the bimetal spring. A stuck automatic choke produces rich running that gradually improves as the engine warms, which is the reverse of most other richness symptoms and catches people out regularly. If the car runs poorly rich when cold and increasingly better as it warms, the automatic choke is the first suspect, not the carburettor itself.
The temperature compensator
Some Strombergs are fitted with a temperature compensator: a bimetal device that bleeds additional air into the mixture when engine bay temperature rises above a threshold, leaning the mixture to account for the reduced density of hot air. In principle this is thoughtful engineering. In practice a compensator that has aged, stuck, or been incorrectly adjusted introduces a lean condition at high operating temperatures that is indistinguishable in symptoms from several other faults.
If the car runs correctly when cold but leans off and runs roughly after extended running or in warm weather, and the diaphragm is sound, cooling system functioning, and ignition timing confirmed, check whether a temperature compensator is fitted and whether it is operating correctly. A compensator stuck open bleeds air continuously and causes persistent lean running. One stuck closed provides no temperature compensation and causes over-richness in hot conditions. Many owners of compensator-equipped cars ultimately disable or remove them, accepting the minor mixture variation that results, on the basis that a known fixed quantity is less troublesome than an unknown variable one.
The overrun valve
Emission-equipped Strombergs often include an overrun valve designed to prevent the engine continuing to fire after the ignition is switched off. The valve is located in the carburettor body and operates on manifold vacuum. A failed valve that sticks open creates a continuous air leak producing persistent lean running. A leaking overrun valve gasket produces identical symptoms. On any emission-equipped Stromberg that runs lean despite correct adjustments throughout, the overrun valve and its gasket should be on the inspection list before the situation becomes an extended and frustrating diagnostic session.
Worn needles and jets: when adjustment is not enough
If the lifting pin test shows correct mixture at idle but the car runs rich on acceleration and lean at light throttle, or the mixture seems broadly correct but varies unexpectedly with throttle position, the needle and jet may be worn beyond the point where adjustment can compensate. The needle wears at its mid-section where it spends the most time in the jet orifice, and the jet wears from round toward oval as the needle vibrates against its walls over many thousands of miles. A worn jet cannot be adjusted into correct behaviour because the clearance between needle and orifice varies depending on the needle’s vertical position, making the mixture different at different throttle openings regardless of the adjustment setting.
Check the needle by rolling it on a flat glass surface: a bent needle will rock rather than roll true. Inspect the mid-section for wear steps or a polished worn area. A bent or worn needle should be replaced and is specified by a code marked on the needle itself. Replace the jet at the same time: fitting a new needle in a worn jet improves matters but does not resolve the fundamental issue, and the next time the carburettor comes apart you will be doing this job again.
Confirm with a plug reading
After completing the full tuning sequence, read the spark plugs after a run that includes some load and varied throttle. A correctly tuned Stromberg should produce plugs with a light tan to grey-brown colour on the insulator nose. Black sooty deposits indicate persistent over-richness. White or very pale grey indicates sustained leanness. The plugs tell you what the mixture has actually been doing across the full operating range in real conditions, not just at the idle point where the lifting pin test was made. Our spark plug diagnosis guide covers the full range of conditions and what each indicates.
Common faults and what they actually mean
- Fuel spitting from the vent pipe or air intake: almost certainly a torn diaphragm. Start here before investigating anything else, without exception.
- Rich running that does not respond to jet adjustment: torn diaphragm, float needle valve stuck open, float level set too high, or automatic choke stuck. Check in that order.
- Lean running that does not respond to adjustment: air leak at the base gasket, inlet manifold gasket, dashpot cover, temperature compensator, or overrun valve. The adjusters cannot compensate for an air leak. Find the leak and fix it first.
- Sticking air valve: carbon or dirt on the air valve OD or in its bore. Clean with paraffin on a clean rag only. Check the needle is straight by rolling it on a flat surface. Confirm diaphragm tabs are correctly aligned and not obstructing movement.
- Idle that will not settle despite apparently correct mixture: incorrectly aligned diaphragm tabs, needle dropped from the adjuster, dashpot cover not seating evenly, or a stripped cover screw allowing an air leak.
- Hesitation on acceleration despite correct idle mixture: empty or low dashpot damper. Check this before anything else. An empty damper is responsible for more unnecessary carburettor rebuilds than any other single fault.
- Needle dropped from adjuster: turned too far toward lean. Remove the dashpot cover, retrieve the needle, reset to one and a half turns from full rich, and approach the lean direction more cautiously this time.
- Runs rich when cold, improves as engine warms: automatic choke stuck or slow to open. Not a mixture adjustment problem.
- Runs correctly cold, leans off when hot: temperature compensator stuck open, or cooling system not maintaining correct temperature. Confirm thermostat function before blaming the compensator.
The Stromberg in honest perspective
The Zenith-Stromberg’s reputation for difficulty is largely inherited from decades of neglected diaphragms and adjustments attempted without the correct tool by owners expecting an SU and getting something different. A Stromberg with a sound diaphragm, correct float level, centred jet, full damper, and properly set needle and jet screw is a composed and effective carburettor that provides smooth, progressive fuel delivery and an idle that settles consistently. The operating principle is elegant and the construction is robust.
The practical maintenance difference from the SU is simply that the diaphragm is a service item requiring periodic inspection and occasional replacement, where the SU’s solid piston just needs keeping clean. Build diaphragm inspection into your annual service, keep a spare for long runs, do not attempt needle adjustment without the tool, and check the damper before suspecting anything complicated. Do these four things and the Stromberg will reward you with years of reliable service. Ignore any of them and it will demonstrate, with unhurried patience, exactly why each one was on the list.
For related reading: our SU carburettor guide covers the HS and HIF units in equivalent detail, our ignition timing guide covers the timing checks that should precede any carburettor setup, our spark plug diagnosis guide covers reading plugs to confirm mixture quality after tuning, and our fuel system guide covers E10 compatibility and fuel line maintenance relevant to all carburettor-equipped classics.