With the ignition right, the carburettors have a fixed target to aim at. Setting up twin SUs is not a dark art — it is a fixed sequence, done in order and never skipped. Here is that sequence, by ear and by the lifting pin. This is the site's single guide to setting up and tuning the twin SUs on a T-Type; for how the SU works in principle, seehow a carburettor works, and for the parts of the carburettor and servicing them, see theSU Type H components and servicing guide. When the carburettors are worn, see the SU carburettor rebuild guide.
First, the golden rule
Do the ignition first, and make sure the engine is mechanically sound and fully warm before you touch a carburettor. Cold, or with an air leak at a worn throttle spindle, no amount of adjustment will come right. Work in this order — float level, jet centring, balance, then mixture — because each step depends on the one before.
When an XPAG runs badly, the cause almost always lies in the ignition (worn points, a tired condenser, a seized mechanical advance or a cap tracking to earth) or in compression (worn rings, burnt or leaking valves, a failed head gasket), and no amount of carburettor adjustment will disguise any of those. Before you touch the carburettors, check that the plugs are the right grade, correctly gapped and a consistent light tan across all four; set the points and the static timing to the Workshop Manual figure; confirm with a strobe that the advance opens up as the revs rise; and run a compression test on a hot engine. All four cylinders should be within roughly 10% of each other, and a low one wants investigating, not tuning around.
1. Float level
The float chamber must hold petrol at the right height, because that sets where the fuel sits in the jet before the engine even draws on it. Too high floods and richens; too low starves. It is checked with the float-chamber lid off, to the dimension in the manual, and set by adjusting the float or its tab. Both carburettors must match — an uneven pair can never be balanced.
2. Jet centring
The jet must sit dead concentric around the needle, so the piston falls freely and the needle never rubs. The classic check: remove the damper and air filter, lift the piston with a finger and let it drop — it should fall with a clean, audible click onto the bridge. A dull or sticky landing means the jet is off-centre. Centring is done with the jet-locking nut slackened, letting the assembly settle true around the needle before nipping it up. An off-centre jet makes correct mixture impossible, so this is not a step to skip.
Needle position and jet depth
With the jets centred, check the needle in each piston: its shoulder should sit flush with the base of the piston, unless the manual calls for a specific offset for your needle profile. Then set the starting jet depth: as a guide, around 0.160 in below the bridge, with both carburettors matched to within about 0.020 in of each other. Matching the pair matters more than hitting the absolute figure; two carburettors that are identical but slightly off will tune sweetly, while two set far apart will never idle cleanly. Confirm the exact figures for your carburettor and needle combination against the Workshop Manual.
3. Balance the pair
Two carburettors must draw exactly the same air at idle, or they fight each other and no mixture setting will feel right. Warm the engine, disconnect the throttle linkage between the carbs, and use a balancing meter (a Unisyn or similar) — or simply listen at each intake with a length of tube — adjusting each carburettor's idle screw until both pull equally. Then reconnect the linkage so they open together. Balance and mixture affect each other, so you will come back and re-check this.
4. Mixture — by ear and lifting pin
Now the fuelling. Mixture is set by raising or lowering the jet with the adjusting nut — down richens, up weakens — a flat at a time. The test is the piston lifting pin: with the engine idling and warm, lift the pin a whisker (about a millimetre) and watch the response. If the revs rise and stay up, it is too rich — weaken it. If the revs drop or the engine stumbles, it is too weak — richen it. If the revs rise very slightly and settle straight back, the mixture is right. Set each carburettor the same way, front then rear, a flat at a time, re-testing after every move.
Damper oil
The oil in each dashpot slows the piston as it rises, which briefly enriches the mixture when the throttle opens: the SU's equivalent of an accelerator pump. SU recommend a 20-weight oil in the dampers of the H-type carburettors fitted to the T-Series. Thicker oil makes the piston rise more slowly and the transient mixture richer; thinner oil does the opposite. Treat damper oil as a fine-tuning tool, not a cure for a mis-set carburettor: start with the recommended grade and leave it alone until everything else is right.
The objective check, reading the exhaust
The lifting-pin test is quick and needs no equipment, but it has real limits: it only judges the mixture at idle, under no load, and how far you lift the pin changes the answer. This is why a professional like Andrew reaches for an exhaust gas analyser, a meter whose probe goes into the tailpipe (or a clamp/sensor on the pipe) and reads what is actually coming out of the engine. Where the pin test infers the mixture from how the engine responds, the analyser measures it directly.
Why the exhaust tells you the mixture
Combustion leaves a chemical fingerprint, and the exhaust carries it. When the mixture is rich there is not enough air to burn all the fuel, so unburned carbon leaves as carbon monoxide, and the amount of CO climbs almost in step with how rich the engine is running, which makes it a superb, sensitive measure of richness. A lean mixture shows the opposite: little CO but excess oxygen. Unburned hydrocarbons (HC) are the other key reading, a spike in HC means fuel is escaping unburnt, the classic signature of a misfire, a leak or an ignition fault. Read together, CO tells the tuner how rich or weak the mixture is, and HC warns whether the burn itself is clean.
Lambda, putting a number on it
Modern analysers boil all this down to a single figure, lambda. Lambda is simply the ratio of the air the engine actually got to the air it would need to burn its fuel perfectly. Lambda of 1.00 is bang on that ideal (stoichiometric) mixture; below 1.00 is rich, above 1.00 is lean, so 0.95 is five per cent rich, 1.05 five per cent lean. A classic engine is usually set a touch rich of 1.00 for a little protection and smooth running, and the tuner adjusts the jets to bring the reading to the target the engine wants. The beauty of it is objectivity: no guessing from engine note, just a number, repeatable every time.
It doesn't replace the basics
An analyser makes mixture-setting precise, and on twin SUs it lets the tuner match the two carburettors exactly and see how the mixture behaves as the throttle opens, something the pin test cannot show. But it sits on top of the sequence, it doesn't skip it: the float level, jet centring and balance still have to be right first, and a healthy ignition still comes before any of it. The analyser is how a professional confirms and fine-tunes what the method has already got close, which is exactly how Andrew uses it on these cars.
5. Idle, and round again
With balance and mixture close, set the idle speed to the book figure on the (linked) idle screws, kept equal. Snap the throttle open and shut a few times to let everything settle, then re-check the balance, and re-check the mixture — the two are interdependent, and a proper set-up means going round the loop once or twice until nothing needs touching. When it idles evenly, picks up cleanly off idle, and the lifting-pin test reads right on both carburettors, you are done.
Quick lean-vs-rich check
Before you reach for the jet nut again, listen and look. Most mixture problems show a clear signature once you know what to watch for. Use this as a roadside sanity check after the lifting-pin test.
| Symptom | Likely direction | First thing to check |
|---|---|---|
| Engine stalls when the throttle is snapped open | Too weak | Lifting-pin test — revs drop or stumble |
| Hesitation or flat spot on acceleration | Too weak | Damper oil level and grade; piston rising too fast |
| Black, sooty plugs; strong petrol smell at idle | Too rich | Float level too high or needle/seat worn |
| Revs hang high and slowly settle back to idle | Too rich | Lifting-pin test — revs rise and stay up |
| Popping back through the carburettor at light throttle | Too weak or ignition miss | Spark first, then mixture; check for air leaks |
| Popping or banging from the exhaust on overrun | Too rich or unburned charge | Mixture and ignition timing together |
Remember that a weak mixture can also be caused by air being drawn in downstream of the carburettor — a split manifold gasket, a loose carburettor flange, or a worn throttle spindle — so always rule out leaks before you keep richening the jet.
When it still will not come right
If you cannot get a clean mixture however you set the jet, stop adjusting and look for a fault, because you cannot tune around one. The usual culprits are an empty or wrong-grade damper (top it up first — it takes thirty seconds), a worn throttle spindle drawing air, a punctured float, a dropped or worn jet and needle, or a needle clipped at the wrong height. Fix the cause, then set up from step one.
Common wear to check before chasing a tune
- Worn throttle spindles. Worn bushes let air past the spindle, an idle air leak no mixture setting can correct. The symptom is an idle that will not come down, or a hunting idle that changes with a squirt of carburettor cleaner around the spindle ends. The cure is to rebush the bodies and fit new spindles.
- Worn needles and jets. A needle that has rubbed on a mis-centred jet develops a shiny flat along one side, and a jet worn oval runs rich whatever the setting. Both are cheap; replace them together.
- Mismatched piston springs. The dashpot spring must match the needle profile for your engine, and two carburettors with different springs will never balance properly.
- Sticking pistons. A piston that will not fall cleanly to the bridge is fouling the needle, gummed with old damper oil, or has a dented suction chamber. Clean it thoroughly; if it still sticks, the dashpot and piston need attention.
A note on modern fuel
Today's pump petrol is not the fuel these carburettors were designed for. Several specialists now offer needles re-profiled for modern fuel; if the plugs come out sooty despite a careful set-up, ask a specialist about one for your engine and camshaft combination. Where possible, use E5 super unleaded rather than E10: it is kinder to old fuel-system rubber.
Common questions
- How do I tune twin SU carburettors on an MG T-Type?
- In order: confirm the ignition and compression are right; set the float level; centre each jet; set the needle shoulder flush with the base of the piston; set jet depth to around 0.160 in below the bridge with the two carburettors matched to within about 0.020 in of each other; balance the airflow; then set the mixture and confirm it with the lifting-pin test. Verify all figures against the Workshop Manual for your carburettor and needle combination.
- What oil goes in SU dashpots?
- SU recommend a 20-weight oil in the dampers of the H-type carburettors fitted to the T-Series. Thicker oil enriches the transient mixture during throttle openings; thinner oil leans it. Start with the recommended grade and only change it once everything else is set correctly.
- Why are my SU carbs running rich?
- Common causes are worn jets, a needle that has been rubbing on a mis-centred jet, damper oil that is too thick, a sticking piston, or a float level set too high. Check the wear items first, then consider asking a specialist about a needle profiled for modern fuel.
- Should I tune the carbs or the ignition first?
- The ignition, always. A worn distributor, incorrect timing or dead compression on a cylinder will make a perfectly set pair of SUs run badly, and no amount of carburettor adjustment will fix it. Get the ignition and compression right, then set the carburettors, and the tune will hold.
With thanks to Andrew at AG Classic Car Tuning, mobile classic-car tuning specialists in North Yorkshire.