A proper MG T-Series tune-up follows a strict order: check compression and valve clearances, sort the ignition (points, plugs, timing), and only then tune the SU carburettors. The carbs are the last step, not the first — SUs are simple and reliable, and most "carburettor" faults actually lie in compression or ignition. Work in sequence and the engine transforms.
This walkthrough covers the XPAG and XPEG engines fitted from the TB onwards. The TA's earlier MPJG engine shares the same philosophy but differs in enough details — bore, valve gear and carburettor sizing — that its figures are not interchangeable, so TA owners should treat the sequence below as a template and verify every number against the MPJG workshop manual.
Every figure quoted here is a guide. Cylinder heads have been skimmed, camshafts changed and distributors swapped for the best part of seventy years, so confirm the specification for the engine actually in front of you before you commit a spanner.
The golden rule: don't start with the carbs
When an XPAG runs rough, the first instinct is usually to reach for a screwdriver and start twiddling the SU jet nuts. Resist it. The SU is one of the simplest carburettors ever made — a piston that rises with airflow, a tapered needle that meters fuel — and once it is set correctly it stays set. Real running faults almost always sit further up the chain:
- Compression — worn rings, tight or burnt valves, a tired head gasket.
- Ignition — pitted points, a worn distributor, fouled plugs, mis-timed static.
The correct running order for any T-Series tune is therefore:
- Compression and valve clearances.
- Ignition — points, plugs, static timing.
- SU carburettors — float, jets, throttles, mixture.
Everything in stages one and two is preparation for stage three. A pair of perfectly-set SUs on an engine with a burnt exhaust valve or 15° of missing advance will still run badly, and you will chase your tail at the carbs for a weekend.
Tools and preparation
Nothing exotic. Assemble it all before you start and lay it out somewhere clean:
- Screwdrivers in a decent range of sizes.
- The correct Whitworth and BSF spanners — metric substitutes round off T-Series fasteners.
- A compression tester with a 14 mm screw-in adaptor.
- Feeler gauges — ideally a go/no-go pair for the tappets.
- A carburettor balancing meter (a Unisyn-type airflow meter or equivalent).
- A Colortune in 14 mm.
- A dwell / multimeter that reads dwell, rpm, points-condition and volts.
- An SU jet-centring tool and jet-adjusting spanner.
- Pince-nez pliers for cotter pins and clevis clips.
- A fresh rocker-cover gasket (nitrile lasts longer than cork) and a handful of new cotter pins.
- Pen, paper and a clean bench. Write down every reading as you take it.
Sources for period tooling and quality spares are listed on the Parts & Specialists page.
Section 1 — Valve (tappet) clearances
Set the tappets with the engine hot — a full run to operating temperature, not just an idle in the drive. The pushrods, block and head must all be at their working length or the setting will be wrong the moment the engine cools.
Use a go/no-go feeler-gauge pair: the "go" size should slide through with a light drag, the "no-go" should refuse to enter. That gives a repeatable setting that a single feeler, which will always feel "about right", cannot.
Clearance guide
Measured on the valve side of the rocker, engine hot:
| Engine range | Clearance |
|---|---|
| TB, TC and TD Mk II up to engine 24115 | 0.019 in |
| TD / TD Mk II from engine 24116, TF, TF 1500 | 0.012 in |
| Unknown head or engine origin | 0.019 in (safer) |
The rule of nines
Find the valve to adjust by adding to nine. Rotate the engine until the reference valve is fully open, then adjust the valve whose number adds up to nine with it:
| Valve fully open | Adjust valve |
|---|---|
| 8 | 1 |
| 6 | 3 |
| 4 | 5 |
| 7 | 2 |
| 1 | 8 |
| 3 | 6 |
| 5 | 4 |
| 2 | 7 |
Working down the list turns the crank in one direction and avoids missing a valve. Slacken the locknut, adjust until "go" enters and "no-go" refuses, hold the adjuster and lock it, then re-check.
Refit the rocker cover with a fresh gasket, tightening the nuts gently and evenly — the alloy pressing distorts easily.
Section 2 — Compression check
Compression tells you whether the tune has anything to work with. It is worth doing every service.
- Run the engine to operating temperature and stop it.
- Remove all four plugs so the engine turns freely.
- Prop the throttle wide open — a partly-shut throttle will strangle the reading.
- Disable the ignition (pull the coil lead to earth or switch off).
- Screw the tester into number one and crank through several compression strokes until the needle stops rising. Record the highest reading.
- Repeat for cylinders two, three and four, writing each figure down.
Compression guide
Indicative only — verify against the workshop manual and adjust for a skimmed head:
| Engine | Typical reading |
|---|---|
| TB, TC, TD | ~100–140 psi |
| TD Mk II, TF, TF 1500 | ~110–150 psi |
| Cylinder-to-cylinder spread | No more than ~20 psi |
Reading the numbers
- A cylinder that builds steadily to a healthy figure is fine.
- A cylinder that comes up low on the first stroke and climbs slowly, never reaching the guide, suggests worn rings. Confirm with a wet test — a teaspoon of clean engine oil down the plug hole, crank again, retest. A jump in reading confirms rings; no change points to a valve or the head gasket.
- Two adjacent cylinders both low? Suspect the gasket between them.
Section 3 — Ignition: plugs, points and condenser
With compression sound, move to the spark side.
Spark plugs
- Remove all four plugs, keeping them in cylinder order.
- Clean the electrodes with a soft brass brush, file the centre electrode flat and gap to 0.025 in on all T-Series.
- Verify the correct reach for your cylinder head — later heads generally take a longer reach; a short-reach plug in a long-reach hole leaves exposed thread that will carbon up.
- Read the porcelain: light tan/biscuit is right; black and sooty is rich (or oil-fouled); wet or oily suggests worn rings or valve stem seals.
Distributor cap, rotor and leads
Check the cap for hairline cracks, tracking and worn HT segments; the rotor for burning at the tip; and the HT leads for cracked insulation. Any of these will lose spark under load without necessarily failing in the driveway.
Points and dwell
The manual quotes a points gap of around 0.010–0.012 in, but the truthful measurement is dwell angle — the number of crank degrees the points are closed. Lucas fitted different distributor cams to T-Series distributors over the years, and each needs its own setting. There is no reliable way to tell them apart short of taking the cap off and looking, so verify before you start turning screws.
| Distributor cam | Points gap (guide) | Dwell angle (guide) |
|---|---|---|
| Symmetric | 0.010–0.012 in | ~45° |
| Asymmetric | 0.010–0.012 in | ~49° |
| High-lift | 0.014–0.016 in | ~60° |
Fit a fresh condenser at the same time as new points — they are cheap and the old one is almost always tired.
Section 4 — Ignition timing (static, on these cars)
T-Series cars are timed statically, not with a strobe. The centrifugal advance mechanism begins to move as low as 300 rpm, so by a normal idle of 700–800 rpm the distributor has already added several degrees. A strobe therefore reads a moving target and cannot give a repeatable static number.
The Colortune method
- Remove the plugs so the engine turns freely by hand.
- Bring number one cylinder to TDC on the compression stroke — rotor pointing at the number one segment, pulley mark aligned with the timing-cover pointer.
- Screw a Colortune into number one plug hole and connect its HT lead.
- Slacken the distributor clamp.
- Rotate the distributor body to retard (in the direction the rotor turns).
- Switch the ignition on.
- Rotate the distributor slowly to advance until the Colortune flashes. That is TDC ignition.
- Lock the clamp without letting the body move, then refit the plugs.
Modern-fuel adjustment (optional)
The XPAG was designed for post-war "Pool" petrol of roughly 70 octane. Modern unleaded is far higher octane and tolerates more advance, so many owners set the static timing a few degrees before TDC — commonly around 5° BTDC — for a livelier engine.
You can mark this on the pulley accurately. The pulley circumference works out to about 11.75 in, so one degree is roughly 0.033 in around the rim and 5° is a little under 0.17 in from the TDC mark, in the direction of rotation.
Section 5 — How the SU carburettor works
A brief detour that makes everything in the next section obvious.
The SU is a variable-venturi, constant-vacuum carburettor. A piston in the suction chamber rises and falls with airflow through the intake, keeping the depression across the jet constant. A tapered needle hangs from the piston into the fuel jet below; as the piston lifts, a thinner part of the needle sits in the jet and more fuel is metered through. There is no separate idle circuit and no accelerator pump — a small oil-damped piston in the dashpot momentarily resists lift on throttle openings, giving a brief enrichment that stands in for one.
The whole thing has almost no moving parts. Once set correctly it stays set. That is why the tuning routine below focuses on setting each element once, in order, rather than fiddling.
Read the dedicated SU carburettor tuning article for a deeper look at needles and profiles; this section is the tune-up sequence itself.
Section 6 — Tuning the carburettors, in order
Work through these steps in sequence. Skipping ahead — for example, setting mixture before synchronising throttles — is the single most common reason a tune won't come good.
1. Float level
Aim for a fuel level about 3/8 in below the jet-bridge. MG's workshop figure and SU's own 7/16 in float-lever spec are both acceptable — either gives a workable level once fuel weight is accounted for. Check the needle valve seats cleanly and shake the float: a rattle means fuel has leaked in and it must be replaced.
2. Piston, needle and jet
- Clean the suction chamber and piston carefully with a lint-free cloth. Never swap pistons between chambers — they are matched pairs machined to a running clearance of a few thousandths of an inch.
- Inspect the needle for bends and scoring; a shiny wear line on one side means the jet has been mis-centred.
- Set the needle shoulder flush with the piston face.
- Confirm the correct needle profile — commonly ES for standard road use — and the standard 0.090 in jet.
- Fit the correct coloured piston spring where required:
| Model | Damper spring |
|---|---|
| TB, TC, early TD | Standard (no colour code) |
| TD Mk II | Red |
| TF, TF 1500 | Blue |
TD Mk IIs and TFs will run badly with the wrong spring — the piston lifts at the wrong rate and the mixture never comes right.
Fill each damper with SAE 20 oil to about 1/2 in from the top.
3. Centre the jets
With the jet properly centred, a piston lifted with the lifting pin and released should drop and hit the bridge with a clean metallic click. A dull thud, or the piston hanging up, means the needle is fouling a mis-centred jet.
Loosen the jet-nut, feed the centring tool up through the jet, tighten the jet-locking bolt, then re-check. Set a starting mixture by winding each jet nut fully up, then down one full turn (six flats).
4. Synchronise the throttles first
With the engine warm, slacken the interconnecting linkage between the two carburettors. Fit an airflow meter over one intake, then the other, and adjust the two throttle-stop screws until both carbs draw the same reading. Set idle to 700–800 rpm, always moving both stop screws equally.
Reconnect the linkage without disturbing the stops.
5. Set the mixture
Working both jet nuts equally, a flat at a time, find the fastest smooth idle. Continue enriching until the idle slows from richness, then back off a flat or two to the fastest point. Rebalance idle speed with the throttle stops as you go.
6. Fine-tune with the lifting-pin test
Each SU carries a small lifting pin under the intake flange. With the engine idling, lift the piston about 1/32 in and let it fall:
- Brief rise, then settle — mixture is correct.
- Rise and stay up — mixture is rich; lean the jet a flat.
- Immediate fall or stall — mixture is lean; enrich a flat.
Because both carbs share a common manifold, an adjustment on one will affect the reading on the other. Expect to go back and forth between the two carbs until both react identically. Refit the air cleaners and re-check — they slightly enrich the mixture.
7. Choke and fast idle
- Set the jet-link and choke-cable slack so both jets drop together on full choke and return fully home on release.
- With full choke applied, set the fast-idle screw clearance to around 0.016 in on the correct screw. This lifts idle for cold running without opening the throttles.
Section 7 — Common traps and how to spot them
A short field guide to the faults that make tune-ups drag on:
- Won't idle slowly (below 800 rpm) — almost always an air leak: worn throttle spindles, split balance-tube ends, tired manifold or carb-to-manifold gaskets. Spray a little carb cleaner around the joints with the engine idling; a change in note reveals the leak.
- Won't go lean enough (jet nuts wound right down) — needle in the wrong position, float level too high, leaky float valve or a worn jet-gland letting fuel past the seal.
- Jet-nut positions differing by more than about two turns between the two carbs — one carb has a float, needle or jet fault; do not "average it out" at the nut.
- Sticking jet — jet not returning fully when choke is released. Usually a bent centring tube or dry, hardened jet gland.
- Poor cold starting — almost always ignition, not carbs: weak spark, wrong plug heat range or a choke that isn't dropping the jets fully.
- Sudden richness after a good tune — thicker damper oil, a piston swapped between chambers, or a needle shoulder no longer flush.
- The classic synchronisation trap — a lean carb reads as rich on the lifting-pin test because its throttle is closed further than its partner. Always resynchronise before diagnosing mixture.
Where this tune-up sits in the wider system
This article is the walkthrough that ties the individual system articles together. When you want to go deeper on a particular stage:
- SU carburettor tuning — needles, profiles and the deeper theory of the H-type.
- Ignition and charging — distributor rebuild, coil polarity, positive earth and dynamo behaviour.
- Cooling and overheating — why a healthy tune runs cooler, and how ignition timing affects head temperature.
- Parts & Specialists — where to source correct plugs, points, needles, jets and Colortunes in the UK.
Work through the sequence as it is written, verify every figure against the workshop manual for your specific engine and camshaft, and the engine will pick up crisply from idle, pull cleanly through the middle of the rev range and settle at cruise without complaint. Do it once a season and it will stay that way.