::standfirst Most XPAG running and overheating troubles trace back to ignition. A worn Lucas distributor, tired points or incorrect timing will make a good engine run badly. These are positive-earth cars with a dynamo and control box, so getting the ignition, timing and charging right transforms how the car runs.
The distributor is usually the culprit
Nine times out of ten, a T-Type that runs roughly, refuses to pull cleanly, or overheats in traffic is not asking for a new radiator or a carb rebuild — it is asking for its distributor to be sorted. Worn shaft and bushes let the cam wander so the points gap varies with every revolution. Weak or seized advance springs and bob-weights stop the timing advancing under load. Pitted contacts, a tired condenser and a fouled cap all conspire to weaken the spark just when the engine needs it most.
A full distributor overhaul — new bushes, refreshed advance mechanism, fresh points, condenser, cap and rotor arm — is one of the most cost-effective improvements you can make to a T-Type. It usually does more for driveability than any amount of carburettor fiddling.
An hour spent on the distributor is worth a weekend spent on the carbs.
The tune-up basics
Treat ignition service as a set, not as individual jobs:
- Points. Renew when pitted or burned; set the gap to the manual figure with a feeler gauge, then verify dwell if you have a meter.
- Condenser. Replace with the points. A tired condenser causes misfires, hot points and hard starting.
- Coil. Original-type coils are usually fine for years; suspect it only after points, condenser and leads test good.
- Spark plugs. Fit the grade specified for your engine — modern Champion or NGK equivalents to the original recommendation — gapped to the manual figure.
- HT leads and cap. Cracked leads and tracking inside the cap cause the classic damp-morning misfire. Replace as a set periodically.
Tell-tales of a tired ignition are rough idle, a misfire that clears with revs (or gets worse with revs, depending on where the weakness is), reluctance to start hot, and a car that pulls unevenly under load.
Setting the timing
Static timing gets the engine running; dynamic timing with a strobe tells you what it is actually doing on the road.
Start by setting static timing to the figure in the workshop manual for your specific engine and distributor number. Then, with the engine warm and idling, check with a strobe: as a guide only, idle timing sits around 12° BTDC and total mechanical advance around 30–32° BTDC above roughly 3,500 rpm. Verify the correct figures for your model and distributor before adjusting — the exact numbers vary between XPAG variants and distributor specifications.
Confirm that the advance actually comes in as revs rise, and that the vacuum unit (where fitted) responds to manifold vacuum. A distributor whose mechanical advance is stuck part-way will feel flat and run hot, no matter how well the static timing is set.
Positive-earth basics
T-Types are positive earth: the positive battery terminal connects to chassis, and the negative terminal feeds the loom. This has real consequences:
- Coil polarity. The coil's SW/+ terminal takes the switched feed; the CB/− terminal goes to the distributor. Fit it the wrong way round and the plugs still spark, but weakly — you lose noticeable performance.
- Electronic ignition. Any electronic ignition module, tacho pickup or solid-state accessory must be a positive-earth part, or wired via a manufacturer-supplied positive-earth kit. Fitting a negative-earth module without conversion destroys it on first switch-on.
- Accessories. Modern radios, USB chargers and LED lamps are almost all negative-earth. Either isolate them fully from chassis or fit an isolated DC-DC converter. Do not "just convert the car" to negative earth casually — the dynamo, control box, ammeter and any surviving original electrics all assume positive earth.
The charging system
The T-Series charges through a Lucas dynamo controlled by a control box (voltage regulator) on the bulkhead. There is no alternator from the factory.
Is it charging? The ammeter is your gauge. With the engine off and lights on, it should show a small discharge; start up and rev gently and it should swing firmly to charge, tapering back towards zero as the battery tops up. A needle that sits stubbornly at zero or discharge under revs means the system is not putting current back.
Common faults:
- Worn dynamo brushes. Cheap and easy to renew; often the sole cause of a lazy charge.
- Sticky or mis-set control box. The cut-out and voltage regulator points can pit, stick or drift out of adjustment. A specialist service or a modern solid-state replacement inside an original-looking box restores charging without spoiling the look.
- Poor earths at the dynamo and control box. A dynamo that cannot earth cannot charge, regardless of its internal health.
Alternator conversion. A discreet negative-earth alternator conversion (with the car re-polarised) gives modern charging at idle and copes better with LED headlamps, heated screens, and other loads. The trade-off is originality and the need to re-polarise the whole car. Keeping the dynamo — properly overhauled — is entirely viable for cars used as intended.
Earths and connections
Before you condemn any component, remake the earths and clean the Lucas bullet connectors. A remarkable share of "electrical gremlins" on T-Types are simply corroded bullets in their brass sleeves, or paint-insulated earth straps between engine, chassis and body.
- Undo, clean and remake each bullet connector — a light smear of dielectric grease keeps them honest.
- Check the engine-to-chassis and chassis-to-body earth straps; renew any that look tired.
- Confirm the dynamo, control box and coil all earth cleanly to a bright metal surface.
Fix the earths first, then diagnose. Half the "faults" will vanish on their own.
Frequently asked questions
Why does my tuned XPAG misfire under load?
Misfire under load is almost always ignition on an XPAG, not carburation. Check the plug gaps and colour first — sooty plugs mean the mixture is rich or the ignition is weak; white or blistered electrodes mean it is lean. Then confirm the points gap and dwell, the rotor arm and condenser, and that the coil is producing a strong blue spark. On a Stage 1 or Stage 2 engine, also check the static timing has not crept too far advanced; a pinking engine often feels like a misfire and will eventually burn a hole in a piston. Finally, balance the SUs again — an unbalanced pair can leave one cylinder running lean enough to misfire at wide throttle.
Why does my XPAG hesitate when I open the throttle?
Hesitation on pick-up is the classic SU weak-mixture signal. The piston in the carburettor is lifting too fast for the fuel to follow, usually because the jet is set too lean, the needle is worn, or the dashpot oil is too thin. On a Stage 1 car with standard 1.25 in SUs, start by checking the damper oil level and weight — SAE 20 is correct — and that the piston rises smoothly without sticking. Then richen each jet by one flat, test again, and confirm with the lifting-pin method. On a Stage 2 car with 1.5 in SUs the same rule applies, but the needle profile is more critical; a worn or mismatched needle will hesitate whatever the jet setting.
Why is my tuned XPAG hard to start when hot?
Hot-starting problems on a tuned XPAG usually come down to fuel evaporation or flooding. A free-flow exhaust and a hot engine bay can boil the fuel in the float chamber and carburettor body, making the mixture over-rich. Check the float level first — it should be at the workshop-manual figure, not simply "looks about right". If the car starts cold but floods hot, try easing the throttle fully open while cranking to clear the cylinders, then start with no throttle. On Stage 2 engines with raised compression the starter also has to turn a higher-compression engine against a hot block; a good battery and clean earth straps matter more than on a standard car.
My Stage 1 XPAG pings on acceleration — what do I adjust?
Pinking means the combustion pressure peak is arriving too early. On a Stage 1 tune running on modern 95 RON petrol, retard the static timing by one or two degrees first and re-test on a warm day under load. If it still pings, check that someone has not already advanced the distributor beyond the Stage 1 figure, verify the compression ratio has not been raised by a previous skim, and make sure you are not running the engine below about 1,500 rpm in a high gear. Do not simply richen the mixture to mask pinking — it helps a little, but the real fix is timing or compression.
After fitting 1.5 in SUs my Stage 2 idle hunts — what is wrong?
An uneven idle after a Stage 2 carburettor swap is usually one of three things: the throttle spindles are worn and drawing air, the inlet manifold is not matched to the head ports and is causing reversion pulses, or the jets and needles are not centred and matched. Re-check the manifold face and gasket match, recentre both jets with the piston-drop test, and confirm both carburettors are drawing the same airflow at idle. Stage 2 engines with a fast-road cam also have less manifold vacuum at idle, so the idle speed may need to be set a little higher than standard — typically 800–900 rpm — to keep the SUs stable.
How do I know if my tune is running lean?
Lean running shows up as a flat spot under load, popping in the exhaust on the overrun, white or grey spark-plug insulators, and elevated exhaust-manifold colour after a run. On a rolling road the air/fuel ratio will read above 14.5:1 at cruise or above 13.0:1 at wide throttle. Fix it by richening the mixture a flat at a time on the SUs, checking for air leaks around the manifold and balance pipe, and confirming the fuel pump is delivering at least 1.5 psi at the carbs. Lean mixture is the fastest way to burn exhaust valves on a tuned XPAG, so do not ignore it.
Related reading
- Cooling the XPAG and XPEG — where under-advanced ignition shows up as an overheating complaint.
- Tuning the Twin SUs — do the ignition first; the carbs come after.
FAQ
What ignition timing does an MG T-Type need? Static timing gets you running; what matters is what happens on the road. As a guide, idle timing sits around 12° BTDC and total mechanical advance around 30–32° BTDC above roughly 3,500 rpm — but verify the exact figures against the workshop manual for your engine and distributor number.
Are MG T-Series cars positive or negative earth? Positive earth from the factory. The positive battery terminal goes to chassis and the negative feeds the loom. Coil polarity, electronic ignition modules and modern accessories all need to respect this.
Why won't my T-Type charge? Most often worn dynamo brushes, a sticky or drifted control box, or a poor earth at the dynamo or control box. Confirm the ammeter's behaviour, then work through brushes, earths and the control box in that order.
Should I fit electronic ignition to an XPAG? It is a legitimate upgrade, provided you use a positive-earth kit and start with a mechanically sound distributor. Electronic ignition eliminates points wear and stabilises dwell, but it will not rescue a distributor with worn bushes or a seized advance mechanism — sort those first.