::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.
Static timing is TDC at full retard, which the Workshop Manual states for the TD and the TF alike. What differs between the two cars is the distributor's own contribution: General Data gives the automatic advance as 32° at the crankshaft on the TD and 25° at the crankshaft at 3,100 rpm on the TF. That is seven degrees between two cars that look the same, and a figure quoted for one is not a figure for the other. Check with a strobe that the advance actually comes in as revs rise, but set the static timing statically. A distributor that has been rebuilt, swapped or re-curved follows neither figure, so establish what is fitted before adjusting anything.
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.
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. The Workshop Manual gives static timing as TDC at full retard on both cars, and the distributor's automatic advance as 32° at the crankshaft on the TD and 25° at 3,100 rpm on the TF. Those are the factory figures for the distributors originally fitted; anything rebuilt or re-curved since will follow its own curve.
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.