Tuning Guide

Spark plugs, coil, HT leads and the firing order

By Jon Cosson

The distributor and timing do the clever work, but the spark still has to be made, carried and delivered, and fired to the right cylinder in the right order. On the Stag, that order, and the cylinder numbering behind it, catch a lot of people out.

The coil, where the spark is made

The coil is a transformer: a low‑voltage primary builds a magnetic field, and when the points break that circuit the collapsing field induces tens of thousands of volts in the secondary, the spark. Many cars use a ballast resistor, so the coil runs on reduced voltage normally but gets full battery voltage while cranking for a fatter starting spark; the classic sign of ballast trouble is an engine that fires on the key at start but dies the moment it returns to run. A replacement coil must match the system, a ballasted car needs a ballast coil, or it runs hot and burns its points.

The ballasted system, and the numbers

Lucas’s own fault diagnosis manual gives figures for a ballasted system, and the archive has had none until now.

Before any of that, establish what your car actually has. The Repair Operation Manual records that a separate ballast resistor, a Lucas 3BR of 1.3 to 1.4 ohms, was fitted only up to commission numbers LD/LE 20001. On later cars the ballast is a resistance wire of 1.3 to 1.5 ohms built into the harness, with no block to find.

With the ignition on and the contact points closed, the voltmeter at the coil positive terminal should register approximately 6 volts on a twelve volt ballasted system. Note the conditions, because they are the whole point: ignition on, contacts closed. With the contacts open the same terminal reads battery voltage, which is how two owners comparing readings can both be honest and arrive at different numbers.

This is a voltage and not a resistance. The archive still has no coil primary resistance specification for the Stag and does not publish one.

Lucas then gives a test worth knowing. With that reading confirmed, temporarily earth the coil negative terminal and crank the engine on the starter. The voltage should increase, because the circuit is arranged to short out the ballast resistor while the starter is operating, letting the coil run slightly above its normal voltage for an easier cold start. A slight decrease instead indicates a faulty solenoid switch, or the lead from it.

Two more from the same procedure. With the contacts open, the coil negative terminal should read battery voltage; no reading means an open circuit in the primary winding, or a short to earth on the lead to the distributor or within it. With the contacts closed it should read zero, and if it reads a voltage Lucas lists the causes: dirty or oily contacts, a bad earth connection, contacts not closing properly, high resistance in the circuit from the coil to the contact breaker, a broken flexible lead, or an open circuit coil to distributor lead.

The contact breaker gap this guide already gives as about 0.015 inch sits inside Lucas’s stated range of 0.35 to 0.40 mm, which is 0.014 to 0.016 inch.

All of these assume a negative earth system. On a positive earth car the supply side of the coil is the negative terminal instead.

The coil that fails when it gets hot

The paragraph above covers a coil mismatched to its ballast arrangement. A coil can also break down when hot with no mismatch at all, and it is one of the harder Stag faults to pin down, because everything looks right while the car is cold.

A thread on the Stag Owners Club forum in September 2026 sets out the pattern. A Mk2 Stag misfires after about twenty minutes’ driving. The coil becomes too hot to touch while the engine temperature gauge sits below half. The car has a ballast coil correctly matched to a ballasted system, a recently fitted electronic ignition module and the timing checked, and the fuel pump is audibly ticking throughout, so fuelling has already been eliminated. Two coils in succession have failed the same way.

A moderator’s reply describes that as a classic presentation of a coil breaking down internally, and offers a rule of thumb worth carrying: a coil will get warm in use, but it should not become too hot to touch.

There is a mechanism behind this that Lucas describe and that is worth understanding. A coil intended for use with a ballast resistor has a primary winding of lower inductance, which lets the magnetic field build more rapidly, and Lucas add that there is less heating effect inside such a coil because the ballast resistor itself dissipates some of the heat produced in the circuit. In other words a ballast coil is designed on the assumption that something else is carrying part of the thermal load.

Remove or bypass the ballast, as happens when an electronic conversion is fitted carelessly, and the coil takes heat it was never designed to take. That is not what happened in the case above, where the system was correctly matched, but it is a documented route to a coil that runs hot and it is worth eliminating before blaming the coil itself.

The archive can now give the factory figures, which it could not when this section was first written. The Repair Operation Manual specifies a Lucas 16C6 coil on earlier models, primary winding resistance 1.43 to 1.58 ohms, and a Lucas 15P6 or 15C6 on later models, 1.30 to 1.45 ohms.

That is worth setting against the case above. The owner reported 1.7 ohms, which is above both factory ranges. It is not proof on its own, because meter accuracy, temperature and the point of measurement all matter, but it is consistent with the coil being at fault and it is the first time the archive has had a figure to compare against.

A free test, at the roadside

Carry a bottle of chilled water. When the misfire starts, cool the coil with it. If the car then runs properly until it heats up again, the coil is the likely cause. It is a diagnosis rather than a repair, and the member offering it says as much, but it costs nothing and it separates a heat affected coil from a dozen other explanations in about a minute.

Two things owners report

A new coil is not an alibi. In the case above two consecutive replacements failed the same way, and another member on the same thread describes going through three new coils before finding one that has since lasted twelve years. Several long standing members say that coils sold today are not what they once were, and two name alternatives they have had better service from. That is owner experience rather than an established fact about any manufacturer’s products, and the archive publishes it as such rather than naming a brand as good or bad.

Mounting matters. One member reports raising his coil so that it no longer sits down against the block, and says it runs cooler for it. He offers that as easier than relocating the coil altogether, which was the original question and was not otherwise answered.

That thread is unresolved. The owner has not reported back, so this is where the evidence points rather than a confirmed diagnosis. Nobody on the thread engaged with his readings, and at the time the archive had no figure to check them against. It does now, and they are given above.

Sources: Lucas Fault Diagnosis Service Manual, Catalog No. XIA116, Lucas Aftermarket Operations, for the ballasted system figures and tests and for the ballast resistor’s share of the heat, consulted as a copy circulated on the Stag Owners Club forum and carrying no publication date. And the Stag Owners Club forum thread on a Mk2 Stag misfiring, September 2026, whose members are described by what they did rather than named. Coil types and primary winding resistances are from the Repair Operation Manual Section 05, Engine Tuning Data, page 05-1, read at source.

HT leads, carrying the spark

The high‑tension leads carry the high voltage from coil to cap and from cap to each plug, eight of them on the V8. Perished or cracked leads let the spark leak to earth, especially in the wet, giving a misfire that comes and goes; a tell‑tale is faint blue tracking in the dark with the engine running. Keep them clean, clear of hot metal, and, the important part on a Stag, routed in the correct firing‑order sequence around the cap.

The spark plug, and how to read it

The plug lets the spark jump a set gap in the combustion chamber. Heat range is how fast it sheds heat, too cold and it fouls, too hot and it can cause pre‑ignition, so use the manual’s grade. The gap sets how far the spark jumps, too small and it is weak, too large and the coil may not fire it cleanly, so set it with a feeler gauge to the book figure.

Reading a plug

Once warm and driven, a plug is a free diagnostic. Light tan or grey is spot on. Dry black sooty deposits mean too rich, too cold a plug, or retarded timing; a wet oily tip means oil past rings or guides; a white or blistered tip warns of running too hot or too weak, or over‑advanced timing. On the V8, pull all eight and compare, an odd one out points straight to that cylinder.

The firing order: and the Stag’s odd numbering

Here the Stag springs a surprise. Most engines number their cylinders tidily and fire number one first. The Stag V8 does neither. Number one sits at the front of the right hand bank, the right hand bank carries the odd numbers and the left hand bank the even ones, both as seen from the driver's seat, and the firing order is 1-2-7-8-4-5-6-3. Stranger still, the timing is set not from cylinder one but from cylinder two, the left front. None of this is a fault, it is simply how Triumph laid the V8 out, but it catches out anyone who assumes the familiar, and it is why a sequence borrowed from a more common V8 will not do. Get the leads out of sequence, or count the cylinders the normal way, and the engine will misfire, kick back or refuse to start. So when the leads come off, mark every one, rebuild strictly to 1-2-7-8-4-5-6-3 around the cap in the rotor's direction, and if in any doubt work from the workshop manual's cylinder diagram rather than from assumption.

Where it all comes together

Coil, leads, plugs and firing order are the delivery half of ignition; the distributor and timing are the control half. Get all of it right, a healthy spark, clean leads, the correct plugs gapped properly, fired in the Stag’s order and timed correctly , and the ignition side is done. Then, and only then, you set the fuelling.