Tuning Guide

Spark plugs, coil, HT leads and the firing order

By Nathan Devonshire

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.

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. Its cylinders are numbered in an unusual pattern across the two banks, 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. 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, not 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.