When you buy a T-Series, the sensible first move is a proper health-check before you use it in anger. A recent example on an MG TF1500 — freshly acquired and given a routine specialist tune — turned up three things worth sharing, because every one of them is common on these cars: a mixture running far too rich, debris in the carburettors despite a new fuel filter, and, most importantly, a charging system pushing dangerously high voltage. Here is what was found, what it means, and what to do about it. (Details anonymised; shared as a teaching example.)
The starting point
The car had recently changed hands and was given a routine health-check and tune to catch anything needing attention before regular use — exactly the approach we recommend in the buying guide. On the face of it the car ran and drove; the health-check is what revealed what was hiding.
Finding 1 — running very rich
On the gas analyser the engine read over 11% CO at idle, against a healthy target of roughly 3.5–4%. The spark plugs told the same story: dry and sooty black, the classic signature of a rich mixture.
The tempting fix is to lean off the jets and move on. The right fix is to find why. Here, the rear carburettor showed fuel "welling" at the jet — a sign of a needle valve not sealing and/or a float level set too high — and the float bowl held debris: fibrous/rubber material caught in the needle-valve gauze, and fine ferrous (rusty) particles that a magnet picked up. A new viton-tipped needle valve, a corrected float height and rebalanced carburettors brought the mixture back to the ~4% CO target.
The lesson: fitting a new in-line fuel filter does not retrospectively clean carburettors that are already contaminated — muck already downstream stays there until the bowls are cleaned out. And the type of debris is a clue: rubber or fibre usually means a fuel hose perishing internally, and ferrous particles mean rust from the tank or steel lines, so a rich, dirty carburettor is often the visible end of a fuel-supply problem worth tracing back. On a twin-carb car it is also worth checking both float bowls and needle valves, not just the one showing symptoms. For what modern petrol does to rubber hoses and tank linings, see the Fuel, Oil & Additives guide.
Finding 2 — ignition, quickly sorted
Less dramatic, but part of a proper tune. The TDC and timing marks were verified first, then dynamic timing — found a little retarded at around 4° BTDC — was set to roughly 7.5° BTDC, a sensible mid-value for modern fuel. The spark-plug gaps were brought back to 0.025", correct for a points system, and the dwell was normal for the distributor fitted. Nothing exotic, but it is the difference between a car that merely runs and one that runs well.
Finding 3 — the charging system, the one that really matters
This is the finding worth dwelling on. With the engine running, the voltage at the battery was not steady — it swung about, and at idle was repeatedly seen above 16V, with occasional spikes past 17V.
That is overcharging, and it is a genuine hazard. It is worth being precise about why, because it is a common point of confusion (and one the Lucas control box guide explains): the Lucas regulator's open-circuit setting is about 16V, which alarms people but is normal — that figure is measured with the battery disconnected. This car's reading was taken on the car with the battery connected and the engine running, where the voltage should sit in the mid-14s and taper as the battery charges. Seeing 16–17V there means the regulator is not controlling the dynamo.
Left unchecked, sustained 16–17V will gas and eventually boil the battery, blow bulbs — especially the small panel and warning bulbs — and stress anything voltage-sensitive. And a wildly fluctuating voltage, rather than a steady-but-high one, points less to a simply mis-set regulator and more to dirty or pitted regulator contacts, or a poor control-box earth, so that the regulator "sees" the wrong voltage and hunts.
The fix follows the sequence in our Lucas control box guide: clean the regulator contacts, make sure the control box has a good earth at terminal E, and only then reset the open-circuit voltage (the adjuster turns anti-clockwise to lower it). It is also worth confirming the dynamo itself is not at fault. Until it is sorted, the car is best used sparingly — every mile at 17V shortens the life of the battery and the bulbs.
The takeaways
A real-world example, anonymised and shared for learning; specifics will vary from car to car. For the how-to behind the charging fix, see the [Lucas control box guide](/technical-library/lucas-voltage-regulator-control-box).