Carburettors & Fuel

The carburettors

The carburetted Stag uses twin Zenith‑Stromberg 175 CD carburettors, “constant depression” (constant‑vacuum), variable‑choke instruments. Instead of the fixed choke and multiple jets of a Weber, a single air valve, the piston, rises and falls on a diaphragm to keep a constant air velocity over the jet, so the mixture stays right across the range rather than being correct at one point and compromised everywhere else. A damper in the top of each carburettor, topped up with light oil, controls how quickly the piston rises: it is what gives the momentary enrichment on a snapped throttle, and it is the single most neglected item on the pair.

For the mechanism in detail, the venturi, the air valve and the needle, see How a carburettor works.

The common faults

Most “carburettor trouble” comes down to a short list, and none of it is exotic:

• Worn throttle spindles and bearings, which let air leak in and make the idle high, erratic or impossible to set, a very common age‑related fault, cured by re‑bushing.
• A split or perished air‑valve diaphragm, which upsets the mixture and running, an inspection‑and‑renewal item.
• The choke (cold‑start) mechanism, a frequent source of poor cold starting and fast‑idle problems.
• A sticking or leaking float valve, causing flooding or starvation.
• The two carburettors out of balance with each other, so they fight one another and no mixture setting feels right.

Setting them up

Tune them warm, and in order: check for air leaks first (worn spindles, manifold joints), set the damper oil, then balance the two carburettors so they draw equally, and finally set the mixture and idle with the jet‑adjusting and throttle screws. Because they are constant‑depression carburettors, balance and air‑tightness matter far more than endless jet‑fiddling, a leaking spindle will defeat every adjustment you make downstream of it. Keep the dashpots topped up with the correct light oil. Confirm settings and damper oil against the Stag or Zenith‑Stromberg workshop manual.

The step‑by‑step sequence, including the warning about the needle adjuster and the diaphragm, is set out in Setting up the twin Strombergs. Do the ignition first: see Ignition timing explained.

Keep them or convert?

Some owners swap to Webers or SUs, but this is not the upgrade it is often assumed to be: Webers in particular can be too much carburettor for a standard engine, need a choke linkage fabricated and rejetting, and bring tuning issues of their own. For most cars, properly rebuilt and balanced Zenith‑Strombergs are the cheaper, sweeter answer. Modern ethanol petrol is hard on old diaphragms and seals, so use ethanol‑tolerant parts when rebuilding, see E10 petrol and the Stag.

And the fuel supply

Whatever the carburettors, they can only be as good as the fuel reaching them: a clean tank, a sound pump, fresh flexible hoses and a clean filter. If a well‑tuned car starves or stumbles under load or when hot, vapour lock, look at the fuel supply before blaming the carburettors. Heat under the bonnet is part of that story too, which is another reason to keep the cooling system in good order.