Tuning Guide

Setting up the twin Strombergs

By Nathan Devonshire

With the ignition right, the carburettors have a fixed target to aim at. Setting up twin Strombergs is a fixed sequence, done in order — and, on a Stromberg, it starts every single time with the diaphragms.

First, the golden rule — and the diaphragms

Do the ignition first, and make sure the engine is mechanically sound and fully warm before you touch a carburettor. Then, before anything else on a Stromberg, check the diaphragms. This is the one that catches SU hands out: the thin rubber diaphragm sealing each air valve is the Stromberg’s critical part, and a pinhole, a tear, or a diaphragm gone hard with age will defeat every adjustment you try to make. Lift each air valve and inspect its diaphragm for perishing, splits or hardening; if in any doubt, renew it. Every Stromberg set‑up starts here. Then work in order — float level, the air valve and needle, balance, mixture — because each step depends on the one before.

1. Float level

The float chamber must hold petrol at the right height, because that sets where the fuel sits before the engine even draws on it. Too high floods and richens; too low starves. It is checked with the float‑chamber lid off, to the dimension in the workshop manual, and set by adjusting the float. Both carburettors must match — an uneven pair can never be balanced.

2. The air valve and needle

The air valve must rise and fall freely for the carburettor to meter properly. Remove the damper and lift the air valve with a finger — it should drop back freely and seat cleanly; a sticky or dull drop points to a binding diaphragm, a dirty chamber, or a bent needle. On the adjustable‑needle Strombergs the needle’s baseline height is set with the top adjuster to the workshop manual’s starting position before any fine‑tuning; on fixed‑needle emission types the needle is not adjustable and the mixture is set at a base orifice instead. Either way, the needle must run true in the jet.

3. Balance the pair

Two carburettors must draw exactly the same air at idle, or they fight each other and no mixture setting will feel right. Warm the engine, disconnect the throttle linkage between the carbs, and use a balancing meter (a Unisyn or similar) — or listen at each intake with a length of tube — adjusting each carburettor’s idle/throttle‑stop screw until both pull equally. Then reconnect the linkage so they open together. Balance and mixture affect each other, so you will come back to this.

4. Mixture — the needle adjuster, and a warning

Now the fuelling — and here the Stromberg parts company with the SU entirely. Instead of a jet nut and a lifting pin, mixture is set by raising or lowering the needle with the adjuster reached through the top of the dashpot: raising richens, lowering weakens, a little at a time. The test is the same idea as the SU’s lifting‑pin — with the engine warm and idling, lift the air valve a whisker and watch the response: revs that rise and stay up mean too rich; revs that drop or stumble mean too weak; a slight rise that settles straight back means the mixture is right. And the warning every Stromberg owner must heed: never turn that adjuster without holding the air‑valve hub still, or you will twist and tear the diaphragm — which is exactly why Zenith made a special tool for the job. Set each carburettor the same way, a little at a time, re‑testing after every move. (On base‑orifice emission types the mixture screw does the same job from below — clockwise is leaner.)

5. Idle, and round again

With balance and mixture close, set the idle speed to the book figure on the linked screws, kept equal. Snap the throttle open and shut a few times to let everything settle, then re‑check the balance, and re‑check the mixture — the two are interdependent, and a proper set‑up means going round the loop once or twice until nothing needs touching.

6. The objective check — reading the exhaust, and the Colortune

The lifting test judges the mixture only at idle, under no load. To go further, the objective check is an exhaust gas analyser: a probe in the tailpipe reads what is actually coming out of the engine. When the mixture is rich, unburned carbon leaves as carbon monoxide — CO climbs almost in step with richness, which makes it a superb, sensitive measure — while a spike in unburned hydrocarbons flags a misfire. Modern analysers boil it down to lambda: 1.00 is the ideal mixture, below is rich, above is lean, with a classic usually set a touch rich for protection. A long‑standing Stag favourite for the same job, cylinder by cylinder, is the Colortune — a spark plug with a glass window that lets you watch the flame as it burns: a rich mixture glows yellow, and as you lean it towards correct the flame turns a clean Bunsen blue. Either way — the meter or the flame — it confirms and fine‑tunes what the method already got close. It never replaces the sequence.

When it still will not come right

If a clean mixture will not come however you set the needle, stop adjusting and look for a fault — and on a Stromberg, look at the diaphragm first, always. The other usual suspects are an empty or wrong‑grade damper (top it up first — thirty seconds), a worn throttle spindle drawing air, a punctured float, a sticking air valve, or a temperature compensator jammed open. Fix the cause, then set up again from the top.