Tuning the AC Aceca

The three engines, and their carburettors

The Aceca left Thames Ditton with one of three engines, each on a triple-carburettor induction system. Which setup applies to your car is determined by the chassis prefix — AE, BE or RS — as set out in the chassis number guide.

AC 2.0-litre Light Six · triple SU carburettors

Fitted to the original Aceca (AE prefix). AC's own 1,991 cc overhead-cam six, in essence a 1919 design, breathes through three SU carburettors. SUs are constant-depression instruments with a spring-loaded piston and a tapered needle; balancing and mixture-setting are relatively forgiving, but damper oil, needle profile and float level all matter. See the AC-engined Aceca article for the wider context of the engine.

Bristol 100-series 2.0-litre · triple Solex 32 BI downdraught

Fitted to the Aceca-Bristol (BE prefix). Bristol's 1,971 cc BMW-328-derived straight-six with its distinctive cross-pushrod valvegear runs three Solex 32 BI downdraught carburettors — one per pair of cylinders. Solexes are fixed-jet carburettors: they are simple and reliable when set up correctly, but ill-suited to being fiddled with blind. Full context in The Bristol Engine and the Solex 32 BI guide.

Ford Zephyr 2.6-litre · Ruddspeed triple carburettors

Fitted to the final Aceca 2.6 (RS prefix). The 2,553 cc Ford Zephyr straight-six was reworked by Ken Rudd's Ruddspeed concern with triple carburettors and produced up to around 170 bhp, making the 2.6 the fastest Aceca. Only 8 were built. Full context on the Aceca 2.6 page.

Tuning triple carburettors

Whatever the engine, the underlying job is the same: get each of the three carburettors drawing the same air, delivering the same mixture, and opening in unison. A car that runs unevenly, hunts at idle or feels lazy through the mid-range is almost always a carburettor problem before it is an engine problem.

Balancing airflow

With the engine warm and the throttle linkage slackened, a balance meter (airflow gauge) is placed over each intake in turn. Each carburettor's individual throttle stop is adjusted until all three draw the same reading at idle. Only then is the interconnecting linkage reconnected. If one carburettor is starved of air, the two on either side will always compensate unequally — no amount of mixture adjustment will fix that.

Setting mixture

Mixture is set carburettor by carburettor to the smoothest, highest idle, then airflow re-checked; the two interact and both must be walked in together. Exact needle profiles (SU), main jets (Solex) and float heights must be taken from the workshop manual.

SU-specific basics

On the SU-equipped AC-engined car, the piston must fall cleanly when lifted 3–4 mm and dropped — a sluggish return means insufficient damper oil, and hunting or over-rich running often traces to a bent or wrong-profile needle. Float chambers must be level and the fuel level at the specified height. The centre of the jet must be concentric with the needle.

Solex-specific basics

On the Aceca-Bristol, worn throttle spindles are the number-one cause of poor running — they leak air past the butterflies and no amount of adjustment cures it. Gaskets between carburettor and manifold must seal fully. Correct jetting is a workshop-manual matter; owners should not substitute jets without documented reason.

Common wear points across all three setups

Throttle spindles, worn needles, tired float valves, split interconnecting rubbers on the throttle linkage, and dirty or corroded internal passages after long standing. On the SU cars, damper oil is a check-every-service item. On the Solex cars, correct linkage geometry — so each butterfly reaches the same angle from the same pedal input — matters as much as the carburettor internals themselves.

Ignition

All three variants use a 12-volt coil-fed ignition with a Lucas distributor, points, condenser and a mechanical (and, on some fitments, vacuum) advance mechanism. The system is simple, but it must be set right for the engine to make its numbers and run cleanly through the rev range.

Points, condenser and dwell

Points gap is set with feeler gauges on a peak of the distributor cam. Where a dwell meter is available, verify the resulting dwell angle against the workshop manual. Wear at the points, a tired condenser, or wear in the distributor shaft bushes are the usual sources of an intermittent misfire.

Spark plugs and gaps

Fit the plug grade specified for the engine, gapped to the manual figure. Different heat ranges suit different engines and uses — do not substitute across variants without checking.

Static vs strobe timing

Static timing uses a 12-volt test lamp on the points, with the engine turned by hand to the specified timing mark. Strobe (or dynamic) timing is done with the engine warm and running at the specified idle, using a timing light to align the crank pulley mark. Strobe is the definitive check; static is a safe start point when a distributor has been out.

Electronic-ignition upgrades

Modern electronic triggers such as Pertronix or Accuspark fit inside the original Lucas distributor in place of points and condenser. They remove points wear and the periodic re-gapping it demands, keep the mechanical advance curve intact, and leave the car outwardly original. Most owners regard this as a worthwhile reliability upgrade; it is fully reversible.

Featured specialist

Triple-carb tuning and Lucas distributor setup are exactly the kind of work many Aceca owners entrust to a specialist. The following independent tuner works on precisely this car's setup.

Frequently asked questions