Rebuilding the SU-type carburettors on a Datsun L-series six

The early L24 runs a pair of side-draught constant-depression carburettors. They have a reputation for being difficult that they do not really deserve — they have very few parts and almost no adjustments, which is exactly why they confuse people used to a fixed-jet carburettor. Before rebuilding anything, understand what they do differently.

01How a constant-depression carburettor works

A conventional carburettor has fixed venturis and compensates for varying airflow with a set of circuits — idle, main, power valve, accelerator pump. Lots of parts, lots of calibration. An SU-type has one variable venturi. A piston rises and falls in the dashpot, changing the throat size to keep the air velocity across the jet roughly constant regardless of throttle opening. A tapered needle attached to that piston moves in and out of the jet, so the fuel flow tracks the airflow automatically. That is the entire principle. It is why there is no accelerator pump and no power valve: the mechanism is inherently compensating. The consequences for anyone working on one:

• Piston free movement is everything. If the piston sticks, the mixture is wrong at every throttle position and no adjustment will fix it.
• Damper oil is a real tuning component, not maintenance trivia. It damps how fast the piston rises, which is what provides the enrichment during acceleration that an accelerator pump would otherwise give you.
• The needle and jet wear as a pair. Replace both or neither.

02The lift-and-drop test

Before you take anything apart, do this. It tells you most of what you need to know. Remove the dashpot cap and the spring, lift the piston with a finger to the top of its travel, and let go. It should fall freely and land with an audible click. If it drops slowly, hangs, or lands softly, the piston is binding. Common causes: a bent needle, a jet that is not centred, dirt in the bore, or somebody having oiled the piston skirt. The piston bore must be dry. Oil on it collects dust and makes it stick. The only oil belongs in the damper tube.

03The rebuild

Work on one carburettor at a time, completely, so you always have an assembled one to refer to. Float chamber. New needle valve and new float valve seat. Set the float height to the specification for your carburettor — usually measured to the bottom of the float with the chamber inverted. Ethanol fuel is hard on the original float material, so check the float for fuel absorption; a float that has taken on fuel sits low and gives a rich mixture that nothing else explains. Jet and needle. Replace as a set. A worn jet is oval and no amount of adjustment gives an even mixture. Check the needle by rolling it on glass — a bent needle is obvious and is a common consequence of somebody forcing the piston in. Centring the jet is the step that matters and the one most often skipped. The needle must pass through the jet without touching it anywhere in its travel. Loosen the jet retaining nut, let the assembly find its own position with the piston dropped onto it, then tighten gently. Redo the lift-and-drop test. If the piston no longer falls freely, the jet is off-centre and you must do it again. Repeat until it drops cleanly. This is worth the patience — everything else depends on it. Throttle shaft. Check for side play by grasping the shaft and moving it. Worn shaft bushings are a vacuum leak that gets worse with rpm and produces a lean condition and a hunting idle that people spend months chasing in the ignition system. Rebushing needs reaming to size; it is a machine shop job and it is worth doing properly. Spindle seals and gaskets all get renewed. They are cheap and they are the reason the thing is apart.

04Setting them up

Fill the dampers with the correct oil to the specified level. Too thick and the engine bogs coming off idle; too thin and it stumbles. The specification is usually a light engine oil or a dedicated damper oil — not ATF, whatever the internet says, unless your manual says so. Balance the two carburettors with a flow meter. This is not optional on a twin setup — two carburettors feeding six cylinders through a shared manifold will fight each other, and an engine that is out of balance runs rough at idle in a way that mimics a dozen other faults. Disconnect the throttle linkage between them, set each to the same airflow at idle, then reconnect the linkage without disturbing either. Then set mixture with the jet adjusting nuts, equally on both, using the piston lift pin: lift each piston about a millimetre and watch what the engine does.

• Rises and stays up — rich.
• Falls immediately — lean.
• Rises slightly then settles — correct.

Re-check the balance afterwards, because mixture changes affect airflow.

05When to stop and replace instead

Rebuild kits are inexpensive and these carburettors last indefinitely if the wear parts are renewed. But if the throttle shaft bores are worn oval, the body is cracked, or the dashpot is damaged, you are past the point where a kit helps. Converting to a downdraught four-barrel is a common answer and it is very rarely the *right* answer on a road car. It typically loses low-end drivability, needs a new manifold and linkage, and puts the engine further from correct for no gain below the rpm anybody actually uses.