The correct carburettor needle is determined by the carburettor specification number, fitting the wrong needle will result in an incorrect fuel mixture across the rev range that cannot be fully corrected by normal jet adjustment alone. The needle's tapered profile determines how much fuel is metered at every point of piston lift, and each specification was carefully matched to the engine's airflow characteristics, exhaust system, and emissions requirements for its intended market. Two needles are required per car (one per carburettor), and both must be the same type. The needle specifications by carburettor type are: AUD150 and AUD341 (home market and most exports): fixed needle ST as standard, with C1W (weak) and SQ (rich) alternatives for tuning.
AUD287 (North American 1968): fixed needle KM. AUD342 (North American 1969): biased needle BAD, requiring a guide kit with spring.
Fixed versus Biased Needles
Fixed needles, used on the AUD150, AUD341, and AUD287 specifications, are locked rigidly in the suction piston by a grub screw and must be perfectly centred in the jet bore by hand during assembly. If the needle is not centred correctly, it will rub against one side of the jet, causing uneven wear on both components and a progressive change in mixture that worsens over time. Biased needles, introduced on the AUD342 specification for the 1969 North American market, are spring-loaded and self-centring within the jet bore, compensating for minor misalignment between the piston and jet and significantly reducing wear on both components. This self-centring action maintains more consistent mixture delivery over the service life of the carburettor, with less need for frequent recentring.
The needle locking screw differs between fixed and biased types and must not be interchanged, using a fixed-type screw on a biased needle prevents the spring from functioning, while using a biased-type screw on a fixed needle allows the needle to move in service.
Jet Hardware
Jet assemblies are colour-coded by position (front and rear carburettor) and must be fitted in the correct locations. Jet bearings are specification-specific and this is a common source of ordering errors: AUD150 and AUD341 use coarse-thread jet bearings with a brass sealing washer, AUD287 uses fine-thread jet bearings with jet-centring and tab washers, and AUD342 uses a different fine-thread jet bearing with a tab washer. Jet adjusting nuts similarly differ between coarse and fine thread, a coarse-thread adjusting nut fitted to a fine-thread jet bearing will cross-thread and damage both components.
When ordering any jet hardware, always specify the carburettor specification number to ensure the correct thread and sealing arrangement is supplied. Needles are always sold individually, two are required per car.
Needle Selection for Tuning
The standard ST needle on the AUD150/AUD341 specification provides the factory-intended mixture for standard engines with the original exhaust system and air filtration. The C1W (weak) alternative leans the mixture slightly, which may be appropriate for engines running in consistently warm climates or with certain performance modifications that improve airflow without significantly increasing fuel demand. The SQ (rich) alternative enriches the mixture, which may be needed for engines with performance exhaust systems, larger air filters, or other modifications that increase airflow beyond the standard needle's fuelling capacity. Any change from the standard needle should ideally be confirmed on a rolling road to verify that the mixture is correct across the full rev range, not just at idle, an incorrect needle profile can give a satisfactory idle mixture while running dangerously lean at higher engine speeds under load.