The SU carburettor meters fuel through a tapered needle in a fixed jet. The needle profile, its diameter at various positions along its length, determines the fuel-air mixture at each throttle position. The factory V8 needle profile is matched to the standard 3528cc engine's fuelling requirement, providing the correct mixture across the full throttle range. Needles are a precision item and should not be substituted casually with other patterns.
Alternative Needle Profiles
Where engine specification has been changed from standard, performance camshaft, performance heads, modified exhaust system, a revised needle profile may be required to maintain correct fuelling. Alternative SU needles in a wide range of profiles are available, identified by a letter or number code stamped on the needle. Selecting the correct needle for a modified engine usually requires running the car on a rolling road or with a wideband oxygen sensor, comparing measured air-fuel ratio against the desired mixture and adjusting needle profile until the result matches.
Jets
The SU carburettor jet is a precision-machined item with a calibrated bore through which fuel is drawn past the needle. Jet bearings (the moving parts that seal the jet to the carburettor body) wear over time and are typically renewed during major service work. Jet diameter is matched to needle profile and carburettor type, substituting jets between different SU specifications can cause significant fuelling issues.
Specification Advice
Selecting needles and jets for a modified engine should be done with technical advice. The parts team can recommend appropriate needle profiles based on the engine specification, and rolling road tuning is usually needed to confirm correct fuelling once the parts are fitted.
MGOC Spares
MGOC Spares supplies standard-specification SU HIF6 needles and jets for the MGB GT V8, alongside alternative needle profiles for modified engines.