The carburettor needle is the precision-machined tapered rod that controls the petrol flow through the SU or Stromberg constant-depression carburettor at different throttle and engine-speed combinations. As the air valve in the carburettor rises under suction, the needle is drawn up out of the jet, exposing more of its tapered profile to the petrol flow and allowing more petrol to mix with the increased airflow. The needle profile, the precise way the diameter tapers from base to tip, determines the air-fuel mixture at every operating condition, and getting the right needle for the engine is one of the most fundamental tuning steps for any SU or Stromberg installation.
Needle Profile and Engine Matching
Each needle profile is identified by a code stamped on the needle itself, typically two or three letters that designate the specific taper pattern. The factory specification for each MG application calls out the needle code originally fitted, and the production cars left the factory with that specification needle in each carburettor. Over the working life of the car, engine modifications, air-filter changes, exhaust changes, and the various other tuning steps can shift the engine's optimum needle specification away from the original, a Stage II head with a freer-flowing exhaust, for example, typically calls for a slightly richer needle than the standard one to keep the mixture correct under the engine's revised breathing characteristic. The needle specification chart for SU and Stromberg carburettors lists the standard needles for each application and the typical alternative needles for various tuning steps.
Standard and Performance Specifications
Standard needles cover the original-equipment specifications for each MG application, the fixed needles fitted to early HS4 carburettors, the spring-loaded biased needles introduced with the 18V-series engines from August 1971, the needles specified for the HIF4 carburettors introduced from November 1973, and the various other needles used across the production range. The original-equipment specification for a specific car depends on the engine prefix and the carburettor type fitted, and the right needle is matched to the chassis and engine numbers at the point of order. Performance needles cover the alternatives suited to engines modified beyond standard specification, typically running slightly richer at part-throttle and full-throttle to match the increased airflow capacity of a tuned engine. The choice of needle is matched to the engine's specification: the standard needle for a standard engine, the appropriate performance needle for a tuned engine, with the precise selection being made on a rolling-road or similar precision-tuning setup where the air-fuel mixture can be measured directly.
Setup and Application Notes
Needle changes are straightforward, the air valve and dashpot assembly are removed from the carburettor top cover, the needle is withdrawn from the air valve (typically held by a single grub screw), the new needle is inserted with the correct shoulder-to-jet-face distance, the air valve is refitted, and the carburettor is then re-tuned using the standard SU or Stromberg setup procedure. The Gunson ColorTune (covered in the Gunson Tools section) provides a direct visual confirmation of the air-fuel mixture in each cylinder, allowing the needle selection to be verified in operation.
For owners running multiple carburettor setups (twin SU on standard cars, twin Weber DCOE on performance conversions, twin SU HS6 on MGC and twin SU HIF6 on MGB GT V8 applications), the needles in each carburettor should be matched, running mismatched needles produces uneven mixture distribution between cylinders. The technical team is available to advise on the right needle specification for a specific engine and intended use.