The SU and Stromberg constant-depression carburettors fitted to the classic MG range are remarkable instruments, accurate, sensitive to small changes in mixture, and capable of running cleanly across an enormous range of operating conditions when properly set up. They are also fundamentally simple, with no jets to block in the traditional sense, but they rely on two consumables to do their job, the dashpot oil that damps the rise rate of the piston, and the cleanliness of the air valve and jet for accurate fuel metering.
SU and Stromberg Dashpot Oil
The dashpot at the top of an SU or Stromberg carburettor contains a small oil reservoir that damps the rise of the air-valve piston during acceleration. As the throttle opens and airflow increases, the piston rises against the suction from the venturi, and the dashpot oil resists that rise, slowing the piston enough to enrich the mixture momentarily and prevent the lean stumble that an undamped carburettor would produce. The viscosity of the dashpot oil determines the rise rate, and getting it right is a precise tuning step, too thin and the carburettor over-leans on acceleration, too thick and it over-richens.
The standard dashpot oil is a 20W/50 engine-grade specification for most SU and Stromberg applications, with thinner and thicker grades available for owners tuning their cars for specific conditions, as a cold winter morning with a too-thick dashpot oil produces a hesitant pickup, while a hot summer day with a too-thin oil causes flat spots under acceleration.
Carburettor Cleaner
Carburettor cleaner is a strong solvent applied through the carburettor air intake or sprayed directly into the throat to strip varnish, gum, and carbon from the inside surfaces of the carburettor body. On SU and Stromberg carburettors it is particularly useful for clearing the air valve guide and the throttle-spindle bushes of accumulated dirt, both of which affect smooth operation. The product is used sparingly, as extended soaking is not necessary and can affect rubber components, and is flushed through with the engine running after application. It is also the right product for a complete carburettor strip-and-rebuild, where the body, top cover, jet bush, and air valve all benefit from a thorough chemical clean before reassembly with fresh seals and gaskets.
Wider Workshop Use
The petroleum-based cleaning products in this section cover the wider workshop applications where a solvent stronger than degreaser is needed, cleaning the throttle linkage and trunnions, freeing a seized choke cable, and the various small jobs around the carburettor and inlet manifold where decades of accumulated heat and oil mist have left a stubborn film. The technical team is available to advise on the right dashpot oil viscosity for a specific carburettor setup and the right cleaning approach for a specific service issue.