page_title: SU carburettor
section_title: SU carburettor types
hierarchical_section_title: SU carburettor / SU carburettor types
caption_attribution_description: Ratbike engineering: an SU carb fitted to an MZ TS250/1 in place of the original BVF carb by the use of handy bits of metal and domestic plumbing fittings, for improved economy and low-speed tractability.
context_page_description: SU carburettors were a manufacturer of constant-depression carburettor. Their designs were in mass production during most of the twentieth century.
  The S.U. Carburetter Company Limited also manufactured dual-choke updraught carburettors for aero-engines such as the Rolls-Royce Merlin and Rolls-Royce Griffon.
context_section_description: SU carburettors were supplied in several throat sizes in both Imperial (inch) and metric (millimetre) measurement.
  The carburettor identification is made by letter prefix which indicates the float type:
  "H": introduced in 1937 in which the float bowl has an arm cast into its base, which mounts to the bottom of the carburetor with a hollow bolt or banjo fitting. Fuel passes through the arm into the carburetor body. The bolt attaches to the carburetor body just behind the main jet assembly.
  "HD": introduced in 1954 with the float bowl mounted with its arm fastening directly below, and concentric with, the main jet. The arm has a flange that fastens with 4 screws to the bottom of the carburetor, and sealed with a rubber diaphragm integral with the main jet.
  "HS": introduced in 1958 the float bowl can be rigidly or rubber mounted to the main body, fuel is transferred by an external flexible pipe to the jet. The jet moves down to richen the mixture for cold starting,when the 'choke' linkage is pulled.
  "HIF": (1972) the float bowl is horizontal and integral (hence the name) Horizontal Integral Float. Example: 1972-1974 MGB.
  "HV" (1929), "OM" and "KIF" types also exist but were less commonly employed.
  The Imperial sizes include 1-1/8", 1-1/4", 1-1/2", 1-3/4", 1-7/8", and 2", although not every type (H, HD, HS, HIF) was offered in every size.
  There were also H models made in 2-1/4" and 2-1/2", now obsolete. Special purpose-built carburetors (Norman) were made as large as 3".
  To determine the throat size from the serial number:
  If the final number (after one, two or three letters, beginning with H) has 1 digit, multiply this number by 1/8", then add 1". For example, if the serial number is HS6, the final number is 6: 6/8 = 3/4", add 1, total is 1-3/4", etc.
  If the final number has 2 digits, it is the throat size in mm. For example, if the serial number is HIF38, the final number is 38, size is 38 mm etc.
caption: An SU fitted to an MZ in place of the original BVF slide carb
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