What is an SU carburettor and how does it work?

The SU (Skinner's Union) carburettor is a constant-depression, variable-venturi carburettor. Unlike conventional fixed-jet designs, it uses a sliding piston and a single tapered needle to automatically regulate fuel delivery in proportion to the engine's air demand. As engine load increases and more air is drawn in, the piston rises, lifting the needle from the jet and allowing more fuel to flow. The result is a smooth, self-adjusting mixture across all engine speeds and throttle positions, with no need for multiple jets, accelerator pumps or complex jetting changes.

How do I identify which SU carburettor I have?

SU carburettors are identified by a letter prefix and a number. The letters indicate the float bowl type: H (introduced 1937), HD (1954), HS (1958) and HIF (1972). The number indicates the throat size. For single-digit numbers, the formula is: multiply by 1/8 inch and add 1 inch. So an HS4 has a 1½-inch throat (4 ÷ 8 = ½, plus 1 = 1½"). For two-digit numbers, the figure is the throat size in millimetres — so an HIF38 is 38mm. The type number is usually stamped on the carburettor body. If you're unsure, contact us with your vehicle details and we'll identify the correct unit.

How do I find the right service kit or rebuild kit for my vehicle?

The quickest way is to use our vehicle-based collections — browse by your make (MG, Triumph, Jaguar, Austin-Healey, Land Rover, Rover, Morris Minor) and you'll find the service and rebuild kits listed for your model. If you already know your carburettor type, you can also search directly by SU type number. Service kits cover routine maintenance items (gaskets, O-rings, float valves), while Major Repair Kits (MRK) include everything needed for a full rebuild. If you're in any doubt, get in touch and we'll point you in the right direction.

What needle do I need for my engine?

Needle selection depends on your specific carburettor type, engine specification and state of tune. The standard needle for your vehicle is usually the correct starting point for a stock engine. If your engine has been modified — different camshaft, cylinder head work, air filters or exhaust — you may need a richer or leaner needle profile to suit the changed airflow. A rough guide: a richer needle (wider taper) if the engine runs lean under load; a leaner needle if it runs rich. We recommend consulting a rolling road or classic car specialist for modified applications. Contact us with your carb type and engine details and we can suggest options.

How much damper oil should I use, and which type?

The dashpot (the threaded cap on top of the carburettor) should be filled to approximately ½ inch (12mm) below the top of the hollow piston rod. Use a lightweight oil — SU specify a 20W grade. Our SU Damper Oil (DAM series) is the correct specification and should be checked and topped up every 3,000–6,000 miles. Running the dashpot dry will cause a flat spot on acceleration, as the piston will rise too quickly and lean the mixture momentarily. Never use engine oil heavier than 20W, as this will damp the piston excessively and impair throttle response.

Are your parts suitable for modern (E10) petrol?

Yes — all carburettor components, service kits and rebuild kits supplied by SU Specialists are manufactured by Burlen to be compatible with modern unleaded fuel including E10 (up to 10% ethanol content). Older original rubber components — float valves, diaphragms, O-rings and hoses — may not be compatible with ethanol-blended fuels, which is why regular servicing and replacement with current-specification parts is particularly important for vehicles used on modern pump fuel. If your vehicle has been standing for some time, a full service kit is highly recommended before returning it to use.

Why doesn't my new carburettor have a piston spring or damper rod?

Not all SU carburettor types use a piston return spring. Early H-type and some HS-type carburettors rely on the weight of the piston and atmospheric pressure to return the piston to the closed position — no spring is required. Similarly, not all types use a separate damper rod; on some units the damper is integral to the piston assembly. If your replacement carburettor or service kit does not include these components and your original unit had them, please contact us — but in many cases their absence is correct for the type.

Why are there no holes drilled on the new throttle spindle for the stop lever?

New throttle spindles are supplied undrilled as a blank, because the hole position varies depending on the specific application and the type of throttle stop lever fitted. The hole must be drilled and tapped to match the original location on your carburettor. This is straightforward work for any competent classic car workshop. If you are unsure of the correct position, compare against your original spindle or consult the relevant workshop manual for your vehicle.

Can I order if I'm in the classic car trade? Is trade discount available?

Yes, we welcome trade enquiries from classic car workshops, restoration specialists and dealers. Please get in touch via our contact page with details of your business and we will discuss account setup and trade pricing. We supply Burlen-manufactured genuine SU parts and can accommodate regular or bulk orders.

Why is my SU carburettor cutting out or losing power under hard acceleration?

This usually points to one of three causes. First, a dry or low dashpot — top up the damper oil and check the level. Second, a worn or incorrectly positioned jet — if the jet is not centred correctly under the needle, fuel delivery will be uneven. Third, a worn needle or jet that has been elongated through years of use. A service kit will address the first two issues; if the needle and jet are worn, they should be replaced as a pair. Mixture setting (the position of the jet relative to the needle) is also worth checking: a jet set too lean will cause hesitation and flat spots.

What is the difference between a service kit, a rebuild kit and a Major Repair Kit?

A service kit covers the consumable items replaced during routine maintenance — typically gaskets, O-rings, float valves and fibre washers. A rebuild kit goes further, adding diaphragms, needle assemblies and other wear items for a more thorough overhaul. A Major Repair Kit (MRK) is the most comprehensive option, providing everything needed for a complete carburettor rebuild from scratch — including throttle spindle seals, jet assemblies, needles, floats and all gaskets. For a carburettor that has not been rebuilt in many years, or where the history is unknown, an MRK is usually the best investment.