SU Carburetter Maintenance and Settings

SU Carburetter Maintenance Overview

SU carburetters are robust and long-lived when properly maintained, but neglected units develop consistent patterns of problems. This guide covers the core maintenance tasks that apply across all SU carburetter types — H, HD, HS, and HIF — with notes where procedures differ between types.

Identifying Your SU Carburetter

Before carrying out any maintenance, it helps to know exactly which type of SU carburetter you have. Look for the stamped identification on the carburetter body:

  • H Type: Separate side-mounted float chamber, sliding jet adjusted by a nut beneath the body. Designation stamped as H1, H2, H4, H6, or H8 (number = bore in ¼-inch increments).
  • HD Type: Similar to H Type but with diaphragm-operated jet and often paired with an Auxiliary Enrichment Carburetter (AEC). Stamped HD4, HD6, or HD8.
  • HS Type: Compact slim body with rear or side float chamber, jet adjusting nut directly below body. Stamped HS2, HS4, HS6, or HS8. The most common type on 1960s–70s British classics.
  • HIF Type: Self-contained compact body with integral (built-in) float chamber accessible via a bottom cover plate. Stamped HIF38, HIF44, or earlier HIF4, HIF5, HIF6.

Needle Valves

The needle valve controls fuel entry into the float chamber and is one of the most common causes of carburetter problems. It consists of a conical brass or synthetic rubber-tipped needle that seats in a matching orifice. When the float rises to the correct level, it presses the needle against its seat and stops fuel flow.

Signs of a worn or failed needle valve:

  • The engine floods — strong smell of petrol, black exhaust smoke, wet spark plugs
  • Fuel drips from the carburetter
  • The float chamber overflows
  • Inconsistent fuel level in the float chamber

To test the needle valve: remove the float chamber lid (or cover plate on HIF types), lift the float to the fully closed position, and blow gently into the fuel inlet. No air should pass if the valve is seating correctly. Any flow indicates wear and the needle valve should be replaced. Always replace the needle valve as part of a standard rebuild — it is inexpensive and a common failure point.

Float Height Setting

The float height determines the fuel level in the float chamber, which in turn affects mixture strength. An incorrectly set float height is one of the most common causes of fuelling problems that do not respond to mixture adjustment.

Setting the float height:

  1. Remove the float chamber lid (H, HD, HS types) or bottom cover plate (HIF type).
  2. With the lid inverted and the needle valve in place, measure the distance from the mating face of the lid to the bottom of the float.
  3. Standard float height for most H, HD, and HS types is 9.5mm (⅜ inch) to 12.7mm (½ inch) from the mating face. Check the specific figure for your application in the workshop manual.
  4. Adjust by bending the float arm slightly — bending it towards the needle valve reduces the height (raises fuel level); bending away from the valve increases it (lowers fuel level).
  5. Always check float height with a new needle valve fitted — a worn valve sits higher and gives a misleadingly low float height reading.

Suction Chamber Oil

The suction chamber dashpot oil is critical to smooth acceleration. Without it, the piston rises too quickly on acceleration, creating a momentary lean condition (flat spot) before the engine picks up. The oil acts as a simple hydraulic damper.

Topping up the dashpot oil:

  1. Unscrew the damper cap from the top of the suction chamber.
  2. Using a small funnel or oil dropper, add SAE 20W engine oil until it reaches within ½ inch (12mm) of the top of the hollow damper rod.
  3. Refit the cap. Do not overfill — excess oil will be expelled into the air intake.

If you prefer slightly crisper throttle response, automatic transmission fluid (ATF) can be used instead of engine oil. Some owners of high-performance applications use a lighter oil. Thinner oil gives quicker piston rise; thicker oil gives slower rise and better flat-spot protection on tired engines.

Piston Spring Rate

The suction piston spring sets the base tension on the piston. Standard springs are colour-coded by rate — the standard spring is usually red, with stronger springs (which give a slightly leaner mixture at low speeds) colour-coded yellow or blue depending on application.

A broken or collapsed piston spring will cause the piston to sit too low, resulting in very rich running at idle and low speed. Always check the spring when rebuilding a carburetter, and verify the spring rate is correct for your application before assuming the carburetter needs re-jetting.

SU Gasket Service and Rebuild Kits

A full rebuild kit for any SU carburetter type will contain the following items as a minimum:

  • Needle valve and seat
  • Float chamber gasket or O-ring
  • Jet O-ring (HS and HIF types)
  • Suction chamber gasket
  • Throttle spindle seals
  • Damper O-ring

Better kits also include replacement throttle spindle bushes and a new jet needle. We recommend using a full kit rather than replacing individual items, as age-hardened rubber components tend to fail in sequence once the carburetter is disturbed.

Throttle Spindle Bushes

Worn throttle spindle bushes are responsible for more persistent idling problems than almost any other single component. As the brass bushes wear, air is drawn past the spindle without passing through the jet, causing a lean condition that cannot be corrected by raising the jet alone.

Diagnosing worn spindle bushes: With the engine idling, spray a small amount of carburetter cleaner around the throttle spindle at each end of the body. If the idle speed changes, air is being drawn in through the spindle — the bushes need replacing.

Replacing spindle bushes: The old bushes are pressed or drifted out from the body. New phosphor bronze bushes are pressed in and then reamed to the correct diameter to accept the spindle with minimal play. This operation requires a reamer of the correct size (typically 5/16 inch for most SU types). Many owners send the carburetter body to a specialist for this work. We offer a professional carburetter refurbishment service if required.

Carburetter Flooding

Flooding — where petrol overflows from the carburetter — has several possible causes:

  • Worn needle valve: The most common cause. Replace the needle valve and seat as a unit.
  • Float with a hole: A punctured float fills with fuel and cannot rise to close the needle valve. Shake the float — any sound of liquid inside means it must be replaced.
  • Incorrect float height: If the float arm is bent incorrectly, the needle valve may not close at the correct fuel level. Check and reset float height.
  • Debris on the needle valve seat: Small particles can prevent the needle valve from seating. Carefully clean the seat with a soft cloth or blow through with compressed air.
  • Excessive fuel pump pressure: High fuel pump pressure can force the needle valve open against the float. If pump pressure is suspected, fit a pressure regulator. Correct SU carburetter fuel pressure is 1.5–3 psi for most applications.

Carburetter Cleaning

Petrol dissolves most deposits in SU carburetters, but varnish from old fuel can build up in jets and galleries. When dismantling for a full clean:

  • Soak metal parts in carburetter cleaner or petrol for 30 minutes
  • Use a lint-free cloth to clean the jet — never a wire or hard instrument which will enlarge the jet orifice
  • Blow through all galleries with dry compressed air
  • Never use abrasive pads on the suction chamber or piston — any scoring will cause the piston to stick