How to Rebuild an SU Carburettor: Step-by-Step Guide

Everything You Need to Know Before You Start

Rebuilding an SU carburettor is one of the most satisfying jobs on a classic car — and one of the most rewarding. A properly rebuilt SU will reward you with smooth, reliable running and accurate fuel delivery. The good news is that SU carburettors are elegantly simple by design, and a full rebuild is well within the capability of any home mechanic with patience and the right parts.

This guide covers the rebuild process for the HS series carburettor, which is the most common type found on British classics. The process for HIF carburettors is broadly similar, with some differences in float chamber access.

What You'll Need

Tools

  • Flat-blade screwdrivers (various sizes)
  • Needle-nose pliers
  • Small spanners or sockets (typically 1/4", 5/16", 3/8" AF)
  • A jet key or large flat-blade screwdriver
  • Carburettor cleaning solvent
  • Clean lint-free cloths
  • Small wire brush

Parts

Before you start, order a Major Repair Kit for your specific carburettor type. A good quality kit (such as those manufactured by Burlen, the official SU parts manufacturer) will include:

  • Needle and needle jet
  • Float chamber needle and seat
  • Float (if replacing)
  • All gaskets and O-rings
  • Diaphragm (for HIF types)
  • Damper O-ring

Browse our Major Repair Kits — we stock kits for every common HS and HIF variant.

Step 1: Remove the Carburettor

Before removing the carburettor, make sure the engine is cold. Disconnect the battery, then:

  1. Disconnect the fuel line from the float chamber and plug it to prevent spillage.
  2. Disconnect the throttle cable and return spring.
  3. On twin-carb setups, disconnect the balance pipe between the carburettors.
  4. Remove the air filter assembly.
  5. Undo the four nuts or bolts holding the carburettor to the inlet manifold.
  6. Carefully lift the carburettor away. Have a rag ready to catch any residual fuel.

Step 2: Disassembly

Remove the Suction Chamber and Piston

  1. Undo the three screws holding the suction chamber (the dome-shaped top of the carburettor) to the body.
  2. Lift the suction chamber away carefully — the piston and spring will come with it.
  3. Remove the piston from the suction chamber. Note the orientation — there is a small pin or lug that locates it correctly.
  4. Remove the needle from the piston. It is held by a small grub screw on the side of the piston. Note which groove the needle is clamped in — this is important for reassembly.

Remove the Float Chamber

  1. Unscrew the float chamber from the side of the carburettor body (three screws on most HS types).
  2. Carefully lift the float chamber away. The float and needle will be visible inside.
  3. Remove the float by sliding out the pivot pin.
  4. Unscrew the needle valve seat from the body using a screwdriver or needle-nose pliers.

Remove the Jet

  1. The jet is the cylindrical fuel-metering component in the centre of the carburettor body.
  2. Disconnect the jet from the mixture adjustment screw and remove it downwards from the body.
  3. Note the jet height before removal if the carburettor was running correctly — this can save time during re-tuning.

Step 3: Cleaning

Thoroughly clean all metal components in carburettor cleaning solvent. Pay particular attention to:

  • The jet bearing — any wear here will cause a rich mixture and poor economy
  • All fuel passages in the body — use a thin wire or pipe cleaner to clear any blockages
  • The suction chamber bore and piston — these must be perfectly clean for the piston to move freely
  • The float chamber — remove any sediment or varnish deposits

Do not use wire brushes on any mating surfaces or soft metal components. Rinse all parts in clean fuel or solvent and allow to dry.

Step 4: Inspection

Before fitting new parts, inspect the following:

  • Piston and suction chamber: Check for wear by inserting the piston into the chamber and checking for play. There should be no visible side-play. If there is, the piston or chamber (or both) needs replacing.
  • Needle: Inspect for wear, particularly in the lower third where it contacts the jet. A worn needle will have a flat spot on one side. Replace if in doubt — needles are inexpensive.
  • Float: Shake the float — if you can hear fuel inside, it is punctured and must be replaced. Check it floats correctly in a bowl of clean fuel.
  • Jet bearing: The jet should slide smoothly in the bearing with no side movement. Wear here is a common cause of permanent rich running.

Step 5: Reassembly

Reassembly is broadly the reverse of disassembly. Key points to remember:

  1. Fit the new needle valve seat and needle. Check the float height matches the specification in the workshop manual for your vehicle.
  2. Refit the float and pivot pin, then refit the float chamber with a new gasket. Do not overtighten.
  3. Fit the new jet and reconnect to the mixture adjustment assembly.
  4. Fit the new needle to the piston in the correct groove. The standard position is the middle groove unless you are adjusting for a known fuel mixture issue.
  5. Refit the piston to the suction chamber, ensuring it is correctly oriented.
  6. Refit the suction chamber to the body and tighten the three screws evenly.
  7. Add clean engine oil to the damper — SAE 20 or dedicated SU damper oil. Do not use ATF or very light oils.

Step 6: Refit and Initial Adjustment

Refit the carburettor to the manifold with a new gasket. Reconnect the fuel line, throttle cable, and return spring. Before starting:

  1. Set the jet to the standard position (flush with the jet bridge or as specified in the workshop manual).
  2. Set the idle speed screw to give approximately 1-2mm of throttle opening.
  3. Prime the float chamber by briefly cranking the engine or manually operating the fuel pump.

Start the engine and allow it to reach operating temperature before making any mixture or idle adjustments. See our SU Carburettor Tuning Guide for detailed guidance on getting the mixture set correctly.

Need Parts?

We stock the complete range of genuine Burlen-manufactured SU service parts — from Major Repair Kits and individual needles to float chamber assemblies and dampers. If you are not sure which parts you need, get in touch and we will help you identify the right kit for your carburettor.