SU Mechanical Fuel Pumps

SU Mechanical Fuel Pumps

Alongside their famous electric pumps, SU also produced a range of mechanical fuel pumps for vehicles where the engine layout was better suited to a cam-driven mechanical unit. SU mechanical pumps were fitted to a number of British vehicles and are known for their robust construction and long service life.

The two main types in regular use on classic vehicles are the AUF 700 and AUF 800 series. Both are diaphragm-type mechanical pumps driven by a pushrod from an eccentric lobe on the camshaft, and both are fully rebuildable with readily available service parts.

AUF 700 Mechanical Fuel Pump

The AUF 700 is a standard-output mechanical pump found on a range of four and six-cylinder British engines. It produces adequate flow for single-carburetter applications and most twin-carburetter setups on engines up to approximately 2.5 litres.

The AUF 700 features a cast aluminium body with an integral sediment bowl protected by a gauze filter. A hand primer lever allows the carburetter float chamber to be primed before starting, which is particularly useful on vehicles that have been laid up.

AUF 800 Mechanical Fuel Pump

The AUF 800 is a higher-output version of the AUF 700, suitable for larger-displacement engines and higher-performance applications. The basic design and service procedure is the same as the AUF 700, but with a larger diaphragm and higher flow capacity.

SU Mechanical Pump Routine Servicing

Mechanical pumps require less frequent attention than electric pumps, but should be inspected during any major engine service. Key items:

  • Sediment bowl and filter: Remove the sediment bowl and clean the gauze filter. Replace the bowl gasket. This should be done annually or whenever fuel contamination is suspected.
  • Diaphragm: The diaphragm is the main service component. Signs of failure include fuel in the engine oil (diaphragm leaking into the pushrod cavity), low fuel pressure, or fuel dripping from the pump body. Inspect during any major service.
  • Output pressure: Correct pressure for SU carburetters is 1.5–3 psi. Excessive pressure (more than 3.5 psi) will force the carburetter needle valve open and cause flooding. Fit a regulator if pressure is consistently high.

AUF 700 Mechanical Pump Dismantling

  1. Remove the pump from the engine. Before removing, note the orientation of the pump body relative to the mounting face — the body can be repositioned in 90-degree increments, and must be refitted in the same position or the outlet will point in the wrong direction.
  2. Remove the sediment bowl bolt, lift off the bowl, and remove the gasket and filter gauze. Note the gasket position.
  3. Mark the upper and lower halves of the pump body in relation to each other before disassembly — the body halves can be rotated relative to each other and must be kept in the original relationship.
  4. Unscrew the body screws and separate the two halves. The diaphragm is clamped between the halves and is attached to the operating rod by a spring-loaded coupling.
  5. Note how the diaphragm is fitted — including the spring arrangement — before removing it from the operating rod.
  6. Remove the inlet and outlet valves for inspection. These are typically held by small screws or are pressed into position.

AUF 700 Mechanical Pump Reassembly

  1. Clean all metal parts thoroughly. Replace all gaskets, the diaphragm, and valve discs as part of the rebuild kit.
  2. Fit new valve discs ensuring they sit flat and undistorted in their recesses. The inlet and outlet valves are not interchangeable — ensure they are fitted in the correct positions.
  3. Fit the new diaphragm to the operating rod, ensuring the diaphragm spring is correctly positioned.
  4. Reassemble the two body halves in the same relative orientation as marked before dismantling. The diaphragm must be evenly clamped — tighten the body screws alternately in a crossing pattern.
  5. Refit the filter gauze and sediment bowl with a new gasket.
  6. Before fitting to the engine, test by pressing and releasing the hand primer — you should feel resistance as the diaphragm creates suction on the intake stroke.

SU Mechanical Pump Fault Diagnosis

Engine not receiving fuel / pump not priming: Check that the pushrod is making contact with the camshaft eccentric. On some engines, the pushrod can fall into the crankcase when the pump is removed. Prime the pump using the hand primer lever while the pump is off the engine to confirm it is working, then refit.

Fuel in the engine oil: This indicates a diaphragm failure. Fuel is bypassing the failed diaphragm and draining into the pushrod cavity and then into the crankcase oil. Change the oil and filter after replacing the diaphragm, as fuel-contaminated oil has severely reduced lubrication properties.

Fuel leaking from the pump body: Typically the sediment bowl gasket or the body joint gasket. The diaphragm can also leak at the clamping edge if the body screws have been overtightened or if the diaphragm has hardened and pulled away from its seat.

Carburetter flooding: Check output pressure. Excessive pump pressure (above 3 psi for most SU carburetters) overcomes the carburetter needle valve and floods the float chamber. Fit a pressure regulator if the pump consistently exceeds 3 psi.

Low power at high rpm: The mechanical pump may not be keeping up with fuel demand at high engine speeds. This is more common on high-performance modified engines running the standard pump. Upgrade to the AUF 800 or fit a supplementary electric pump.

SU Mechanical Pump Replacement Parts

We stock rebuild kits for AUF 700 and AUF 800 mechanical pumps, including diaphragm assemblies, valve disc sets, sediment bowl gaskets, and body gaskets. Complete replacement pumps are available where rebuild is not practical. Pressure regulators for high-pressure applications are also stocked.