How to Test an SU Mechanical Fuel Pump (AUF 700 and AUF 800)

October 31, 2026

SU fuel pumps and repair parts on a workbench

SU mechanical fuel pumps were fitted to a wide range of BMC and Austin Rover cars, including the Mini, Metro, Allegro, Maxi, Maestro and Montego. They're simple and reliable, but when a car starts misfiring under load or cutting out on a hot day, the pump gets the blame. These tests show whether it's really at fault.

AUF 700 and AUF 800: what's the difference?

Both are diaphragm pumps driven from an engine cam, either directly or through a push-rod.

The AUF 700 mounts on the crankcase on two 5/16in UNF studs, with a heat-insulating block and two gaskets. Its outlet cover is held by three long screws and can be removed, giving access to the filter and valves.
The AUF 800 was developed for the higher under-bonnet temperatures of later cars. It's a sealed unit with good vapour-handling and a capacity of 11 gal/hr. It mounts on two 8mm (5/16in) studs with an insulating block between two gaskets.

Test 1: flow check on the car

Disconnect the fuel pipe at the carburettor and put the end into a suitable container.
With the ignition switched off, turn the engine over on the starter.
Watch the flow:
- Good, steady flow: the pump is fine. Look at the carburettor instead — a sticking or blocked float chamber needle valve is the usual culprit.
- Good at first, then falls away: the tank isn't venting. Take off the filler cap and try again. If the flow recovers, clear the tank vent.
- Weak flow: check for a choked filter in the pump or the tank.
- Bubbles in the fuel with the pipe end submerged: there's an air leak in the inlet line. Check and tighten every connection between the tank and pump.
- No flow and no bubbles (AUF 700): strip the pump and check the valves and diaphragm.

Do not blow compressed air through the pump to clear it. You'll damage the valves.

Test 2: suction (vacuum) test on the bench

Use these tests when you suspect the pump, or after you've overhauled one. You need a pressure/vacuum gauge (the SU service gauge is 18G 1116).

Clamp the pump in a soft-jawed vice and connect the gauge to the inlet.
Operate the pump lever through three full strokes.
Read the vacuum and watch how fast it falls:
- AUF 700: at least 152.4mm (6.0in) Hg, falling no faster than 50.8mm (2.0in) in 15 seconds.
- AUF 800: at least 228.6mm (9.0in) Hg, falling no faster than 76.2mm (3.0in) in 5 seconds.
Disconnect the gauge before continuing.

Test 3: delivery pressure test

With the pump still in the vice, connect the gauge to the outlet.
Operate the lever through two full strokes.
Read the pressure and watch how fast it falls:
- AUF 700: at least 0.21 kg/cm² (3.0 lb/in²), falling by no more than 0.035 kg/cm² (0.5 lb/in²) in 15 seconds.
- AUF 800: at least 0.42 kg/cm² (6 lb/in²), falling by no more than 0.140 kg/cm² (2 lb/in²) in 5 seconds.

A pump that won't reach these figures, or loses vacuum or pressure quickly, has leaking valves or a damaged diaphragm. On the sealed AUF 800 the answer is a new pump. The AUF 700 can be stripped and rebuilt.