SU Carburettor & Fuel Pump Fault Finding Chart

Use this chart to narrow down a problem with your SU carburettors or SU fuel pump. It's based on the fault diagnosis tables in the SU workshop manual, with links to the parts that fix each cause. Before you start on the carburettors, make sure the ignition timing, plugs, points and valve clearances are right — a lot of "carburettor" problems are really ignition problems.

H, HD and HS carburettors

Symptom Likely cause Remedy
Erratic running, stalling at idle, lack of power, high fuel consumption Sticking piston — dirty piston and suction chamber Clean both with petrol or methylated spirit. No abrasives. See our piston drop test guide
Jet out of centre Re-centre the jet so the piston drops freely with the jet fully up
Bent needle Fit a new metering needle
Hesitation or flat spot when accelerating Low damper oil level Top up with SU damper oil, SAE 20
Wrong oil grade (too thin) Drain and refill with the correct grade
Too rich at idle, or fuel leaking from the jet (H type) Faulty top jet gland Fit new glands, or a service kit
Dirt under the top gland washer Clean and reassemble
Faulty bottom gland Fit new
Float chamber or jet flooding Incorrect fuel level Check and reset — see float level settings
Dirty or worn float chamber needle valve Clean or renew the needle valve and seat, and flush the fuel system
Punctured or fuel-soaked float Fit a new float, or a StayUp float kit
Idle won't settle, twin carbs won't balance Air leak past worn throttle spindles Replace spindles and bushes — see our spindle bush guide

HIF carburettors

The HIF suffers from the same faults as the HS: sticking pistons, low damper oil and worn needle valves. Two HIF-specific checks:

  • Float level: the float should sit 1.0mm ± 0.5mm below the chamber face with the body inverted.
  • Bi-metal jet lever: the jet head must move freely in the bi-metal cut-out, or the mixture won't compensate for fuel temperature.

StayUp floats are available for HIF: AUE 895 left hand and AUE 896 right hand.

SU electric fuel pumps

Symptom Likely cause What to do
No fuel at the carburettor, pump silent No electrical supply, or dirty or burnt contact points Check for voltage at the terminal. On points-type pumps, remove the cover and check the tungsten points are touching. Clean or replace with a pump repair kit
Pump ticks but delivers no fuel Air leak on the suction side, or dirt under a valve Check every connection from tank to pump. Remove and clean the valves, taking care not to damage the valve discs
Flow falls away after a few seconds, pump slows Tank not venting Remove the filler cap. If flow recovers, clear the tank vent
Reduced flow with rapid pumping Blocked filter, air leak on the suction side, or faulty valve washers Clean the filter, check the inlet union and O-rings, renew the valves. Fit an inline fuel filter
Noisy pump Air leak on the suction side Run the outlet into a container with the pipe submerged. Steady bubbles confirm a leak. Check the connections, the inlet union seals and the coil housing screws
Pump stops and needs a tap to restart Stiff diaphragm or rocker mechanism Overhaul the pump, or use our fuel pump refurbishment service

Never blow compressed air through an SU pump — it will damage the valves. You can clear a blocked pipe between tank and pump with compressed air, but only with the pump disconnected.

SU mechanical fuel pumps (AUF 700 and AUF 800)

Symptom Likely cause What to do
Normal flow at the carb but still starving Sticking or blocked float chamber needle valve Clean or replace the needle valve
Flow good at first then falls away Tank not venting Remove the filler cap and re-test. Clear the vent
Low flow Choked pump or tank filter Clean the filters
Bubbles in the fuel from the outlet pipe Air leak in the inlet line Check and tighten the inlet connections
No flow, no bubbles (AUF 700) Valve or diaphragm failure Strip and inspect the pump, or fit a replacement pump

For how to test a mechanical pump with a gauge, see how to test an SU mechanical fuel pump.

Still stuck?

Send us your carburettor spec number and a description of the symptoms and we'll help you work out what's wrong. Or let us do it for you with our carburettor refurbishment and pump refurbishment services.