A smell of petrol in the garage, a damp patch under the engine, and an engine that starts black-smoking rich or will not start at all: a flooding SU is one of the few T-type faults that is genuinely urgent, because the fuel is landing near a hot manifold. The good news is that the cause is nearly always one of four things, and three of them are cured with the carburettor still on the car.
Owner photograph to follow
Why an SU floods
An SU carburettor has no jets to meter fuel by pressure — it relies on the fuel in the float chamber sitting at a constant height relative to the jet. The float chamber is a simple cistern: fuel enters through a needle valve, the float rises with the level, and at the correct height the float pushes the needle onto its seat and shuts the supply off. Flooding means that valve is not closing, so the pump keeps pushing fuel in until it escapes past the lid, out of the overflow, or through the jet into the inlet.
That gives four candidates, in the order you will actually meet them: dirt on the needle-valve seat, a worn valve, a punctured or fouled float, and a float level or pump pressure that is simply wrong.
Working through it
- 01
Make the car safe first
Petrol running over a hot exhaust manifold is a fire, not an inconvenience. Stop the engine, switch off the ignition so the SU pump stops delivering, let the engine cool, and work outdoors or with the doors open. Have an extinguisher within reach before a spanner touches the carburettor.
- 02
Find where the fuel is actually coming from
Wipe everything dry, then switch the ignition on without starting and watch. Fuel from the float-chamber lid joint or its overflow means the chamber is filling past its level. Fuel weeping from a union or the pipe to the chamber is a leak, not flooding. Fuel running out of the mouth of the carburettor or dribbling down from the jet is an overfull chamber finding the lowest way out.
- 03
Tap and clean the needle valve
The commonest cause of all is a speck of dirt or a piece of old fuel hose holding the needle valve off its seat. Give the float-chamber lid a light tap with a screwdriver handle: if the flooding stops, dirt is confirmed. Take the lid off, lift out the float and needle valve, and clean both, the seat and the chamber. Look for a ridged or grooved needle tip and a worn seat — either means renewal, and a modern ball-type replacement valve is a well-proven cure for repeat offenders.
- 04
Check the float
Shake the float next to your ear. Fuel sloshing inside means it is punctured and no longer floats high enough to close the valve, and it needs renewing. Brass floats can be re-soldered but rarely stay fixed. Check too that the float and its hinge move freely, with nothing fouling the chamber wall.
- 05
Set the float level
With the lid inverted, the float should just hold the needle valve closed with a small, even gap under the float — the level is adjusted by carefully bending the hinged lever, or by the adjustable arrangement on later carburettors. The gap is specified as a gauge or bar diameter that differs between SU types and between models, so take the figure from the Workshop Manual or SU data sheet for your carburettor rather than working by eye.
- 06
Check the pump pressure
An SU electric pump that delivers more than the carburettor's float valve can hold shut will flood it however clean the valve is. Excess pressure usually means the wrong pump has been fitted, or a modern replacement is over-specified. If flooding returns on a clean valve and a sound float, check the delivery pressure against the figure for your pump and consider whether the fitted pump is correct for the car.
- 07
Look for the after-effects
Persistent flooding washes the bores and dilutes the sump. If the car has run while flooding, change the engine oil rather than leaving petrol-thinned oil in the engine, and check the plugs — soot-black, wet plugs confirm how long it has been running rich.
Reading the symptom
| What you see | Most likely | First move |
|---|---|---|
| Floods once, then behaves for weeks | Dirt intermittently holding the valve open | Clean the valve, chamber and the in-line filter; check the tank for debris |
| Floods steadily whenever the pump runs | Worn needle valve or punctured float | Shake the float; renew valve and float as needed |
| Rich running, black plugs, no visible leak | Level slightly too high, or a worn jet and needle | Re-check float level, then mixture setting |
| Fuel from a union or hose, chamber level correct | Perished hose or slack union — a leak, not flooding | Renew the hose; check every union from pump to carburettor |
| Pump ticks continuously and the carburettor floods | Valve held open, or too much delivery pressure | Cure the valve first, then verify the pump is the right one |
The continuously-ticking pump is worth dwelling on: the same symptom appears when the pump has a leak on its delivery side, so read it alongside the fuel-pump article rather than assuming the carburettor is at fault.
Keeping it cured
Almost all repeat flooding comes from debris, and almost all the debris comes from two places: a tank that has been left part-full and has begun to shed rust and varnish, and old rubber fuel hose breaking up internally where modern petrol has attacked it. If you have cleaned a needle valve twice, do the boring jobs — renew the hose in ethanol- tolerant modern specification, fit or clean the in-line filter, and deal with the tank — rather than the interesting one a third time.
It is also worth remembering that a car standing for months with a full float chamber will gum the valve with varnish as the light fractions evaporate. Turning the fuel off and running the chamber dry before a long lay-up avoids a spring full of flooding.
Read next
This guide is a general introduction and is not a substitute for the official Workshop Manual for your car.