Fuel Pump Location and Over-Fuelling (AC Engine)
AC-engined cars run SU carburettors, and owners fighting a persistent over-fuelling problem often wonder whether the fuel pump is in the wrong place. Pump position does matter, but not usually for over-fuelling. Here is where the pump is best mounted, and where flooding really comes from.
Confirm against your car
This applies to AC-engined cars with SU carburettors; Bristol-engined cars run triple Solex carburettors and are covered separately. Petrol is highly flammable, work in ventilation, away from ignition sources, and clean up spills. Confirm against your own car and consult a specialist for persistent fuelling faults.
Mount the pump low, and near the tank
SU and Facet-type pumps push fuel better than they pull it, so they run best mounted low and well back, near the tank, at or below the mid-depth of the tank, so the pump stays primed with liquid rather than drawing air. Owners mount them variously, on the rear bulkhead, low in a rear corner of the boot, but the common principle holds: low, rearward, and kept primed. Keeping the tank reasonably full helps a low-mounted pump stay fed.
Over-fuelling is usually not the pump's position
If the carburettors are flooding, the cause is more often in the float chambers than in where the pump sits. The usual suspects are a float height set wrong, needle valves not sealing on their seats, worn or with grit on the seat, or fuel pressure too high. SU and other early carburettors are low-pressure, high-volume: pressure in the region of only a few pounds per square inch (around 4 psi).
So before moving the pump, check the float settings and the needle valves and seats, test the floats for leaks, a fuel-laden float loses buoyancy and lets the level rise, and can be checked in hot water, and confirm the pump pressure is in the low range these carburettors expect. A regulator set too high will overwhelm the needle valves whatever the pump's position.
In short
Mount an AC-engined car's fuel pump low and near the tank so it stays primed, since these pumps push better than they pull. But if the carburettors are over-fuelling, look first to float height, needle-valve sealing and fuel pressure, SU carburettors want only a few psi, rather than the pump's location. Fix the float chambers and the pressure, and the flooding usually goes with them.
Sources. This article draws on the experience of owners in the AC Owners' Club community, including advice from the SU specialist Burlen. No endorsement is implied.
A general guide to fuel-pump mounting and over-fuelling on AC-engined cars, not a substitute for the workshop manual or a carburettor specialist. Petrol is flammable; confirm details against your own car and get persistent fuelling faults checked.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.