::standfirst The SU fuel pump's reassuring "tick" means it's working; when it goes silent and the engine starves and stalls, the internal points have usually stuck. A gentle tap on the pump often frees them enough to get you home — but the real fix is cleaning or renewing the points, or fitting a good low-pressure replacement.
How it should behave
Switch the ignition on with the engine off and the pump should rattle away rapidly for a few seconds while it fills the float chambers. Once the needle valves close on full bowls, the ticking slows to the occasional single click as the pump tops up against the closed valves. On the road, you should barely notice it — a soft, intermittent tick under load, near-silence at rest.
Rapid ticking on priming, then near-silence: that is a healthy SU pump.
The classic failure — it goes silent
The engine stumbles, coughs, and dies. You turn the ignition off and back on and there is no reassuring rattle — just silence. The points inside the pump have stuck closed or dirtied, and the coil no longer pulls the diaphragm.
The traditional roadside fix is a firm but gentle tap on the pump body — the handle of a screwdriver is about right, not a hammer blow. The jolt is usually enough to unstick the points, the pump starts ticking again, and you can drive home. Treat it as a get-you-home measure, not a repair: a pump that has stuck once will stick again, and the next time may be somewhere less convenient.
Roadside checklist
SU fuel pump — quick diagnosis
Step 1 of 5
1.Ignition on. Is the pump silent — no reassuring tick, tick, tick?
The other complaint — it never stops ticking
The opposite fault is a pump that ticks on and on at idle and never settles. That means fuel or pressure is bleeding away somewhere and the pump keeps topping up to compensate. Work through the likely leaks in order:
- Float-bowl gaskets. A weeping gasket between the float-bowl lid and body will let fuel seep out and drop the level; the pump refills endlessly.
- Fuel line and unions. Perished rubber hose, porous flexible sections, or a loose union between the pump and carbs will lose pressure between ticks.
- Needle valves not seating. A worn or dirty needle valve lets fuel through even with the float up, flooding the bowl and running fuel out of the overflow.
- The points themselves. Worn or dirty contacts can stop the pump latching properly at the end of a stroke, so it cycles continuously instead of resting.
Check gaskets and lines first — they are cheap and visible — then work back to the pump.
Proper repair and replacement
Once you are home, do the job properly rather than trusting the next tap.
Clean and adjust the points. Remove the bakelite end cover, ease the points apart, and clean the contact faces with a strip of fine abrasive paper or a points file. Clean the pedestal underneath and check the throw of the rocker. Set the contact gap to the figure in the workshop manual for your pump.
Renew perished fuel line. Old rubber hose degrades from the inside out on modern ethanol-blended petrol. Replace any suspect sections with modern ethanol-resistant hose rated for continuous fuel service, and use fresh clips.
Choose a correct low-pressure pump. The SUs on a T-Type want around 2.5–3.5 psi. Your realistic options are:
- an original-type SU pump, rebuilt with fresh points and diaphragm;
- a good reproduction SU with modern solid-state or standard points;
- a low-pressure Facet-type electric pump rated in the same 2.5–3.5 psi range.
Positive-earth note
These are positive-earth cars. Wire the pump accordingly — the switched feed goes to the pump's live terminal and the pump body earths through its mounting to chassis positive. Reversing the polarity on a solid-state SU replacement will destroy the electronics on first switch-on.
Frequently asked questions
Why does my tuned XPAG misfire under load?
Misfire under load is almost always ignition on an XPAG, not carburation. Check the plug gaps and colour first — sooty plugs mean the mixture is rich or the ignition is weak; white or blistered electrodes mean it is lean. Then confirm the points gap and dwell, the rotor arm and condenser, and that the coil is producing a strong blue spark. On a Stage 1 or Stage 2 engine, also check the static timing has not crept too far advanced; a pinking engine often feels like a misfire and will eventually burn a hole in a piston. Finally, balance the SUs again — an unbalanced pair can leave one cylinder running lean enough to misfire at wide throttle.
Why does my XPAG hesitate when I open the throttle?
Hesitation on pick-up is the classic SU weak-mixture signal. The piston in the carburettor is lifting too fast for the fuel to follow, usually because the jet is set too lean, the needle is worn, or the dashpot oil is too thin. On a Stage 1 car with standard 1.25 in SUs, start by checking the damper oil level and weight — SAE 20 is correct — and that the piston rises smoothly without sticking. Then richen each jet by one flat, test again, and confirm with the lifting-pin method. On a Stage 2 car with 1.5 in SUs the same rule applies, but the needle profile is more critical; a worn or mismatched needle will hesitate whatever the jet setting.
Why is my tuned XPAG hard to start when hot?
Hot-starting problems on a tuned XPAG usually come down to fuel evaporation or flooding. A free-flow exhaust and a hot engine bay can boil the fuel in the float chamber and carburettor body, making the mixture over-rich. Check the float level first — it should be at the workshop-manual figure, not simply "looks about right". If the car starts cold but floods hot, try easing the throttle fully open while cranking to clear the cylinders, then start with no throttle. On Stage 2 engines with raised compression the starter also has to turn a higher-compression engine against a hot block; a good battery and clean earth straps matter more than on a standard car.
My Stage 1 XPAG pings on acceleration — what do I adjust?
Pinking means the combustion pressure peak is arriving too early. On a Stage 1 tune running on modern 95 RON petrol, retard the static timing by one or two degrees first and re-test on a warm day under load. If it still pings, check that someone has not already advanced the distributor beyond the Stage 1 figure, verify the compression ratio has not been raised by a previous skim, and make sure you are not running the engine below about 1,500 rpm in a high gear. Do not simply richen the mixture to mask pinking — it helps a little, but the real fix is timing or compression.
After fitting 1.5 in SUs my Stage 2 idle hunts — what is wrong?
An uneven idle after a Stage 2 carburettor swap is usually one of three things: the throttle spindles are worn and drawing air, the inlet manifold is not matched to the head ports and is causing reversion pulses, or the jets and needles are not centred and matched. Re-check the manifold face and gasket match, recentre both jets with the piston-drop test, and confirm both carburettors are drawing the same airflow at idle. Stage 2 engines with a fast-road cam also have less manifold vacuum at idle, so the idle speed may need to be set a little higher than standard — typically 800–900 rpm — to keep the SUs stable.
How do I know if my tune is running lean?
Lean running shows up as a flat spot under load, popping in the exhaust on the overrun, white or grey spark-plug insulators, and elevated exhaust-manifold colour after a run. On a rolling road the air/fuel ratio will read above 14.5:1 at cruise or above 13.0:1 at wide throttle. Fix it by richening the mixture a flat at a time on the SUs, checking for air leaks around the manifold and balance pipe, and confirming the fuel pump is delivering at least 1.5 psi at the carbs. Lean mixture is the fastest way to burn exhaust valves on a tuned XPAG, so do not ignore it.
Related reading
- Tuning the Twin SUs — once fuel delivery is sorted, the carbs come next.
- Cooling the XPAG and XPEG — the other classic "it stopped by the side of the road" story.
FAQ
Why does my SU fuel pump keep ticking? Continuous ticking means fuel or pressure is leaking away between strokes. Check float-bowl gaskets, fuel lines and unions, and the needle valves before blaming the pump itself.
My MG won't start and the fuel pump is silent — what do I do? Tap the pump body firmly but gently with a screwdriver handle. If it starts ticking, the points had stuck; drive home and clean or renew them properly. If tapping does nothing, check the pump has power and a good earth before condemning it.
What pressure should an SU fuel pump run at? The SUs on a T-Type are designed for roughly 2.5–3.5 psi. Higher pressures overwhelm the needle valves and flood the carbs.
Can I fit a modern fuel pump to a T-Type? Yes, provided it is a low-pressure type in the 2.5–3.5 psi range — a low-pressure Facet-type is a common choice. Avoid high-pressure pumps intended for fuel injection, and be sceptical of pressure regulators sold to reduce them.