SU carburettors are elegantly simple devices, but like any precision instrument they wear, and they give clear signals when something is wrong. Most problems reduce to one of a handful of root causes — and once you know what to look for, diagnosis is usually straightforward. This guide covers the most common SU carburettor faults, how to identify them, and what to do about them.
1. Flooding — Fuel in the Intake, Rich Running or Petrol Smell
Symptoms: Strong fuel smell, black smoke from the exhaust, fouled spark plugs, fuel pooling below the carburettor, difficulty starting when hot.
Cause: Flooding almost always comes from one of two sources: a worn or contaminated needle valve that can't seal off the fuel supply, or a float that has developed a leak and is sitting too low in the fuel. Both allow too much petrol into the float chamber, which overflows into the intake.
Fix: Start with the needle valve — it's the more common culprit and the easier fix. Remove the float chamber lid and inspect the needle tip for a circumferential groove or flat spot worn into the rubber sealing surface. Even a slight groove is enough to let fuel bypass. Replace with a matched needle valve and seat kit: the WZX 1102 (0.096" bore) suits most HS and H-type carburettors. While the chamber is open, shake the float close to your ear — any sloshing means fuel has entered and the float needs replacing too. Our StayUp Float kits use a sealed, fuel-resistant construction that eliminates the ingress problem permanently.
2. Flat Spot on Acceleration
Symptoms: The engine hesitates or stumbles when you open the throttle, particularly from low speed or after a period of light throttle. The car may feel like it briefly cuts out before pulling cleanly.
Cause: An acceleration flat spot on an SU carburettor is almost always caused by insufficient damping in the piston assembly. The hydraulic damper is designed to slow the rise of the piston on throttle opening, briefly enriching the mixture as the throttle opens. Without adequate damping — whether from low oil level, wrong oil viscosity or a worn damper — the piston rises too quickly, the mixture goes lean momentarily and the engine stumbles.
Fix: Remove the damper cap from the top of the suction chamber and check the oil level — it should be approximately 0.5 inches above the top of the hollow piston rod. Top up with SU Damper Oil SAE Grade 20. Do not use engine oil, ATF or any other substitute — the viscosity must be correct. If the damper rod or cylinder is worn, it will need replacement; contact us with your carburettor type and we can advise.
3. Rough or Lumpy Idle
Symptoms: The engine idles unevenly, hunts (cycles between high and low rpm), or won't hold a steady idle at all. The problem may worsen when the engine is warm.
Cause: A rough idle on an SU carburettor typically points to an air leak or an incorrectly set mixture. Air leaks are the more insidious cause — they introduce unmetered air that leans the mixture at idle, where the engine is most sensitive to mixture strength. The most common leak points are the throttle spindle (where it passes through the carburettor body) and the joint between the carburettor and the inlet manifold.
Diagnosis: With the engine at idle, spray a small amount of carburettor cleaner around the throttle spindle ends and the base of the carburettor. If the idle speed rises or the note changes, you have an air leak at that point.
Fix: Minor spindle wear can sometimes be addressed with new seals. Significant wear requires a new spindle or a full body replacement. If the idle is simply set too lean — a common result of previous owners adjusting the mixture screw — use a jet adjusting spanner to raise the jet (richen the mixture) in small increments until the idle improves. As a rule of thumb, the correct mixture gives the highest idle speed for a given throttle setting; lower the idle speed with the throttle stop screw after each adjustment.
4. Black Smoke and Poor Economy — Running Rich
Symptoms: Black sooty exhaust smoke, high fuel consumption, sooty or oily spark plugs, strong fuel smell in the exhaust.
Cause: Persistent rich running (distinct from the flooding described above) is usually caused by the jet sitting too low — delivering too much fuel — or by a needle that is worn thinner than its original profile, metering more fuel than intended. It can also be caused by a sticking piston that fails to rise when the throttle opens, keeping the needle too deep in the jet.
Fix: First check that the piston moves freely — lift it with your finger through the dashpot and it should rise and fall smoothly with a gentle thud as it seats. If it sticks, carefully clean the piston and bore with a lint-free cloth (no abrasives). Check the needle for a wear step at the point where it sits in the jet at idle; a worn needle needs replacing. Our SU Needle Profile Chart identifies the correct needle for your carburettor and application. If the mixture has simply been set too rich, adjust the jet upward (leaner) using a jet adjusting spanner.
5. Fuel Pump Not Priming or Ticking Continuously
Symptoms: On switching the ignition on, the SU electric fuel pump should tick rapidly then slow and stop as pressure builds and the float chambers fill. Continuous ticking means it can't build pressure (a leak or pump fault). No ticking at all means the pump isn't operating.
Cause: Continuous ticking most often indicates a fuel leak somewhere between pump and float chamber (preventing pressure build-up), a worn pump diaphragm, or deteriorated pump points on older mechanical-points type pumps. No operation can indicate a failed coil, dirty points or an electrical fault.
Fix: For points-type pumps, inspect and clean or replace the contact points — this resolves the majority of intermittent pump faults. If the diaphragm has hardened or cracked, a repair kit will include both. We stock repair kits for all standard SU pump types including the EPK series and AZX series pumps. If the pump is beyond economic repair, our electronic replacement units are a direct swap and eliminate the points maintenance requirement entirely.
6. Hard Starting from Cold
Symptoms: The engine cranks but takes many attempts to fire, particularly on cold mornings. Once running it clears quickly.
Cause: Cold starting on an SU carburettor relies on the choke (strangler) providing a temporarily rich mixture and on the carburettor being correctly set up. If the carburettor is lean at idle (jet set too high), the choke may not provide enough enrichment to compensate. A weak fuel pump, empty damper or sticking piston will all compound the problem.
Fix: Work through the checklist — damper oil level, needle valve condition, piston movement, mixture setting and fuel pump operation. If all of these are correct and starting is still difficult, check that the choke mechanism is operating fully and returning to the open position when released (a partially closed choke is a common cause of poor hot running after a cold start).
When to Consider a Full Carburettor Refurbishment
If your carburettor has multiple faults — worn spindle, worn needle and jet, poor piston sealing and a history of fuel leaks — individual part replacement can become a process of diminishing returns. A full SU carburettor refurbishment addresses all wear items in one go: the unit is fully stripped, inspected, cleaned, and rebuilt to OEM specification with new seals, spindles and any worn components. It's the most reliable route to a carburettor that performs as it did when new.
Need Help Diagnosing Your Carburettor?
If you're not sure which fault you're dealing with, or which parts are correct for your specific carburettor, our team is happy to help. Get in touch with your vehicle details and a description of the symptoms — we have decades of hands-on SU carburettor experience and can usually point you straight to the cause and the solution.
Browse our full range of genuine SU carburettor parts, all manufactured to the original OEM specification.