The Most Common Faults — and What Causes Them
SU carburettors are robust, long-lived units that will run for decades with minimal attention. But when something does go wrong, the symptoms can be confusing. This guide covers the faults most commonly encountered on HS and HIF series carburettors — what causes them, how to diagnose them, and what to do about it.
1. Rich Running (Black Smoke, Sooty Plugs, High Fuel Consumption)
Symptoms
Black smoke from the exhaust, sooty or oil-fouled spark plugs, excessive fuel consumption, rough idle, strong petrol smell.
Causes and Fixes
- Jet not centred: The jet must sit centrally in the jet bearing for the needle to pass through without friction. A miscentred jet restricts needle movement and causes rich running. Re-centre the jet following the procedure in the workshop manual.
- Worn jet or needle: A worn jet bore or a needle with a flat spot from prolonged contact will allow excess fuel through. Replace both as a pair — they are a matched wear item.
- Float chamber flooding: If the float is punctured or the needle valve is worn or stuck open, the float chamber overfills and floods the engine. Check float buoyancy and replace the needle and seat.
- Mixture set too rich: If the jet has been lowered too far during adjustment, the mixture will be permanently rich. Raise the jet (clockwise on the adjustment screw) and reset.
- Damper oil missing or wrong grade: The dashpot damper slows piston lift to provide an acceleration enrichment effect. Without oil, the piston lifts too quickly and the mixture goes momentarily rich then lean. Top up with SAE 20 or dedicated SU damper oil.
2. Weak Running (Hesitation, Popping, Flat Spots)
Symptoms
Hesitation on acceleration, popping or spitting through the carburettor, flat spots in the power band, difficult warm starting.
Causes and Fixes
- Air leak at the manifold: A leaking gasket or cracked manifold allows unmetered air into the engine. Check with carburettor cleaner sprayed around the manifold face — a change in idle speed indicates a leak. Replace the manifold gasket.
- Mixture set too weak: If the jet is raised too high, the mixture will be permanently lean. Lower the jet (anti-clockwise on the adjustment nut) by half a turn at a time and re-check.
- Needle in wrong groove: The needle position in the piston affects mixture throughout the rev range. Check the needle is in the correct groove for your application — standard is usually the middle groove.
- Blocked jet: Varnish or debris in the jet restricts fuel flow. Remove and clean the jet in carburettor solvent or replace it.
- Weak or broken piston spring: The spring controls piston return and affects mixture at idle. A weak spring causes a weak, erratic idle. Replace the spring.
3. Erratic or High Idle
Symptoms
Idle speed too high, idle speed that hunts or fluctuates, engine will not settle at a consistent speed.
Causes and Fixes
- Throttle spindle wear: On high-mileage carburettors, the throttle spindle bore wears oval, allowing air to bypass the throttle plate. This air is uncontrolled and causes a permanently high, erratic idle that cannot be adjusted away. The fix is to replace the carburettor body or have it professionally resleeved.
- Idle speed screw set too high: Check the throttle stop screw is not holding the throttle plate open further than necessary. Back it off until the idle drops, then re-set to the correct speed.
- Sticking throttle cable: A stiff or routed throttle cable can hold the throttle open. Check the cable runs freely and is not kinked.
- Air leak: As above — air leaks cause high, uncontrollable idle. Check all vacuum connections and the manifold gasket.
4. Fuel Leak from Float Chamber
Symptoms
Petrol dripping from the float chamber, petrol smell, fuel pooling under the car when parked.
Causes and Fixes
- Worn needle valve and seat: The most common cause of float chamber flooding. The needle valve controls fuel entry into the float chamber. When worn, it no longer seals properly and allows continuous fuel entry. Replace the needle and seat as a pair.
- Punctured float: A float that has developed a pinhole will absorb fuel and sink, holding the needle valve open. Shake the float — if you can hear fuel inside, replace it.
- Float chamber gasket or O-ring: A cracked or hardened gasket will allow fuel to seep from the joint. Replace the gasket when rebuilding.
5. Piston Sticking or Moving Sluggishly
Symptoms
Flat spot immediately off idle, poor throttle response, engine stumbles when accelerator is applied quickly.
Causes and Fixes
- Dirt or varnish in the suction chamber: The piston must move completely freely. Any contamination on the piston or chamber wall will cause sticking. Remove the suction chamber and clean thoroughly.
- Bent or damaged needle: A bent needle will catch on the jet and restrict piston movement. Inspect the needle carefully and replace if not perfectly straight.
- Insufficient damper oil: As noted above, missing damper oil affects piston behaviour. Check and top up.
- Suction chamber screws over-tightened: If the suction chamber screws are overtightened, they can distort the chamber and cause the piston to bind. Re-torque evenly to the correct specification.
6. Carburettor Leaking Oil into Air Filter
Symptoms
Oil present in the air filter or intake, oily residue around the carburettor top.
Causes and Fixes
- Damper O-ring worn or missing: The damper O-ring seals the dashpot cap against the suction chamber. A worn O-ring allows damper oil to be drawn into the intake. Replace the O-ring — it is included in most Major Repair Kits.
- Too much damper oil: Overfilling the dashpot will cause oil to be expelled. Fill to the correct level — typically 5-6mm below the top of the hollow piston rod.
Need Parts to Fix Your Carburettor?
Most of these faults are resolved with genuine Burlen-manufactured service parts. Browse our range of Major Repair Kits, needles and jets, and individual components. Not sure what you need? Contact us — we are happy to help diagnose the fault and recommend the right parts.