Classic Car Carburettor Problems: Flooding, Hesitation, Rich Mix
Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent
A carburettor is a simple mechanical device, which is why it is so easy to blame and so easy to mis-adjust. On many classics we see, the carb is fine and the real problem is points, timing, an air leak or old fuel. A methodical approach saves money and frustration.
Check the basics before the carb
- Ignition: points gap, dwell, timing, plugs, leads, condenser and rotor arm. A weak spark feels exactly like a lean or rich carb.
- Fuel supply: is the pump delivering steadily? See SU fuel pump problems if you have an electric SU pump.
- Air leaks: manifold gaskets, worn throttle spindles, perished vacuum pipes and brake servo hoses.
- Fuel age: petrol that has sat over winter goes stale and leaves varnish in jets.
- Air filter: a blocked element makes the mixture richer.
If you are thinking about converting to electronic ignition to rule spark out long term, read points vs electronic ignition first.
Flooding
Signs: fuel dripping from the carb, a strong petrol smell, hard hot starting, and black wet plugs.
Common causes:
- Sticking or worn float needle valve, often from dirt or varnish holding it open
- Incorrect float height, letting fuel rise too high in the chamber
- Punctured float, which sinks and fills with fuel. Brass floats can be shaken to listen for fuel inside
- Fuel pump pressure too high for the needle valve, common after fitting the wrong aftermarket pump
Flooding is a fire risk, particularly with a hot exhaust manifold underneath. If fuel is dripping, do not run the engine, keep any ignition sources away and leave the bonnet open to ventilate.
Classic car needs some care?
Servicing, recommissioning and mechanical restoration for older cars across Kent.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Hesitation and flat spots
Signs: the car stumbles, coughs back through the carb or bogs down when you press the throttle, then picks up.
- SU carbs: low or missing oil in the damper, a sticking piston, a worn needle or jet, or the wrong needle profile
- Weber, Dellorto and Solex: a worn or split accelerator pump diaphragm, blocked progression holes or blocked idle jets
- Air leaks leaning the mixture off at small throttle openings
- Carburettor icing in cold damp weather, which our carburettor icing guide explains
Rich running
Signs: sooty black tailpipe and plugs, strong fuel smell, poor economy, lumpy warm idle and sometimes black smoke.
- Choke sticking on, the most common and the simplest. Check the cable returns fully
- High float level or a leaking needle valve
- Worn jets and needles after decades of use
- Mixture set too rich, often to cover an ignition or air leak problem
- Blocked air filter
Twin carbs and balancing
Many British classics run twin SUs or twin Webers. If the carbs are out of balance, one is pulling harder than the other, giving rough idle and poor pick-up. Balancing needs the linkage set correctly, mixture set on each carb and airflow matched. Our guide to carburettor tuning basics goes through the order to do it in.
Modern fuel and old carbs
E10 petrol contains more ethanol, which can attack some older rubber, cork and plastic parts and some float materials. It also absorbs water if left sitting. Read classic car E10 petrol for the practical choices, and use fresh fuel after winter storage.
When to stop
Switch off and do not restart if you smell strong petrol in the cabin, see fuel dripping or hear spitting back with flames from the intake. Carburettor fires on classics can spread quickly. Keep a suitable extinguisher in the car.
How we can help
We work on classic cars at the customer's home and in our workshop near Aylesham. We start with ignition and fuel supply, check for air leaks, then strip, clean and set up the carbs, including float height, jets, needles, accelerator pumps, damper oil and twin carb balance. We will tell you honestly whether a carb needs a rebuild kit, new parts or just setting up. Labour is £30 to £50 per hour depending on the job, and we quote before starting.
Send your car details and postcode on WhatsApp 07563 350335 or use our contact page.
Classic car needs some care?
Message us on WhatsApp with your reg and postcode, or use our contact form, for a price before any work starts.
WhatsApp for a free quote Call 07563 350335 Contact form
Labour £30 to £50 per hour depending on the job. Send your reg number and we will reply with a price before any work starts.
Frequently asked questions
Why does my classic car carburettor keep flooding?
The usual causes are a sticking or worn float needle valve, an incorrect float height, a punctured float or dirt in the needle seat. Fuel pump pressure that is too high can also overwhelm the needle.
What causes hesitation on a carburettor engine?
A weak mixture is the most common cause, from air leaks, a worn accelerator pump, low SU damper oil or a blocked jet. Ignition faults often feel the same, so timing and points should be checked too.
How do I know if my carburettor is running rich?
Signs include black sooty plugs and tailpipe, a strong fuel smell, poor fuel economy and lumpy running when warm. A sticking choke is a common and simple cause.
Does E10 petrol affect classic carburettors?
It can. E10 can attack some older rubber and seal materials and may affect float and seal parts, so it is worth checking the materials in your fuel system and using the right fuel for your car.
