Carburettor Tuning Basics for Classic Cars

Written by Petro Olekhnovych, mobile mechanic in Aylesham, Kent

Carburettor tuning means setting the idle speed and the fuel-air mixture so the engine idles smoothly and pulls cleanly. It only works on an engine that is already healthy, so the ignition, valve clearances and air leaks must be checked first. Most “carb problems” turn out to be something else.

This guide explains the basics for owners of classic cars, using the common SU carburettor as the main example.

Check the engine before you touch the carburettor

A carburettor can only be set correctly on an engine in good mechanical condition. A classic car guide to SU carburettors lists the checks to do first:

• Ignition timing, and the points or electronic ignition.
• Spark plugs and the air filter.
• Valve clearances, on engines that need them adjusted.
• Manifold and carburettor flange seals.

Air leaks matter most. Any extra air getting in past a worn gasket or a cracked vacuum hose upsets the mixture, and makes correct setting impossible. A guide to vintage carburettor tuning also warns that leaking or misrouted vacuum hoses can make the idle and deceleration too lean.

The same guide says ignition timing should be set before the idle mixture. If you adjust the carburettor first, you may end up chasing a problem that is really in the ignition. If your car still has contact breaker points, see our guide to points vs electronic ignition.

Signs your mixture is too rich or too weak

The mixture is the balance between fuel and air. Burlen, who make SU carburettors, describe how the engine sounds at idle:

• Too weak (lean): an irregular note, a “splashy” misfire and colourless exhaust.
• Correct: a regular, even note.
• Too rich: a regular or rhythmical misfire with blackish exhaust.

Other signs of running rich include black smoke and high fuel consumption. Flat spots when you accelerate can point to a weak mixture, a worn needle or a low damper oil level.

A rich mixture is not always a tuning problem. A punctured float can cause flooding, and a misplaced needle can also upset the mixture. Parts like these need fixing before any adjustment will hold.

How carburettor tuning works on an SU

Here is the outline of Burlen’s procedure for a single H-type SU. Other SU models and other makes, such as Weber, Solex and Zenith, are set up differently, so always follow the instructions for your carburettor.

1. Warm the engine fully. Mixture should only be set when the engine is at normal running temperature.
2. Set the idle speed with the throttle stop screw. Do not try to raise the idle by making the mixture richer.
3. Adjust the mixture. On an H-type SU, raising the jet adjusting nut weakens the mixture and lowering it richens it. Burlen says to aim for the fastest idle that still runs evenly, then reset the idle speed.
4. Check with the lifting pin. Gently lift the piston about 1/32 inch (0.8 mm) with the lifting pin and listen.
5. Top up the damper. Burlen recommends thin SAE 20 oil, filled to just below the top of the hollow piston rod.

Make small changes and give the engine time to respond. One SU guide suggests turning the adjusting nut one flat at a time.

Reading the lifting pin test

Burlen gives three results:

• Engine speed rises considerably: the mixture is too rich.
• Engine speed rises very slightly: the mixture is correct.
• Engine speed drops immediately: the mixture is too weak.

Twin carburettors

Twin SUs must also be balanced so both draw the same amount of air at idle. This is usually done with a flow meter such as a Unisyn. Each carburettor’s mixture is then set separately, and the balance rechecked, because each setting affects the other. Twin carbs are also common on some air-cooled VW engines. See our guide to air-cooled VW servicing.

Safety and fuel

Work in the open air or a well-ventilated space, never with the engine running in a closed garage. Keep naked flames well away from petrol and the carburettor.

Fuel also matters on older cars. GOV.UK says classic, cherished and older vehicles may not be compatible with E10 petrol. Owners of vehicles not listed in the government’s checker should keep using E5 super grade (97+ octane). Read more in classic cars and E10 petrol.

When to get a mechanic involved

Basic idle and mixture setting is within reach of a careful owner with the right instructions. It is time to get help if:

• The engine will not hold a setting, or the settings keep drifting.
• You smell strong petrol or see fuel leaking. Stop and do not run the engine until it is checked.
• The car runs badly even with the mixture set as above.
• The carburettor needs dismantling, new jets, needles or floats.

These usually point to wear, leaks or an ignition fault rather than a simple adjustment.

How we can help

We are a mobile mechanic based in Aylesham, Kent, and we service and repair classic and vintage cars. We can check your ignition, look for air leaks and set the carburettor at your home or garage. If the car is not running right, our fault finding will find the cause and we will tell you what it needs before any work starts.

We cover Aylesham, Canterbury, Dover, Deal, Walmer, Sandwich, Ash, Eastry, Folkestone, Wingham, Littlebourne, Bridge, Barham, Shepherdswell, Eythorne, Elvington, Nonington, Adisham, Whitfield and Sturry. Labour is £30-£50 per hour depending on the job. Parts are extra, and the price is agreed on WhatsApp first.

Has your classic been off the road for a while? Read about classic car recommissioning before you start it.

Frequently asked questions

How do I know if my carburettor needs tuning?
Uneven idle, black smoke, high fuel use or flat spots on acceleration are common signs. Check the ignition and look for air leaks first, as these cause the same symptoms.

Should I tune the carburettor with the engine cold?
No. Set the mixture only when the engine is fully warm.

What oil goes in an SU damper?
Burlen recommends thin SAE 20 oil, filled to just below the top of the hollow piston rod.

Can I raise the idle speed by richening the mixture?
No. Set the idle with the throttle stop screw and use the mixture adjustment only for the mixture.

Need help? Send a WhatsApp to 07563 350335 with your car’s make and model and we’ll come to you.

Sources

• Burlen: H Type Carburetter Tuning - Single
• Classic Car Hub: Setting up and tuning SU carburettors
• Motales: Tuning carburetors on vintage cars
• GOV.UK: E10 petrol explained