CARBURETTOR BASICS

Welcome to It's My Classic's introduction to the subject of carburettors, where engine novices can gain an insight into this deceptively complex, but vital, little device.

The carburettor (often shortened to 'carb', though Australians also use the term 'carby') is a device that sits on the side of the engine, sucking in fuel and air and mixing them in a variable ratio, ready for the engine.

The carburettor is linked to the hand throttle or accelerator pedal via a cable. As the throttle is increased, the throttle valve opens wider, increasing the volume flowing through it.

 

The carb is also (in older vehicles) linked to a choke knob. The choke valve reduces the amount of air getting in and so increases the ratio of fuel to air, making it 'richer' and easier to ignite on cold days or when the engine is cold.

Carburettors were mostly phased out in Europe by the late 1980s in favour of fuel injection (Mercedes Benz had been using mechanical fuel injection since the 1950s).

Weak Mixture or Lean Mixture

This is a term you may often hear when people talk about setting up their engine.

This is when a carburettor is feeding a fuel-air into the engine's combustion chamber that has too much air in it. The result can be backfiring and uneven acceleration as your engine searches for the fuel it needs to run evenly.

The opposite of a weak mixture is a 'rich mixture', when the engine can be overfuelled. Neither scenario is particularly good for your engine, though running ever-so-slightly lean is  preferable to over-fuelling.

Single SU carburettor parts

Single SU (HS) carburettor

Popping Back

What is popping back? This is the term used to describe what happens in a carburettor when a weak mixture (insufficient fuel in the air-fuel mix) burns slowly so that combustion is still taking place when the inlet valve opens. A portion of this still-burning mixture travels down the induction pipe, exiting through the carburettor air intake. This causes a slight bang that is commonly referred to as 'popping back'.

Persistent popping back can be dangerous as it can result in carburettor  fires where the petol is ignited, particularly when there has been excessive flooding of fuel in the carburettor.

One of the ways that carburettors can be categorised is by the direction of the air flowing through them.

  1. Up or vertical draught

  2. Down or inverted draught

  3. Horizontal or side draught 

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There are several makes of carburettors, each working in a slightly different way, including:

  • Amal

  • Holley

  • Solex

  • Stromberg

  • SU (Skinners Union)

  • Webber

  • Zenith

Down draught Solex carburettor diagram from Newnes Aut

Down-draught Solex carburettor (Copyright Newnes Automobile Repair)

It's My Classic - the free online classic car and motorcycle magazine for people with a passion for heritage vehicles.