All About Carburation

Last month we covered ignition systems, but there is no point in a spark unless you’ve got something to light. So this month we’re covering the fuel side of things.

To get a nice big bang out of an engine, you want fuel and air to be really well mixed up. You get a far better explosion when the fuel is suspended in the air (oxygen) than if it’s sitting in a puddle at the bottom as you get more surface area on the fuel - the really tiny units of fuel are surrounded by oxygen-rich air. As petroleum comes in liquid form, we need to produce a fine mist of that fuel and mix it with the air that process is called carburation.

Starting with the most basic part of its operation, the SU carburettor (fitted to pre-injection Minis as standard) has a small hole called a ‘jet’. In this hole there is a conical pin called the ‘needle’. As the air flows past this arrangement, fuel is sucked out of the jet and past the pin in a fine mist.

As more fuel is required, the needle is pulled further out of the jet; allowing more fuel past. Rather than just linking the needle to the throttle pedal, the SU carburettor uses the air flow caused by the engine to lift a piston, attached to the needle. As the needle lifts more fuel is allowed to enter the engine, corresponding to the amount of air that is being pulled in. This piston movement is damped by a smaller piston in oil to reduce the chance of the piston moving too fast and an excess of fuel entering the engine and causing flooding. This is the dashpot oil which is why the consistency of this oil is important.

THE RIGHT NEEDLE