Carburettors

The carburettor is the bit of the motorbike which mixes the fuel with air. The fuel comes the tank, through a fuel tap which lets you turn the fuel on or off, then into the carburettor. Normally, we leave it up to gravity to get the fuel from the tank into the carburettor - we don't need to pump it. This wouldn't work in an aeroplane which has to fly upside down, but bikes spend most of their life right-way-up so it works fine. The air is sucked in from the outside world, and passes through an air cleaner before reaching the carburettor. There's a few different types of air cleaner, but their purpose is the same. They remove grit, dust and other undesirable gunk from the air. You don't want little bits of grit flying into your engine where they'd cause havoc.

The carburettor has its own little reserve of fuel called the float chamber. This work in the same way as a toilet cistern. When the fuel level drops, a value is opened which lets more fuel in. When the fuel level is high enough, the valve closes. You can get an idea of how much fuel is stored in the float chamber by turning the fuel tap off while the bike is still running. It'll keep running for a minute or so.

The main purpose of the carburettor is to 'spray' the fuel into the air stream which passes through the carburettor. This creates a very fine mist, so that each bit of fuel is surrounded by lots of air which it can react with once it reaches the cylinder. It's no good having big blobs of fuel, because only the fuel on the surface will be exposed to oxygen.

Let's look at how the carburettor sprays the fuel into the air. There's a fairly wide passage runs through the carburettor which is where the air passes through. We make a little hole in the side of this passage, and connect it to the float chamber. The idea is that the passing air will suck the fuel out of the hole, and it'll then mix with the air.

We can make this work much better if we make the air passage narrow before it reaches the hole, and then widen afterwards. This creates a pressure difference (the Venturi effect) which acts to suck the fuel out into the air stream.

Clearly, the size of the hole is going to have a big effect on the resulting fuel/air mixture, so we use a carburettor jet. A jet is like a screw with a hole running down the middle of it. You screw the jet into the carburettor body, and the fuel goes up through the hole in the middle. You can buy jets with different sizes of hole if you want to change your fuel/air mixture.

There's a problem with this description though. There's not yet any way for the rider of the bike to control the fuel/air mixture. That's going to be a problem, because the rider isn't going to be happy with an engine which only runs at one speed. We can introduce speed control by arranging for a metal needle to sit above the jet. We can slow the engine down by moving the needle down, so that it partially blocks the hole in the jet. This throttles the fuel supply, and so it's known as the throttle control. The right-hand grip on a motorbike simply controls the height of the needle. When you're at full throttle, the needle is totally out of the jet. When you're going very slowly, the needle will be blocking the jet more.

So, that's the a pretty simple carburettor. The people who first built these things pretty quickly realized that they weren't perfect. For a start, they didn't work very well if the engine was running slowly. If the engine is running slowly, the piston is sucking air in at a relatively slow rate. If the air is moving past our jet very slowly, it doesn't suck out much fuel so we end up with an overly lean mixture. We can fix this problem by putting a second, larger, hole in the carburettor called the pilot jet. This is only operational when the engine is running slowly. It's larger size ensures that enough fuel is mixed in, even at slow speeds. At higher speeds, it's not used.

So, what can go wrong with carburettors? Since they've got lots of very small fuel passages, they can get blocked quite easily. That's why you have a fuel filter (usually in the fuel tap). That's also why it's a bad idea to run the bike with very little fuel. Chances are, you've got little bits of sediment in your fuel tank which have sunk to the bottom. If they get into the carburettor, everything is going to stop. You'll also get problems if the fuel leaves behind any residue inside the carburettor. This gums up the insides of the carb, and you need pretty noxious cleaning fluids to get rid of it.

If it's a cold day then starting the bike can be difficult. If the walls of the carburettor and cylinder inlets are cold, then the fuel will condense on the walls. The same thing happens if you breath out on a cold window - the water vapour in your breath condenses on the cold surface. If your fuel is all condensing before it reaches the cylinder, the mixture will be too lean to burn. You can remedy this by providing some means of making the mixture much richer when you're starting the bike. This is what the choke lever controls. You can also see why it's bad to run the bike with the choke lever on, since you'll end up with an overly rich mixture. Also, if you put the choke on when you're starting the bike on a warm day then you'll get an overly rich mixture too, which will stop the bike starting. Taken to extremes, the fuel will start condensing in the cylinder and stop your spark plugs from doing their job.

A final carburettor problem is called carb icing. This is usually only a problem when it's quite cold outside and there is high humidity (lots of water vapour in the air). When any liquid evaporates and turns into vapour, it takes in heat from it's surroundings. That's why you feel colder if you get wet - the evaporating water is sucking all your heat away. The same thing happens in the carburettor when the volatile fuel turns into fuel vapour. This makes the surfaces of the carburettor colder - by as much as 20 degrees celcius. This isn't normally a problem, but if the carburettor is already cold then the cooling effect of the vapourizing fuel can make the carburretor body drop below freezing. If this happens on a humid day, then as we suck in air we will also be getting lots of water vapour. The water vapour can condense on the cold interior surfaces of the carb, and ultimately freeze. The ice can block the jets and stop the carb from working.