hi guys, i took this from another forum. have a read...
Below explains the function of carburettors ... find out what pilot
jets are! 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.
hope this helps! naturally all this won't apply to new big bikes
with funky electronic fuel injection (EFI). 
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rock hard... ride free...
