Your carburetors are basically responsible for mixing
gas with air at the right precise ratio. This ratio varies as you
"lay on the gas". When the pistons in the engine go down, the
intake valves open up (in 4-cycle engines) and allow the piston to suck
air through the carbs from the air filter or filter box. As the air rushes
through the throat (or Venturi) of the carb, it creates a vacuum that
sucks gas up through some small nozzles and atomizes it to be pulled into
the engine and burned. The amount of vacuum created is primarily
controlled by the throttle, officially called the butterfly valve.
Next time you have a straw in a glass of something, try blowing fast
over the top of the straw while it’s in the glass. What happens?
Depending on the size of the straw and the force of your blow, liquid is
sucked up the straw, just like a carb. The smaller the straw, the higher
you can draw up the liquid. The carbs are kept full with fuel by the floats
and the needle valve. It works like your toilet – after you flush, the
float falls down, letting more water into the toilet until it reaches a
specific line when the valve turns the water off. The main difference is
that your carbs aren’t supposed to let that liquid level drop as much as
your toilet does. Otherwise, they wouldn’t be able to effectively suck
up the gas.
Carbs vary a bit by how they are designed...some have two jets (most older
and more basic carbs) and some have 3 (see the picture on the left).
The carb on the left has a pilot jet, a needle jet, and a
main jet for a total of 3 separate fuel metering systems. To see a
picture of a carb with just 2 jets, check out Project
CB750's carburetor page.
Bottom View with the float bowl off