Basic Explanation
  In-Depth Explanation


Carbs / Fuel System


Basic Explanation

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



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Last Update: Wednesday, December 14, 2005 11:53 PM