Post by DOCTOR FIELDS on Sept 11, 2010 14:34:06 GMT -5
Carburetors seem to be the most misunderstood component of the fuel system, many just go by "hear say" when it comes to tuning. A properly tuned carb. can make or break (literally) a good race engine.
This will be the first of several tech articles i'll write up as time allows. I don’t claim to know it all, just trying to help clear up a few myths and misconceptions people have about carburetors.
In order to properly tune a carb. you must first fully understand how they work.
In liquid form, gasoline does not burn, therefore it needs to be vaporized. The main function of a carb. is to start the process of turning gasoline from a liquid to a vapor. Fuel is pumped into the fuel bowl through a needle and seat assembly. The fuel is then drawn through main jets by a pressure differential created by a venturi effect. A venturi is hourglass shaped, which increases air speed (velocity) at the smallest section, creating a low pressure area. It is this reduction in air pressure that enables fuel to be PUSHED, from the fuel bowl into the intake manifold. You may have been taught a carburetor "sucks" fuel out of the bowl, but in technical terms, atmospheric pressure (14.7 psi) pushes down on the fuel in the bowl through a vent. As airflow increases through the venturi, the effect causes the pressure to drop around 1 psi (13.5 psi), allowing the "heavier" atmospheric pressure to push the fuel out of the bowl. Boosters located in the venturis, act like a venturi within a venturi, further increasing the pressure drop (carb. signal), without significantly compromising airflow.
After the fuel is pushed from the bowl through the main jets, it goes through emulsion tubes / jets. Without emulsification, fuel droplets would be too large for atomization. These tubes or "jets" as in some higher end performance carbs, preatomize the fuel by mixing it with air channeled in through air bleeds. This effect is similar to drinking through a straw with holes in the sides. This process ensures thorough atomization and therefore complete vaporization by the time it reaches the intake valves, yielding thorough combustion and increased power.
That’s the basics on how a carburetor works. In part 2, we’ll discuss the different circuits and how each one works.
This will be the first of several tech articles i'll write up as time allows. I don’t claim to know it all, just trying to help clear up a few myths and misconceptions people have about carburetors.
In order to properly tune a carb. you must first fully understand how they work.
In liquid form, gasoline does not burn, therefore it needs to be vaporized. The main function of a carb. is to start the process of turning gasoline from a liquid to a vapor. Fuel is pumped into the fuel bowl through a needle and seat assembly. The fuel is then drawn through main jets by a pressure differential created by a venturi effect. A venturi is hourglass shaped, which increases air speed (velocity) at the smallest section, creating a low pressure area. It is this reduction in air pressure that enables fuel to be PUSHED, from the fuel bowl into the intake manifold. You may have been taught a carburetor "sucks" fuel out of the bowl, but in technical terms, atmospheric pressure (14.7 psi) pushes down on the fuel in the bowl through a vent. As airflow increases through the venturi, the effect causes the pressure to drop around 1 psi (13.5 psi), allowing the "heavier" atmospheric pressure to push the fuel out of the bowl. Boosters located in the venturis, act like a venturi within a venturi, further increasing the pressure drop (carb. signal), without significantly compromising airflow.
After the fuel is pushed from the bowl through the main jets, it goes through emulsion tubes / jets. Without emulsification, fuel droplets would be too large for atomization. These tubes or "jets" as in some higher end performance carbs, preatomize the fuel by mixing it with air channeled in through air bleeds. This effect is similar to drinking through a straw with holes in the sides. This process ensures thorough atomization and therefore complete vaporization by the time it reaches the intake valves, yielding thorough combustion and increased power.
That’s the basics on how a carburetor works. In part 2, we’ll discuss the different circuits and how each one works.
