A Carburetor Does Not "Suck" The Fuel Out Of The Float Bowl...

The Carburetor Chronicles...
If you want to start an argument at the next car club meeting, just ask someone to explain how a carburetor works. Most likely you will get the explanation of how the mechanical fuel pump fills up the float bowl, and the gas is "sucked" out of the float bowl and into the intake manifold where it is mixed with the air at a ratio of somewhere around fifteen parts air to one part gasoline.

Well...most of that explanation is correct except the part about the carburetor "sucking" the fuel out of the float bowl. That part is incorrect, here is what really happens. By the way, this explanation is also true of 2bbl and 4bbl carburetors. I am using the 1bbl example to make things easier for you to understand.

Before I explain this... you need to understand something called Bernoulli's Principle. Bernoulli's Principle says that the total energy of a particle in motion (like a particle of gasoline) remains constant at all points along the path that it is traveling. Therefore if the gasoline is expected to travel at a higher velocity, (such as when traveling thru the intake) the pressure around the particle of gasoline must be reduced. Less resistance, more speed. Lost already? Read on.

Suppose you have a formation of soldiers marching down the street twenty abreast. They march until they come to a narrow alley that they can only march ten abreast. Because of the narrow alley, the soldiers will have to march twice as fast as they marched in the street in order to maintain the same rate of travel as they had before.

This same explanation is what happens in automotive fuel applications as well. Keep in mind that a carburetor is a little more complicated than this... but for our discussion, we are just looking at how the air and fuel mix together.

Air is drawn into the top of the carburetor and down into the bottom of the carburetor, into an area known as the Venturi. The Venturi contains a tube that is smaller in diameter than the air intake area so a low-pressure area is created, just like when the soldiers got to the narrow alley.

We know that atmospheric pressure is present inside of the carburetor (and is all around us) which means there is atmospheric pressure inside of the float bowl.  A low-pressure area is created inside of the Venturi of the carburetor because the Venturi tube is smaller in diameter than the carburetor air intake.

The outlet of the float bowl is located inside the Venturi so what's going to happen to the fuel inside the float bowl as a result of the low pressure created by the Venturi?

The fuel is going to be pushed out of the float bowl by the atmospheric pressure into the Venturi of the carburetor, where it will be mixed with the outside air to form the fuel mixture. It is that fuel mixture that then travels down inside of the intake into the cylinders. The intake manifold is a big opening and a high-pressure area so the difference in pressure is what helps draw the air/fuel mixture thru the carburetor and into the intake manifold.

So the fuel in the float bowl is being pushed out of the float bowl by atmospheric pressure. It is not being "sucked" out of the float bowl as many car owners believe. 

This Carburetor needs an insulator and a heat riser

And Another Thing...
If the incoming fuel has a high humidity rate (water vapor often from cheap gas) and the outside temperature is cold enough the cooling effect will cool the carburetor base temperature to below 32 degrees Fahrenheit, which will cause the base of the carburetor to frost over. Most of us have experienced that first hand.

Now you know what that ceramic insulator spacer (and the heat riser) is for, that mounts under the base of the carburetor...to prevent the base of the carburetor from freezing up which will cause the air/fuel mixture to stall, resulting in poor engine performance.

The density of the air in the air/fuel mixture will also affect engine performance. If you remember back in your younger days...how much better your car seemed to run on the way home at midnight from your girlfriend's house... than it did when you drove to her house at five o'clock. Chances are the air was much denser at midnight (and cooler) so there were more molecules available to mix with the gasoline. Your car really did have more power... it wasn't just the thoughts of your cute girlfriend that made the trip home seem to go faster.

So now you know how a carburetor really works, why the base of your carburetor sometimes frosts over, and most important of all you understand Bernoulli's Principle, which you should have already known and understood if you paid any attention in physics class. But if you were like me a Hot Rod Magazine was a better textbook than any physics book.

All that doesn't matter now because you learned what you needed to know here, and you learned how you can apply what you learned to something auto-related. They never did that in physics class!!