carburetor Tune Up Guide

previous page: The Carburetor and Its Purpose

CARBURETOR CONSTRUCTION AND CIRCUITS

Since all late model cars and light trucks are using downdraft carburetors, our text will apply only to them. However, the principles of carburetion do not change. The only difference between the terms updraft and downdraft, is the direction in which air enters the carburetor throat or air horn. It either enters from the bottom and travels up into the engine, or from the top down into it. In as much as updraft carburetors are in the minority as far as equipment on present day cars and trucks is concerned, it is of little value to discuss them in this book. All downdraft carburetors regardless of size and shape operate in the same general manner. This being the case, we can break their construction into five separate circuits for easier discussion and understanding. The circuits are as follows:

1. Float
2. Low Speed
3. High Speed (and power on Zenith and Stromberg)
4. Pump
5. Choke

Since we have listed the circuits in sequence, let us discuss them in this manner. In the part of the carburetor known as the body is located the float bowl or chamber. This chamber is used for the storage of a certain quantity of gasoline. It serves two purposes, namely, to keep all the other circuits of the carburetor supplied with the amount of fuel they need and to absorb the pulsation of the fuel pump, as it delivers the gasoline to the carburetor. Though its construction is simple, it plays a very important part in the proper functioning of the engine. The float system consists of the following: float chamber or bowl, fuel inlet, needle valve and seat, float, float pin and on some carburetors a float pin retainer, and the float chamber or bowl cover which contains the float chamber vent.

next: Float Circuit