Cars used to come with carburetors as the fuel delivery system before fuel injection technology had been developed. To carburate means to mix air and fuel to the proper air/fuel blend in order to create combustion within a motor. In order to achieve this mixture there are a couple of basic systems within a carb that are necessary to know.
1. Float Level
The Carb collects fuel in the bowl area once fuel has been delivered to it. Every carb has one or more bowls attached. The importance of the float level is to ensure that your engine is provided with enough fuel but also to make sure that it does not receive to much. Fuel flows into the bowl until it reaches the proper float level and then gets cut off by what's called the needle and seat. It prevents any more fuel from entering the bowl area. Without this device restricting the amount of flow the fuel would just dump into the motor causing it to run rich or stall the motor. Also, without enough fuel within the bowl the carb does not have enough fuel to deliver to the motor when necessary which will cause it to run lean or stall as well. Once the float level has been set correctly it usually won't need to be adjusted again.
2. Idle Circuits
In order for a car to run properly the idle circuits need adjusted first. A set screw on the body of the carb allows you to adjust the flow of fuel being drawn through the idle circuit into the motor by the vacuum of the motor. The picture shows you a typical Holley carb set screw. This particular model has 4 of them. Same as the float level, too much fuel and it will run rich, and with too little it will run lean. This adjustment is important to prevent the motor from stalling as well as maintain idle quality and off idle response.
3. Accelerator Pump
Unlike the idle circuit the accelerator pump is a little diaphragm which stores fuel to pump extra fuel through the carb when the throttle is opened to help compensate for the increase of air flow. This also helps with the off idle response especially when quickly going from idle to wide open throttle. Again the idea is to provide enough fuel so the motor will not stumble but also not to much to cause it to run rich.
4. Main Jets
The main jets are what keep the motor running at that precise air/fuel ratio from idle to wide open throttle throughout the entire RPM range, ensuring it will always be getting the correct amount of fuel. Fuel gets drawn by the airflow through the venturis out of the bowls and through the main jets. The bigger the jet the more fuel and vise versa.
5. Power Valve
The Power valve circuit is a vacuum sensing system which is activated once the vacuum in the motor drops to a certain value. This generally happens when the engine is under load causing a need for more fuel. When opened the valve allows extra fuel to run through the main metering circuit, and once again, too big a valve will make it run rich and too small a valve will cause it to run lean.