Carburetors or carbs were once commonly used on almost all car engines. Carbs come in all different types. The purpose of all carbs is simply to control the fuel going into the engine. The downdraft or draw through throttle plate carb was most widely used on passenger cars. These carbs usually consist of a low speed circuit used for low rpm or idle. Sometimes on the slightly more advanced carbs there are mid range circuits. The accelerator pump circuit assists in acceleration. The main jets are used the most and are very important to full throttle tuning. All production vehicle carbs (quadrajet, carter)usually have a choke circuit for warm up assist. Let’s start with the carburetor’s choke system. The choke operates usually by restricting the air into the engine. Along with changing the fuel to air ratio the manifold vacuum increases significantly, pulling even more fuel from the low speed circuit. The carb needs to run rich (alot of fuel) when the car first starts up because the engine isn’t warm enough to help vaporize the fuel. The carb uses idle, low speed or pilot jets (which are the same things) and air bleed screws to control the amount of fuel being sucked in at idle. The air bleed screws lean out the mixture as they are rotated out. Idle tends to be slightly richer than stoichiometric (perfect fuel to air ratio) of 14.7 to 1.