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CarburetorThe carburetor controls how much air and fuel go into the motor. Its control lies in its ability to mix and atomize the fuel before it enters the crankcase. Carbs only control what they've been given permission to control- by the tuner. With that permission, it'll work with the conditions it's given, including altitude, temperature, humidity, barometric pressure and fuel quality.
Carbs work within the laws of physics - physics tied to pressure, negative pressure (vacuum) and speed. Here are some carb terms to know:
• The Venturi - This is the large intake hole, often referred to as the horn, that directs air into the motor.
• The Float Bowl - Located on the bottom of the carburetor, the float bowl is basically a reservoir in which a certain fuel level is maintained. The closer the fuel level is to the carb's throat, the smaller the pressure differential (known as ?P [delta P] which is the difference between the pressure inside the float bowl and atmospheric pressure). To move fuel from the float bowl into the air steam that is moving through the venturi, there must be a pressure differential between the float bowl and venturi. The pressure in the bowl must be larger than that of the venturi to push the fuel into the air stream. Float bowls are vented to the atmosphere, thus there has to be negative pressure (vacuum) inside the venturi. This negative pressure pulls the fuel up from the float bowl and into the air stream.
• Pilot Jet - The pilot jet and air screw control fuel mixing from the closed throttle position to a 1/8 throttle position. The pilot jet throttle position function overlaps to the 1/4 throttle position when another fuel circuit takes over. Pilot jet sizes come in increments of five (20, 25, 30 etc.).
• Throttle Slide - The throttle slide has many important functions. Its opening determines how much air enters the motor. The slides come in three configurations: roundslide, flatslide and D-slide. Throttle slides have a cutaway that controls the air and fuel mixture from idle to 1/3 throttle position; it overlaps to half throttle. Throttle slides are available in different sizes and cutaway patterns.
• Jet Needle - The jet needle controls the air-to-fuel mixture from 1/4 to 3/4 throttle position. Needle jets come in different sizes for different tuning needs. Each needle jet has a four digit code that identifies length and tapers for each end. The upper taper affects how the fuel and air mixes at 1/4 to 1/2 throttle position, whereas the lower taper affects fuel and oxygen mixing at 1/2 to 3/4 throttle position.
• Needle Jet - This jet, along with the previously mentioned jet needle, controls the air-to-fuel mixing between 1/4 to 3/4 throttle position. Needle jet tuning comes to play when changing E-clip position. Lowering the E-clip raises the needle, and raising the E-clip lowers the needle.
• Main Jet - Located at the bottom of the needle is the main jet. This jet controls fuel flow to the needle jet. At the 3/4 throttle position, the opening between the jet needle and needle jet becomes so great that it exceeds the main jet's size. At this point, fuel flow is wholly determined by the main jet - which is full-throttle.
Information for this report was obtained from Olav Aaen's Carb Tuning Handbook and Cutler's Performance Center Carburetor Handbook.