Carburetors are important for controlling the speed of engines. They are responsible for mixing and metering the air/fuel mixture at an ideal ratio for different throttle positions, engine loads, or RPMs. To tune the carburetor, you must understand the pressure changes that impact air and fuel flow in the intake tract. This article is part of a three-part series on carburetion, which will cover throttle position, RPMs, tuning tricks, and alternative fuel systems like electronic fuel injection.
During the intake cycle, the engine piston moves towards top dead center (TDC), creating a vacuum in the crankcase. Atmospheric pressure air behind the reed valve flows through the intake manifold, carburetor, flame arrestor, and engine compartment to equalize the pressure. A certain volume of air is ingested by the engine at a given power output, measured in cubic feet per minute (CFM). As this air travels through the intake tract, it may speed up or slow down to keep pace with the rest of the column. When the air passes through the carburetor venturi, it speeds up, causing a drop in pressure, known as the Bernoulli Principle.
Carburetors use pressure differentials to move fuel from the fuel tank to the fuel pump, regulator chamber, fuel circuits, and carb bore. The fuel pump, powered by a hose vented into the crankcase, moves the diaphragm back and forth to draw fuel through the one-way check valve. The fuel in the chamber is pushed against the check valve and closes it. Fuel then passes through the needle and seat, which requires overcoming the spring pressure on the needle to open. The low-speed outlet, bypass holes, and high-speed outlet all receive fuel from the fuel chamber.
The low-speed outlet operates when the throttle plate is almost closed, exposing it to the low pressure in the intake manifold. Fuel in the fuel chamber is at atmospheric pressure, so it flows through the low-speed jet, around the mixture screw, and out the outlet. In the next article, we'll discuss tuning tricks for adjusting the pop-off pressure of the needle valve, and explore the bypass holes and high-speed outlet for full throttle. The final article will explore alternative fuel systems such as electronic fuel injection and heavily modified carbs.
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