All About the Carburetor - Part 2

The carburetor under high engine load

Higher engine loads demand more fuel. The carburetor handles this by increasing the amount of fuel through the power valve controlled by the intake manifold vacuum. Manifold vacuum travels from the base of the carburetor to the power valve through a passage. The manifold vacuum under normal running conditions holds the valve closed because the vacuum is at its greatest. As the engine load is increased the vacuum drops and the power valve starts to open. The valve will be completely opened when the engine is under a heavy load resulting in very low vacuum.

The carburetor during engine acceleration

When an engine initially accelerates the balance of air and fuel is thrown off balance because fuel is heavier than air. This results in more air than fuel, an overly lean mixture. To solve this, the accelerator pump is used to supply more fuel to the engine. This maintains the balance until the fuel air ratio reaches proper levels. The accelerator pump is operated by a linkage and does not rely on vacuum to operate.

The carburetor under cold engine starting conditions

Cold engine starting uses more fuel because the fuel is not fully vaporized due to less air in the carburetor. To increase the amount of fuel during starting the choke plate is used. When the choke plate is closed the vacuum in the carburetor increases and pulls more fuel from the fuel nozzle and both the idle ports. The choke is usually thermostatically controlled but can be controlled manually. Once the engine gets started the amount of extra gas for starting needs to reduced. This is done by using the vacuum in the intake manifold, which pulls the choke partially opened to stop too much gas from flowing in the engine and allowing more air flow while the engine starts. As the engine warms up the thermostat control of the carburetor opens the plate until the engine reaches normal temperature. At normal running engine temperature, the plate will be fully open.