Lesson 1/learning Event 2
When the throttle is closed, the accelerator piston is in the up position. As the throttle is opened, the piston
moves downward, pressurizing the fuel in the pump cylinder. The pump intake check valve closes, and the
fuel in the pump well is forced out through the pump discharge valve and the pump nozzle in the carburetor
If the pump piston is moved down suddenly, the fuel under it cannot be immediately discharged through the
small discharge nozzle. For this reason, the operating linkage does not push directly on the piston. Instead,
the linkage compresses the pump spring, and the spring pressure moves the piston down. This feature slows
down the action of the pump and allows the fuel to flow out through the pump discharge nozzle for several
seconds. The discharge tapers off as the movement of fuel in the main metering circuit increases. The
accelerating pump circuit refills with fuel when the throttle closes. During the upward movement of the
piston, the pump intake check valve opens, permitting fuel from the carburetor bowl to enter the pump
Other accelerating pump circuits operate differently. One type makes use of manifold vacuum and spring
pressure to operate the pump. Another type uses a pocket of air between the piston and fuel to eliminate the
need for an accelerating pump spring.
It is a well-known fact that a richer mixture is needed when an engine is cold. This is because all the fuel
supplied to the cylinders will not vaporize when cold. Therefore, a larger amount of fuel must be supplied to
get enough vaporized fuel for combustion. To supply the extra fuel, a choke circuit is used.
The choke is a round, flat disc (similar to the throttle valve) in the carburetor air horn above the venturi.
Closing the choke valve creates a high vacuum in the carburetor throat. This causes large amounts of fuel to
flow freely from both the low- and high-speed circuits. Chokes may be operated manually by the driver, or
they may operate automatically.
To operate a manual choke, the driver usually pulls a cable that closes the choke valve. Closing the valve
completely shuts off the supply of air. However, gasoline must have some air in order to burn. The needed
air is admitted by one of two methods.