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Difficulty: MediumFuel and Air Intake Systems

In a carbureted fuel system, what is the primary purpose of closing the choke plate during a cold engine start?

  1. A
    To increase intake airflow to create a leaner air-fuel mixture during cold cranking
  2. To restrict intake airflow and create a richer air-fuel mixture needed for cold engine ignitionAnswer
  3. C
    To lean out the incoming charge by admitting extra air into the cylinder on the compression stroke
  4. D
    To divert liquid fuel directly into the exhaust manifold before the intake stroke begins

Answer

Restricting intake airflow to create a richer air-fuel mixture needed for cold engine ignition
When an engine is cold, liquid gasoline does not vaporize easily and tends to condense on cold metal surfaces. Closing the choke plate restricts the incoming air entering the carburetor throat. The engine's downward intake strokes create a strong manifold vacuum beneath the closed choke plate, which draws a larger quantity of fuel through the carburetor discharge nozzles. This creates a fuel-rich air-fuel mixture necessary for reliable cold starting.

Step-by-Step Solution

1
Identify the physical state of fuel and air in a cold engine during startup.
Cold cylinder walls and intake passages cause fuel to condense into liquid droplets, hindering proper vaporization and ignition.
Gasoline must be vaporized to burn efficiently, so a higher proportion of fuel (a rich mixture) is required to ensure enough vapor is present for combustion.
2
Determine how the choke plate alters carburetor airflow and pressure differentials.
Closing the choke plate restricts incoming air at the top of the carburetor air horn, creating high intake manifold vacuum below the plate.
This high vacuum pulls additional liquid fuel from the main and idle fuel circuits into the reduced airstream, supplying the necessary rich air-fuel mixture.

Key Concept

Carburetor choke plate operation and cold-start air-fuel mixture enrichment
Estimated Time:45s
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Topics

Fuel and Air Intake Systems

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