A technician is troubleshooting a float-type carburetor and needs to explain to an apprentice why fuel flow increases as engine speed increases. Which principle best explains this?
- AAs airflow through the venturi increases, pressure at the venturi throat decreases, drawing more fuel from the discharge nozzle due to the greater pressure differential
- BHigher engine speed increases fuel pump RPM proportionally, forcing more fuel directly into the venturi
- CThe float rises with engine speed, opening the needle valve further to allow more fuel flow
- DIncreased manifold pressure pushes fuel backward through the discharge nozzle into the venturi
Show answer & explanationAnswer & explanation
Correct answer: A. As airflow through the venturi increases, pressure at the venturi throat decreases, drawing more fuel from the discharge nozzle due to the greater pressure differential
In a float carburetor, air velocity through the venturi increases with engine speed, which lowers pressure at the venturi throat according to Bernoulli's principle. This creates a greater pressure differential between the float chamber (near atmospheric) and the venturi throat, drawing progressively more fuel out of the discharge nozzle.
Why the other options are wrong
- B. The engine-driven fuel pump maintains pressure to the carburetor bowl but doesn't directly meter fuel based on venturi airflow.
- C. The float level is unrelated to engine speed; it only regulates fuel level in the bowl.
- D. Manifold pressure does not push fuel backward through the nozzle; this describes the wrong direction of flow.
Venturi Fuel Metering
A float carburetor uses the pressure drop created by airflow through a venturi to draw fuel from the discharge nozzle in proportion to engine airflow demand.
- Higher airflow = lower venturi pressure = more fuel drawn
- Based on Bernoulli's principle
- Float chamber remains near atmospheric pressure
- Fuel discharge nozzle is located at the venturi throat
Memory trick: Faster air, lower pressure, more fuel sucked up — Bernoulli's bar trick