Full Throttle Operation

At full throttle, vacuum at power fuel tube is so great that an additional amount of fuel is drawn from float bowl through fuel passage. This extra amount of fuel supplies a rich enough mixture to allow engine to reach maximum power output. Vacuum at lower engine speeds is not sufficient enough to draw fuel from power fuel tube.

Fig 1: Full Throttle Operation

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.

Fig 2: Automatic Choke System

Courtesy of VOLKSWAGEN GROUP OF AMERICA, INC.