The secondary high speed circuit is different from the primary high speed circuit, although the construction is quite similar. The secondary high-speed circuit is composed of the secondary main jet (3), main air bleed (1), small venturi (6), and main nozzle (5). The venturi is of a double type, the same as the primary side. When the engine requires power, the accelerator pedal is depressed, and the primary throttle valve is opened more than 5° (this is called the “secondary-touch” angle of the primary throttle valve). At this point, the secondary throttle valve is forced open about 2°.
In addition, the diaphragm (7), actuated by the vacuum in the primary and secondary venturis, opens and closes the secondary throttle valve.
When the primary throttle valve opening angle exceeds 5° (“secondary-touch”) the secondary throttle valve is forced slightly open by the link mechanism. This prevents delay of opening that would result from the throttle valve being sunk into the bore or impeded by frictional resistance with the bore.
At low engine revolution, the vacuum does not sufficiently actuate the diaphragm to operate the secondary throttle valve. Therefore, the fuel mixture required by the engine is supplied only by the primary side. For this reason, the air velocity in the primary venturi is increased, properly atomizing the fuel resulting in even distribution to each of the cylinders without reduction of engine torque.
As the engine revolution gradually increases, the intake frictional resistance increases only in the primary circuit. This results in the venturi vacuum becoming stronger, and gradually opening the secondary throttle valve, causing the secondary side of the fuel supply to start its function. In other words, the secondary throttle valve opens, not to obstruct the engine intake efficiency when engine speed increases, but to function together with the primary and secondary high speed circuits. Also, when the secondary throttle valve opening angle is small, the volume passing through the secondary venturi is small. Since sufficient vacuum does not exist to discharge the fuel from the secondary main nozzle, the low speed circuit is provided in the secondary side for compensation.