Low speed circuits are provided in both primary and secondary systems, the former supplying the fuel at light load and slow speed. and the latter at heavy load and slow speed. In other words, when the secondary throttle valve is opened up by the “Kick-up” device, the actuating vacuum on the secondary side main nozzle, being slight, draws monstly air into the secondary side low speed circuit, resulting in a lean fuel mixture.
To prevent this, the low speed circuit is provided in the secondary side to obtain a correct fuel mixture. The construction of the secondary system is more simple than the primary side, not having the air bleeder (1), economizer jet and idle port. The gas which is regulated by the secondary main jet (3) is further minimized by the slow jet (2), and mixes with the air entering from the air bleeder (1), then is drawn into the manifold from the slow port (4) located near the throttle valve.