Fuel for idle operation is taken from a lower or “isolated” section of the well after passing through the main jet (7). The idle fuel passage (F) connected to this well is restricted by a small drilled hole near the bottom of the passage. The fuel from the lower well travels up this channel and through the idle jet (9) calibration to be mixed with air originating from around the sides (2) of the main venturi and flowing through the air inlet port (G). The amount of air admitted through the air inlet port for idle operation is controlled by the idle adjusting needle (10). The idle system functions only at idle and low speeds, or when the throttle plate (1) is almost closed and there is a very strong suction at the edge of the throttle plate to draw the mixture of fuel and air from the idle jet and idle adjusting needle to be discharged into the air stream through the priming plug (11). Turning the idle adjusting needle IN (clockwise) increases the suction on the idle jet for a richer mixture. Turning the needle OUT (counter-clockwise) increases the amount of air admitted and thereby reduces the suction on the idle jet resulting in a leaner mixture.