Fuel uder pressure is supplied to the fuel bowl through the fuel inlet fitting, fuel valve (needle and seat) to the float chamber. The floats automatically control the opening of the fuel valve to maintain the proper level of fuel in the fuel bowl under all conditions of engine speed and load. With the throttle closed, the manifold vacuum (suction) below the throttle plate is applied through the priming plug, idle channel, idle jet, idle fuel passage to the compensating jet. The idle jet receives its fuel through the idle fuel passage from the compensating jet. After being measured through the idle jet, the fuel is mixed with air; the amount of air being regulated by the idle adjusting needle. This mixture of fuel-air then passes through the idle channel and is discharged into the air stream through the priming plug when the throttle plate is closed enough to cause a high suction at the plug. The priming plug opening is of a size and shape to give smooth running at low engine speeds. Since the idle adjusting needle is an air adjustment, turning the needle in (clockwise) gives a richer mixture. Turning the needle out (counter-clockwise) results in a leaner mixture. At speeds above idle, the compensating jet discharges fuel through the lower section of the idle fuel passage to the well around the cap jet base.

The fuel then flows through the holes in the cap jet base inot the annulus (located between the cap jet base and the main jet). As the throttle is opened slightly, this fuel is discharged into the air stream through the slot in the secondary venturi after passing through the cap jet calibration and the cap jet tip. The cap jet calibration (by means of the size and shape of its opening) determines the rate of fuel discharge when progressing from idle speed to higher engine speeds. Since air is admitted through the wellvent, into the well to be mixed with the fuel from the compensator, the effect of the compensating jet will be reduced as engine speed increases and engine vacuum is reduced.

The main jet, often referred to as the “high speed jet,” exerts its principal influence at the higher speeds. Its fuel is discharged into the air stream through the slot in the secondary venturi. The main jet adjustment needle regulates the amount of fuel passing through the main jet. Turning the main jet adjusting needle in (clockwise) reduces the amount of fuel. Turning the needle out (counter-clockwise) increases the amount of fuel. The size of the main jet determines the maximum amount of fuel which may be admitted to the engine.