As fuel under pressure is supplied to the carburetor through the fuel inlet fitting, fuel valve and seat into the fuel bowl, the float rises and shufts-off the fuel supply at the fuel valve needle and seat when the fuel reaches a given level in the fuel bowl. The float then automatically maintains this level of fuel in the fuel bowl regardless of engine speed and load.

At idle speeds, the throttle plate is advanced slightly to expose about one half of the area of the idle discharge port to engine suction (manifold vacuum). This suction is transmitted through a vacuum passage and the idle jet to the idle fuel passage. Fuel from the fuel bowl then flows through the main jet into the main discharge jet and from the discharge jet through the idle feed holes into the idle fuel passage and from here it is metered through the calibration of the idle jet.

As fuel leaves the idle jet and enters the vacuum passage leading to the discharge port, it is mixed with air admitted through the air intake through the permanent idle air bleed. This calibrated air bleed prevents the syphoning off of fuel into the intake manifold through the idle system if for any reason the idle air adjusting needle should be placed on its seat. An additional variable amount of air is admitted past the idle adjusting needle and seat to be mixed with the fuel-air mixture in the idle vacuum passage to be discharged into the engine manifold at the idle discharge port. Turning the idle adjusting needle in (clockwise) increases the suction on the idle jet and causes a richer mixture as well as a reduced volume of air admitted past the idle adjusting needle. Turning the idle adjusting needle out (counter-clockwise) results in a leaner idle mixture.

As the throttle plate is oepned wider, the delivery of fuel-air through the high speed system supplements the idle system to increase the fuel-air supply. Although the delivery of fuel through the idle system diminishes as the throttle plate approaches the wide open position, it continues to deliver a small amount of fuel-air to the engine.

NOTE: In the 228 series the idle system operates in the same manner as in the 28 series except that the discharge of idle fuel-air mixture into the air stream is controlled directly by the idle adjusting needle located in the throttle body at the lower idle discharge hole. Turning the idle needle valve in (clockwise) results in a leaner mixture of fuel-air since less of the fuel-air mixture is discharged into the air system through the idle discharge hole.

In the curb idle position additional air is admitted through the upper idle discharge hole to be mixed with the fuel-air in the vacuum passage and discharged through the lower idle discharge hole. As the throttle is opened slightly the fuel-air is discharged through both the upper and lower idle discharge holes to increase the fuel-air supply to the engine. With the throttle plate opened to the cruising range, fuel from the fuel bowl is metered through the main jet to the base of the main discharge jet. To maintain the proper air-fuel ratio a small amount of air is admitted through the well vent or high speed bleeder through a hole below the fuel level. By introducing air into the discharge jet as a point below the fuel level, the surface tensions of the fuel is reduced to help the fuel flow at low suction. This bleed also restricts the fuel flow through the main jet under high suction. A ring land in the main discharge jet just above the lower bleed holes separates the idle fuel supply from the high speed system.

With the throttle plate advanced to a point just above the idle range, the air passing through the carburetor lowers the pressure at the discharge nozzle to cuase the fuel to flow from the fuel bowl through the main jet into the main discharge jet. The air admitted through the bleed holes in the discharge jet and measured by the well vent is mixed with fuel. This mixture of fuel-air from the main discharge jet passes through the discharge nozzle into the upper part of the secondary venturi. As it passes through the main venturi, this fuel-air mixture is further broken up or vaporized before it enters the intake manifold of the engine.

With the throttle opened from 3/4 to wide open position, the intake manifold vacuum drops below 6” of mercury which permits the spring to force the power valve piston downward to open the power jet valve. Fuel from the fuel bowl is then free to flow through the power jet valve (at a rate determined by the calibration at the bottom of the valve) into the main discharge jet where it is added to that metered through the main jet and discharged into the air stream at the discharge nozzle. As the load decreases and the manifold vacuum builds up above 6”, the vacuum piston is lifted to cut off the extra fuel supply.

NOTE: When a speed governor is installed between the carburetor and the intak emanifold, the power system vacuum passage must be connected directly into the intake below the speed governor. This is accomplished by installing a hollow by-pass screw “A” in the threaded end of the vacuum passage “C” at the throttle flange to close off the side outlet “B”.

The hollow screw will leave a vertical passage “C” open at the bottom. Most speed governors are designed with a vacuum channel in the governor body that will line up with the vertical passage in the carburetor. To complete the installation, a special flange gasket with cutout for the vacuum passage is necessary when making an installation with a speed governor.

When the throttle is opened suddenly, the pump lever attached to the throttle shaft moves the accelerating pump piston down to supply additional fuel instantly to counteract a momentary leaning out of the fuel-air mixture caused by the air rushing in through the air cleaner. As the pump piston moves down, the inlet pump check valve closes to trap fuel in the pump cylinder under pressure. The pressure built up in the pump cylinder then lifts the outlet ball check and weight above it to close the air inlet check valve. Fuel from the pump cylinder is then forced through the fuel channel and accelerating jet to be discharged into the air stream. The pump assembly controls the amount of fuel that is delivered to the accelerating jet. This may be varied by adjusting the pump stroke. Three grooves at the upper end of the pump rod provide a means of varying the pump stroke. When the cotter pin is placed in the bottom groove, the full stroke of the pump is available to discharge the largest volume of fuel. A spring is used between the pump piston and the pump rod to reduce the pressure build-up on the down stroke of the pump.

The check valve located at the bottom of the pump cylinder, supplies fuel to the accelerating sytsem. Any pressure from the pump piston will cause the small disc to seat and prevent the flow of fuel back to the fuel bowl. The refill check valve which is made up of a ball, a weight and a retainer washer is to facilitate instant refilling of the pump cylinder when the throttle is allowed to close and raise the pump. The final check valve is to provide a break in the accelerating channel by admitting air to the channel after the pump charge is dissipated. This prevents a syphoning action through the accelerating system.

To start a cold engine, the choke valve must be closed to create an extremely high suction on all systems of the carburetor thereby delivering an extra supply of fuel to the engine. To avoid over-choking or flooding of the engine, the choke must be partially opened as soon as the engine begins to operate. This is accomplished by a poppet valve located in the choke plate which opens automatically as soon as the engine starts to supply enough air for a running mixture. As the engine warms, the choke should be opened gradually until it reaches the fully open position.