This carburetor has duplicate idle systems for both of its barrels. In this description, reference is made to only one of the systems for purposes of illustration. At idle and low speeds, the pressure drop in the venturi is not sufficient to operate the main metering system. The high manifold vacuum at low rpm is used to provide a pressure difference which will operate the idle system.
The high pressure of the air in the float chamber forces fuel through the main jet into the bottom of the main well. The fuel enters the idle tube through the narrow tip at its lower end. The tip of the idle tube is a calibrated restriction which meters the flow of fuel into the idle system. The fuel leaves the idle tube through holes in the head of the tube and flows up an angle channel in the nozzle bar, where air from the idle air bleed enters the fuel flow.
The emulsion of fuel and air passes around a circular groove in the air bleed plug and down through the other side of the nozzle bar. It then flows down pas the two idle transfer holes which inject additional air into the fuel, and is discharged from the idle discharge hole into the strong manifold vacuum below the closed throttle plate.
The idle adjusting needle governs the mixture delivered at idle only. A small hole in the air bleed plug vents the idle system to dissipate any siphoning effect which may occur at higher speeds or when the engine is shut off.
During off-idle operation, this system functions similarly, except that, as the throttle plate is opened past the idle transfer holes, they cease to function as air bleeds and begin discharging fuel. The flow from the idle system tapers off as the main metering system begins to operate with an increase in engine speed.
First set the idling speed by adjusting the throttle stop screw located on shaft operating housing.
The idle mixture is controlled by the idle adjusting screw. NOTE: These screws regulate fuel flow and are not air bleed screws. Turning these screws in or to the right makes the mixture leaner, and turning them out or to the left makes the mixture richer. Screws should be turned in until they just touch the seat, then backed off one full turn. Care should be taken not to jam the screws against the seat tight enough to groove the points. If this occures, the needles will have to be replaced before a satisfactory adjustment can be made. The screws should be adjusted with the fingers to avoid damage.
When the engine has warmed up, turn both idle screws in until the engine starts to slow down, then turn both idle screws out until the engine starts to slow down. A point half way between these two will be very close to the correct idle mixture. After arriving at the correct mixture, it may be necessary to reset the throttle stop screw to obtain the correct idle speed. Changing the speed may make a slight change in the idling mixture. If this occures, readjust the screws as outlined above. When adjusting the idle mixture it is important that both idle screws be set at approximately the same number of turns open, otherwise, the off idle performance may be erratic.
Failure to idle properly after adjusting - Be sure motor is tuned to specifications. Check for air leaks in intake manifold gaskets, carburetor body, bowl and flange gaskets.
If the idle is erratic and not sensitive to adjusting, check the float level in the float bowl. Check the idle tubes (10), Figure A They must be tight on seat in nozzle bar castings (15). Check the economizer valve (30), Figure B.
