The fuel line from the cabruretor bowl (12) passes through the main metering jet (13) into the idle tube (10). Air is introduced into the fuel stream by idle air bleed (8). This fuel and air mixture then travels through the idle passage (1) through the chamber around the bleeder plug (16) and to the continuation of passage (1) then discharges at idle transfer hole (2) and idle discharge hole (3).
When the engine is set to idle at a speed of approximately 350 RPM, the mixture is discharged out of the lower hole (3) only. As the throttle plate (5) is opened and the engine speed and airflow increases, the upper idle transfer hole (2) starts discharging in addition to the lower hole at about 450 RPM. The action and timing are such that the discharge from the upper hole reaches a maximum at approximately 750 RPM and then gradually becomes less effective as the main nozzle (15) begins to flow.
The lower discharge hole (3) is provided with an idle adjusting needle (4). Turning this needle out (to the left) gives a richer mixture and turning in (to the right) a leaner mixture. The idle adjustment should be set with a vacuum gauge for the highest and steadiest vacuum reading or the smoothest running and maximum idle RPM. When seating the idle adjustment needle (4), excessive force should never be used as this may cause a groove to form on the needle point. If this occurs, the adjusting needle will have to be replaced in order to obtain a satisfactory idle adjustment.
Failure to idle properly after adjusting - Be sure motor is tuned to specifications. Check for air leaks in intake manifold gaskets, carburetor body, around the throttle shaft, and at bowl and flange gaskets.
If the idle is erratic and not sensitive to adjustment, check the float level in the float bowl. Check the idle tubes (10). They must be tight on seat in nozzle bar casting (15).