Fuel for idle and early part throttle mixtures is metered through the low speed, or idle circuit.
Gas enters the idle wells through the metering rod jets. The low speed jets meter the fuel at the lower end of the tube. Fuel flows up through the tube into the passage in the bowl cover where air, metered through the by pass, mixes with the fuel. Both air and fuel then pass through the economizer, and move downward in the passage where additional air, metered by the idle bleed, mixes with the fuel to attain an ideal combustible mixture. This idle mixture is then discharged into the manifold below the throttle plates through the idle port opening and the idle adjusting screw port.
Adjusting the idle mixture screw controls the amount of mixture discharged into the manifold. Turning the idle screw toward its seat reduces the amount of mixture, turning the screw out increases the amount of mixture admitted to the manifold.
The idle port is slot shaped and as the throttle valves are opened, more of the idle port is uncovered to the low pressure in the manifold which causes a calibrated increase in the amount of idle mixture to flow to the manifold.
The idle jet, by pass, economizer, idle bleed, and idle port hole as well as the bores of the carburetor are all metering points and must be free from dirt and carbon deposits. Restrictions will cause poor low speed performance. Worn or damaged idle adjusting screws should be replaced.
