Since we are discussing the idle/off idle circuit, perhaps now is the time to discuss what some might call semantics:
Generally, changing the adjustment of the idle mixture screw does NOT change the idle mixture!!!
The idle circuit in many (virtually all of those with which I am familiar) carburetors is FIXED. The idle circuit generally consists of the following:
(1) the low speed jet (a.k.a. idle jet, idle tube, etc.) FUEL
(2) the idle bypass AIR
(3) the idle economiser (a.k.a. idle restrictor)
(4) the idle air bleed AIR
(5) the idle mixture screw (really should be called a volume screw)
(6) the ports (lower port and transition slot)
Without getting into atmospheric pressure (in the bowl) and negative pressure (vacuum) in the ports:
Fuel passes from the bowl through the low speed jet into a passage. Further along this passage, air is drawn into the fuel through the idle bypass, and mixing begins. The air fuel mixture is now directed through the idle economiser (restrictor). As the name implies, the idle restrictor is a necked-down diameter in the passage. As the dynamic volume is reduced, the velocity of the air fuel mixture increases, adding to the mixing of the air and fuel. Past the idle economiser, the passage diameter is again increased, and additional air is bled into the fuel via the idle air bleed. The idle mixture is now complete and it travels into the final passage to the ports. The first port encountered is the transition slot. IF the throttle plate is not completely closed, air passing by the throttle plate creates a negative pressure in this port, causing some mixture to be delivered into the throttle area. If the plate is closed, no mixture will pass though this port. The remaining mixture continues to the the idle mixture screw, where the position of the screw controls how much of the mixture is passed through the lower idle port.
I like to use the analogy of an old-fashioned shower with three controls (hot water faucet, cold water faucet, and volume). Think of the hot water faucet (think fuel) and the cold water faucet (think air) being controlled by the carburetor design engineer, and the volume or pressure control (idle mixture screw) as being controlled by the person taking the shower.
The user CANNOT change the hot water or cold water setting in the shower!
However, IF NECESSARY, the mixture in the carburetor MAY be changed by replacing either the low speed jet with a larger or smaller one or changing the value of the two air jets.
Jon.
