Carburetor Basics
If you want to know how amazing carburetors are, just consider all the do-dads that fuel injection has to have to emulate carburetors even partly successfully. By the same token, if you want to know how lousy carburetors are, you will come to realize how much of their adequacy they owe to ignition, that is, how much ignition "covers" for carburetion, actually makes it look good. But that is another subject.

Many people view carburetors as primarily fuel instruments. But they aren't The verb "carburet" means, "to permeate with carbon." The carburetor spikes the intake air with flammable hydrocarbons. Air is the working medium in combustion, and so it is in carburetion. Try this. Start and warm up your gasoline-engined lawnmower. Shut it off and carefully but quickly remove the carburetor. Pour about a teaspoon of gasoline into the manifold and immediately pull the start rope--but keep a hand on the kill switch! The engine will rev gloriously wide-open for a few seconds. Very little quantitative relationship exists between fuel and rpm, but a lot exists between air and rpm. Air is the thing. The internal combustion engine runs on air. The fuel is of secondary importance--it comes along for the ride, so to speak, to heat the air and create the pressures that perpetuate more air movement. This is in keeping with everything about the engine's internal combustion. It's all about air.

In the middle of the carburetor is an area called the venturi. This is Grand Central Station for the caburetor, where all the action is. From this zone a brass tube is plumbed down into a fuel reservior. Intake air rushing through the venturi creates a drop in air pressure at the top of this tube, and the fuel is pushed up the tube by the atmospheric pressure at the bottom. However, this fuel enters the venturi in sheets. The engine will struggle to convert it into a mist, so the fuel tube has some lateral holes drilled in it that froth the fuel, and the venturi and thus the engine receive managable droplets. The engine's heat then turns the resulting droplets into a vapor, which the engine can burn. A motor vehicle carburetor has two or three such tubes, each progressively larger in diameter, and each with the "air bleed" just described. A choke system is needed before the engine is warm so that as poor as vaporization is with a cold engine, nearly twice as much fuel thrown at it can make up for it--even cold, some of that fuel vaporizes and the engine starts.

Engine rpm is controlled by an air valve called the throttle. As the throttle is opened, first the smallest tube then the next largest and then the next is deployed by the airstream and its effect on negative pressure in the venturi, and fuel discharge happens. This carburetor is called a "fixed venturi" carb because in contrast to later designs its venturi size never changes, only how much air is exposed to it. The drawback is that even if its size is correctly matched to the engine by the manufacturer and thus enjoys a fairly strong pressure drop, changes in throttle opening result in very marked changes in that negative pressure, too much change for good throttle response--the fuel tubes lag behind in their delivery. To make this work, vehicle manufacturers made the venturi smaller than it really needed to be so that even when the orrssure drop was weak it was strong enough, or they made sure more than one cylinder would pull on the carb at any given time, which smoothed out these negative pressure variances. They also added an accelerator pump, a one-shot fuel pump that shot a stream of fuel down the intake manifold, to further compensate for the temporary loss of negative pressure signal at quick throttle opening.

The smallest fuel tube supports the engine at idle, when the engine is its least efficient and thus most susceptible to changes in conditions. Thus this tube must have a tapered screw inserted into it to facilitate easy adjustment of the output for different environments. The idle tube is the only one that absolutely must have this screw, though manufacturers have a few times put similar screws on the other tubes--such as on personal watercraft carburetors.

The idle circuit in addition to being the most sensitive is also the most active circuit in the carburetor in all contexts outside of racing. The fixed venturi carburetor starts the engine on the idle circuit. The carb’s first transition function from idle to acceleration is supported by the idle circuit. If the carb has an anti-afterburn valve it too is part of the idle circuit. The idle tube is also the only one of the three tubes that never shuts off, meaning that it augments the other tubes at all times, though its relative percentage of contribution to the total fuel delivery naturally diminishes with more throttle opening. In a large bore bike while in the higher gears, the idle circuit is actually doing a large part of the fueling. The idle circuit, being the smallest, is also that which plugs up the quickest due to fuel varnish.

And, due again to its tenuous functionality, the idle system comes into the most scrutiny by the EPA. Initial government oversight over the idle circuit resulted in a change to its adjusting screw. The original part was simply a tapered screw tapped into the idle air bleed passage. Turning the screw inward reduced the idle circuit's access to its air bleed, while turning the screw outward increased the access, with fuel volume being the intended change but with the obvious but incidental changes to the mixture strength of the discharge at the same time. The feds weren't happy with this variability in the air/fuel ratio and not just the volume. When the 1978 Clean Air Act revision for the first time added motorcycles, the screw had to be removed from the idle air bleed and placed into the idle discharge passage where the air and fuel ratio would stay constant and only its volume would be changed by the adjustment. At the same time the screw's tip was made much more pointed because now atomized fuel was involved and not just air, requiring much finer adjustment increments.

After moving the idle mixture screw, some manufacturers also more or less hid its location on the carburetor either by putting it (as Keihin did) upside-down or by covering it with a disguising piece of aluminum or plastic and in many cases added a stop flag so that its adjustment could not be changed by more than 7/8 turn. Owners of Keihin VB series carbs know that this flag was super-glued to the screw and is thus easy to remove. But you might also discover that Keihin gave the screw underneath the flag an exceptionally narrow break-off neck to thwart the unwary. Dirty pool, that.