Idle Circuit Function and Calibration
Functionally, the idle circuit is very similar to the main metering system and may be thought of as a main system for low-speed use. Under normal circumstances, idle flow begins when manifold vacuum acts on the curb idle discharge port and creates a fuel delivery signal; the action is much like the one that activates the main system at higher airflow rates. Vacuum pulls fuel out of the main well through an idle feed restriction, and then through a series of passages, where it is ultimately emulsified with air admitted at the idle air bleed. Once emulsified, the air/fuel mixture is drawn past the mixture-adjusting needle and out of the idle discharge port. The routing of idle fuel and fuel emulsion differs between carburetor models, but conceptually, the idle circuit is virtually identical from one carburetor type to another. In most cases, idle passages lead fuel up toward the top of the carburetor, where bleed air is admitted, before bringing it back down to the discharge port. This is done in order to raise idle fuel above float-bowl level. Without this type of routing, the discharge port would serve as a float bowl drain, siphoning fuel from the float bowl into the manifold.
Traditional designs have idle mixture adjusting screws protruding into the discharge ports. Some carburetors, however, represent a departure from this practice since the mixture screws are located at some distance from the actual port. In operation, the two designs are identical. The adjusting needle simply controls the amount of fuel emulsion, which is allowed to reach the port. Placement of the needle is actually of little consequence.
Making the Adjustment
Contrary to popular belief, the mixture needle does not control idle circuit air/fuel ratio. Although this might just be semantics, there is a difference between the amount of air/fuel emulsion in the idle circuit mixture and the actual mixture that ultimately reaches the engine. Internal air/fuel idle circuit calibration is determined by the diameters of the idle feed restriction and idle bleed. Note that an idle restriction is nothing more than a metering jet for the idle system and that the air bleed serves as a primary airflow-regulating orifice. As the air/fuel emulsion exits the idle port, it is mixed with air that flows past the throttle plates. Varying the throttle plate opening increases or decreases both the amount of air that is allowed to flow into the intake manifold and the volume of air/fuel emulsion discharged at the idle port. Since these two idle mixture components are both affected by the degree of throttle opening, the ratio of air to air/fuel, once set, is not drastically altered by changes in idle speed. Turning the idle mixture screw varies the volume of air/fuel emulsion discharged into the manifold, not the ratio of air to fuel in the emulsion. Therefore, when a satisfactory idle cannot be achieved by tweaking the mixture screws richer or leaner, it can be assumed that the carburetor idle fuel calibration is not within the range demanded by that particular engine.
Use of dual four-barrel carburetors is often an excellent example of idle circuit incompatibility. With such an installation there are two primary idle systems and the volume of air/fuel emulsion is effectively double that available from a single four-barrel mounted on the same engine. Even when the mixture screws are adjusted toward lean, the requirement for uniform fuel distribution may simply cause too much overall fuel flow to permit a clean idle. In such cases, the idle-feed restrictions of both carburetors should be reduced, to lean the idle mixture to an acceptable level.
A Free Transfer
Just as the term “economizer valve” is a misnomer, “idle system” is somewhat anomalous. The circuit does function at idle, but also feeds the transfer slot, which serves to bring an engine off idle and into a higher rpm range. As the throttle plates are opened, the portion of the transfer slot exposed to manifold vacuum increases, bringing about a corresponding increase in fuel flow. Eventually, as the gas pedal is depressed further, the throttle plates will be opened sufficiently to actuate the main system, and flow through the idle circuit will diminish when the main system fuel demands starve the idle circuit. The purpose of the transfer slot is as its name implies, to smoothly transfer fuel control from the low-speed idle system to the high-speed main system.
Although there is no provision to shut off idle fuel once the main system is activated, that is effectively what occurs. Once the throttle plates are open far enough to allow passage of a significant quantity of air, the amount of vacuum presented to the idle port and transfer slot is reduced. Concurrently, a strong signal has been developed at the main discharge nozzle, so fuel that might otherwise reach the idle system is stolen from the main well by the main circuit. Since idle fuel is neither in demand nor in supply, flow effectively ceases.
Chokes and Cold Starting
When cold weather strikes, it isn’t just we mortals who experience discomfort. Our friend and ally, the internal combustion engine, is right beside us, shaking and shuddering. It is a simple fact of automotive life that gasoline does not readily vaporize when both it and the passageways through which it must flow are very cold. Add to these considerations the comparatively low speed at which a starter spins an engine, and it can be seen that cold starting does not provide ideal conditions for engine operation. Without a good strong signal, very little fuel is drawn out of the carburetor, and most of what flows returns to liquid state as soon as it contacts a cold manifold runner.
A choke plate is the device used to alleviate problems associated with coaxing a cold engine to life. When the plate is closed, a vacuum is created immediately below it and fuel is drawn out of both the idle and main system. With vaporization assisted by an over abundance of fuel, conditions are more conducive to combustion, and an engine will usually spring to life. Once this occurs, there is sufficient vacuum to initiate normal fuel flow and cause a reasonable amount of vaporization. Fuel will still condense on the intake manifold walls, but a richer than normal mixture is continually supplied until the manifold warms up and the choke is fully open.
The choke plate functions in conjunction with a fast idle cam that raises engine speed when the choke is in operation. Most automatic choke systems are also fitted with a pull-off diaphragm, which partially opens the choke when manifold vacuum is developed, preventing an over-rich mixture and subsequent stalling.
The carbs on most race engines have no choke, so starting a race engine when temperatures are relatively low can be a challenge. Cold starting is typically accomplished by depressing the gas pedal several times so that the accelerator pump squirts a healthy volume of raw fuel into the intake manifold. This will enable the engine to start, but it may run very erratically until it warms enough to allow normal vaporization. Careful maintenance of fast idle is a good practice as multiple restarts can lead to excess amounts of raw fuel washing lubricant from the cylinder walls.
Secondary Metering Systems
Depending on fuel bowl configuration, a four-barrel carb may or may not have a secondary idle circuit. If the fuel bowls are arranged on a primary/secondary configuration, a secondary idle circuit is necessary. If no fuel was taken from the secondary side at idle, and if a driver were overly conservative in his application of the accelerator pedal, fuel in the secondary bowl would be allowed to stagnate. After a prolonged period of non-use, varnish, gum and foreign matter would begin to block various passages; and when the secondaries were finally called into action, fuel delivery would be inadequate or totally blocked. By including a secondary idle circuit (on carburetors with separate primary and secondary float bowls), fuel continuously flows through the float bowl and out the constant feed discharge hole (the secondary equivalent of the curb idle discharge port). This ensures that the secondary fuel bowl is always filled with fresh fuel, eliminating varnish buildup. Secondary idle fuel can also help improve fuel distribution at low engine speeds.
Rather than being essential to proper operation, a secondary idle circuit is supportive and generally has no provision for mixture adjustment. An exception may be found on some race carburetors where adjustment capability is included to ease tuning engines with long duration camshafts. Except for the fact that the secondary idle system passes less fuel and is not adjustable, it is identical to the primary version.
Secondary Main Metering
The secondary circuits of most four-barrel carburetors are actually quite simple since they are only called into action during periods of high load. Most carbs have relatively rich secondary fuel calibrations and may or may not incorporate a power enrichment circuit.
Vacuum Secondary Actuation
If the secondary throttle plates were linked directly to the accelerator pedal, fuel economy would be unnecessarily reduced. Constant use of all four-barrels is not only wasteful, but it would also keep most engines in a state of over-carburetion. In order to correlate the degree of secondary throttle opening to actual engine requirements, carburetor manufacturers use one of several forms of venturi-vacuum activation. Depending on carburetor model and manufacturer, actuation may be by a vacuum diaphragm, counter-weighted or spring-loaded air valve. Irrespective of the method employed, the concept is designed to precisely tailor carburetor airflow capacity to engine requirements, rather than providing too much too soon, as can be the case with mechanical secondary opening.
As the name implies, mechanical secondaries operate through a direct link to the accelerator pedal linkage. A 1:1 direct linkage is used on a few race-only carburetors. Usually, mechanical actuation is controlled by a progressive linkage system. “Progressive” pertains to the opening rate of the secondaries relative to the primaries. Progressive linkage will usually allow the primary throttle plates to open 40-45 degrees before the secondary plates begin to open. But because the primaries are already open 40 degrees or more before throttle opening progresses to the secondaries, the opening rate for the secondaries must be very quick so that both primaries and secondaries reach wide-open throttle at the same time.
Under most operating conditions, rapidly rotating the secondary throttle plates from closed to fully open would result in a severe stumble. On the primary side, an additional squirt of fuel from the accelerator pump prevents this situation, and it is solved the same way on almost all mechanical secondary systems, with a secondary accelerator pump. The two pump circuits are virtually identical. There are some mechanical-secondary carburetors, however, with low airflow capacities and specially designed booster venturis that operate satisfactorily without a secondary accelerator pump. But all high-performance units require a secondary pump circuit to overcome loss of signal during rapid transition from part- to full-throttle operation.
It’s difficult to do any irreversible damage to a carburetor unless you use a drill, hammer or impact wrench. The majority of adjustments and light modifications can be made with hand tools and can be easily changed if necessary.
Text and Photos by Dave Emanuel