TPS: Throttle Position Sensor. Mechanically coupled to the throttle shaft, the TPS signal is critical to proper idle function and acceleration enrichment.
Enrichment CircuitWhen an engine is under heavy load, the carb's main metering system may not be able to supply it with enough fuel. Carb manufacturers have come up with different styles of enrichment circuits, each designed to trim the air/fuel ratio to meet the engine's needs. Each of these enrichment circuits "sees" the engine load as manifold vacuum. Quadrajets and Carter AFB-style carbs use metering rods to restrict the flow of fuel through the main jets. The rods are supported on small springs that hold them away from the orifices in the jets. Under light loads (cruising), engine vacuum pulls the rods into the jet orifices, restricting fuel flow. Under a heavy load, engine vacuum drops, allowing the springs to force the rods away from the jets and allow more fuel to flow through the jets. Swapping in metering rods with different sized tips alters the amount of fuel enrichment. Modular-style Holley and Demon carbs use a power valve to supply more fuel under load. Vacuum holds the valve closed against an internal spring, and when the engine vacuum drops below a certain level, the spring forces the valve open and allows additional fuel to flow. Think of it as a vacuum-controlled jet.
EFI is already a step ahead of the game. The main fuel map accounts for engine load, so the additional fuel that a carb feeds via its load-sensing enrichment circuit is already accounted for. Easy, huh?
ChokeOften shed for performance reasons, the choke is still an important part of a street car that's driven in cold conditions. A simple device, the choke butterfly closes off most of the air from the primaries, creating a rich condition to compensate for poor atomization of the fuel when the engine is still cold to the touch. Choke pull-off is controlled by a manually operated cable, electric resistance, or the heat of the intake manifold, depending on the design.
As you might have guessed, there's no choke butterfly attached to a multiport EFI throttle body-it's all done with electronics, and common sense will guide you through. Tweaking the "Cranking Fuel" graph changes how long the injectors stay open, and consequently, how much fuel the engine is fed when you blip the key (pumping the pedal won't do a damn thing). After the engine fires, you'd like to use up some of that excess cranking fuel before the injectors kick on, so you enter how many revs the engine must turn before the Afterstart enrichment is added. Afterstart enrichment gives the richening effect of the choke, so you open another map and make sure that extra fuel completely dies out by the time the engine warms up.
Idle CircuitThe venturi effect is nice and predictable, but it won't draw fuel out of the boosters if the pressure drop isn't sufficient. At near-idle speeds, as we'd expect, the main metering circuit is useless. Carburetors use a separate idle circuit that bleeds a small amount of air and fuel, mixed in a specific ratio, below the throttle blades where it's drawn into the engine. Idle screws restrict the amount of mixture that is drawn into the engine (although they have some impact on the idle air/fuel ratio, the range is determined by the size of the idle air bleeds and idle fuel feed restrictions). As the throttle blades are opened, the idle transition slot is "uncovered" and exposed to engine vacuum, drawing more air and fuel until the main metering circuit takes over. Under heavy-right-foot conditions, the accelerator pump shoots a large volume of fuel into the venturis to bridge the time delay until the main metering system takes over.