The essential fuel delivery systems are:
#1- The Pilot Circuit (also called the primary, low speed or idle circuit) consists of a brass fuel jet, called the pilot jet (in the float bowl), the pilot mixture screw (PMS), and the pilot air-correction jet (in the perimeter of the carb’s “mouth”). The Pilot circuit delivers its air/fuel mixture through a small hole in the carb’s “throat”, just downstream of where the throttle plate’s lower edge almost touches the carb bore. The pilot circuit regulates the fuel mixture at idle and small throttle openings, typically under one-quarter throttle. The pilot air
correction jet (the small brass piece in a recess to the left of a bigger hole at the bottom of the carb “mouth” in the photo above) admits air to the pilot system, through a channel cast into the carb body, above the pilot jet, and it serves as a fuel/air ratio modifier and emulsion improver. This system can only deliver fuel to the engine by utilizing a strong intake vacuum to “suck” the fuel from up the float bowl.
#2- The Midrange Circuit, which is actually a component of the Main system (below), is comprised of the needle, needle jet, slide assembly and throttle plate assembly. The slide has a diaphragm attached to its top, which serves to isolate the chamber above the slide from atmospheric conditions below it. SU brand carbs and some early motorcycle (Honda) and automotive (Datsun) CV carburetors had a piston-shaped top on the slide, which ran up & down in a machined “cylinder” in the carb top-half. It did the same thing as today’s diaphragm, but it was heavy, more expensive and less responsive to throttle input. The needle, which hangs from the bottom of the slide and moves up & down within the orifice of the needle jet, acts as a “fuel-throttle”, by having a tapered shape to nearly close the needle jet’s opening when the slide is at its lowest position and then to allow full gas flow at its highest position. The midrange system regulates the air/fuel mixture between approximately one-quarter throttle and near-wide open throttle (WFO) and, like the Main Circuit, of which it is a component, it relies on the Venturi Effect to draw fuel up from the float bowl. (Keep reading)
#3- The Main Circuit’s ultimate components include the entire midrange system (above) PLUS the main jet, emulsion tube (between the main jet and the needle jet) and the main-air correction jet (in the perimeter of the carb’s “mouth”, opposite the pilot air correction jet). The function of the main jet is to limit the total amount of fuel available to the engine at wide-open throttle. The main air correction jet admits air to the main system, through a cast-in channel that connects to the emulsion tube directly above the main jet, and that air also acts as a fuel/air ratio modifier and emulsion improver. While the midrange system uses fuel delivered through the main jet and air from the main correction jet, those jets have little-to-no effect on metering the fuel/air mixture at less than wide open throttle.
#4- The Starter or Enrichener Circuit: There is no true “choke” in the Road Star carb, or in most modern motorcycle carburetors. That’s because, rather than strangling the intake tract of its air (as real chokes do), it has a circuit that infuses extra fuel directly into the intake tract, there by enrichening the fuel/air mixture. The enrichener system (we’ll call it a choke for simplicity from now on) requires high intake vacuum downstream of the throttle plate to work. So opening the throttle during startup will actually reduce the choke’s ability to do its job. If the throttle is opened significantly, the “choke” may completely stop delivering any extra fuel, until the throttle is closed enough to regain a high vacuum downstream of the throttle plate. Essentially, if the engine is cold enough to need “choke” to start, leave the throttle grip alone when you hit the starter button.
#5- The deceleration enrichener system is a small device mounted to the side of the carb, containing a small diaphragm and spring. It adds an additional measure of fuel during the very high intake vacuum that exists during closed-throttle deceleration at road speeds. Its function is to help reduce exhaust backfiring during deceleration. It is not common to all modern motorcycles and it has no readily adjustable functions.
#6- The accelerator pump is just what it sounds like. It is a small plunger which gives a squirt of raw gas into the intake tract, when the throttle is applied from idle or near idle. (The brass accelerator pump nozzle protrudes laterally into the carb intake, in the photo above) This extra shot of gas is intended to compensate for
a momentary lean condition, which occurs when the throttle plate is suddenly opened, causing air velocity through the carb to drop too low to draw sufficient fuel from the main system. That momentary lean period can be problematic, so the accelerator pump serves to “take up the slack”. Knowing the above carburetor systems and their functions becomes more relevant as you understand their theory and adjustments, which follows.