Fuel Control Unit The fuel control unit is attached to the regulator assembly  and contains all metering jets and valves.  [Figure 2-25]  The  idle and power enrichment valves, together with the mixture  control plates, select the jet combinations for the various  settings (i.e., auto-rich, auto-lean, and idle cutoff). The purpose of the fuel control unit is to meter and control  the fuel flow to the discharge nozzle. The basic unit consists  of three jets and four valves arranged in series, parallel, and  series-parallel hookups.  [Figure 2-25]  These jets and valves  receive fuel under pressure from the regulator unit and then  meter the fuel as it flows to the discharge nozzle. The manual  mixture control valve controls the fuel flow. By using proper  size jets and regulating the pressure differential across the  jets, the right amount of fuel is delivered to the discharge  nozzle, giving the desired fuel/air ratio in the various power  settings. It should be remembered that the inlet pressure to the  jets is regulated by the regulator unit and the outlet pressure  is controlled by the discharge nozzle. The jets in the basic fuel control unit are the auto-lean jet, the  auto-rich jet, and power enrichment jet. The basic fuel flow  is the fuel required to run the engine with a lean mixture and  is metered by the auto-lean jet. The auto-rich jet adds enough  fuel to the basic flow to give a slightly richer mixture than  best power mixture when the manual mixture control is in  the auto-rich position. The four valves in the basic fuel control unit are: Figure 2-25.  Fuel control unit. 2-18 1.  Idle needle valve 1. The idle needle valve meters the fuel in the idle  range only. It is a round, contoured needle valve, or a  cylinder valve placed in series with all other metering  devices of the basic fuel control unit. The idle needle  valve is connected by linkage to the throttle shaft so  that it restricts the fuel flowing at low power settings  (idle range). 2.  Power enrichment valve 3.  Regulator fill valve 4.  Manual mixture control 2. The manual mixture control is a rotary disk valve  consisting of a round stationary disk with ports leading  from the auto-lean jet, the auto-rich jet, and two  smaller ventholes. Another rotating part, resembling  a cloverleaf, is held against the stationary disk by  spring tension and rotated over the ports in that disk  by the manual mixture control lever. All ports and  vents are closed in the idle cutoff position. In the auto- lean position, the ports from the auto-lean jet and the  two ventholes are open. The port from the auto-rich  jet remains closed in this position. In the auto-rich  position, all ports are open. The valve plate positions  are illustrated in  Figure 2-26 . The three positions of  the manual mixture control lever make it possible to  select a lean mixture a rich mixture, or to stop fuel flow  entirely. The idle cutoff position is used for starting or  stopping the engine. During starting, fuel is supplied  by the primer. 3. The regulator fill valve is a small poppet-type valve  located in a fuel passage which supplies chamber C  of the regulator unit with metered fuel pressure. In  idle cutoff, the flat portion of the cam lines up with  the valve stem, and a spring closes the valve. This  provides a means of shutting off the fuel flow to  chamber C and thus provides for a positive idle cutoff. 4. The power enrichment valve is another poppet-type  valve. It is in parallel with the auto-lean and auto-rich  jets, but it is in series with the power enrichment jet.  This valve starts to open at the beginning of the power  range. It is opened by the unmetered fuel pressure  overcoming metered fuel pressure and spring tension.  The power enrichment valve continues to open wider  during the power range until the combined flow  through the valve and the auto-rich jet exceeds that  of the power enrichment jet. At this point the power  enrichment jet takes over the metering and meters fuel  throughout the power range. 5.  Carburetors equipped for water injection are modified  by the addition of a derichment valve and a derichment  Auto lean jet Power enrichment valve Auto rich jet Power enrichment jet Manual mixture control Regulator fill valve Idle needle valve The functions of these valves are: