73-00-00
AMM
Spring Vent To Atmosphere
SECTION A-A
Figure 1: Engine Fuel System Schematic
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Engine Fuel and Control
AMM
Air density decreases as the airplane climbs. The air diaphragm does not measure the air density and the system gives a continuously rich mixture. The pilot can adjust the mixture strength with the manual mixture control.
If you move the mixture control lever aft, then the mixture becomes leaner. If you move the mixture control fully aft, then all fuel flow is cut off and the engine stops.
Different mixture strengths are necessary for different power settings. A linkage connects the throttle lever to the idle valve. It controls the fuel flow at idle to give a rich mixture. When the throttle is opened, the idle valve changes the outlet from the metering jet to give the correct mixture strength.
The pressures acting on the air diaphragm at idle are very small. A small spring in the air diaphragm makes sure that the ball valve stays open and the system supplies sufficient fuel for idle.
The injector has two adjustments:
) Idle speed. You can adjust the idle speed with the stop screw on the throttle lever. Turn the stop-screw clockwise to increase the idle speed.
) Idle mixture. You can adjust the idle mixture with the thumb-screw. The thumb-screw is in the link between the throttler lever and the idle mixture lever. The thumb-screw has right-hand threads on both ends. One thread is coarse and the other thread is fine.
If you turn the thumb-screw so that the coarse thread is moves into its end-block, then the link becomes shorter and the idle mixture becomes leaner. If you turn the thumb-screw so that the coarse thread moves out of its end-block, then the link becomes longer and the idle mixture becomes richer. Section 71-00 tells you how to do an engine run-up to set the mixture strength.
B. Fuel Flow Divider
The fuel outlet of the injector connects to the fuel-flow-divider. The fuel-flow-divider is on top of the engine crankcase and has an outlet for each cylinder.
The fuel-flow-divider has a diaphragm. Fuel injector outlet pressure connects to the bottom of the diaphragm and atmospheric pressure connects to the top of the diaphragm. A hollow needle-valve attaches to the diaphragm and a spring above the diaphragm pushes the needle-valve downwards.
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AMM
Fuel pressure from the fuel injector pushes the diaphragm up against the spring and opens a port in the needle-valve. The port connects to the outlet pipes for the cylinders. When fuel flows out the pressure drops and the needle-valve closes a small amount to keep up the injector pressure. At idle the needle-valve can close between each pulse of fuel. At the usual power settings the needle-valve stays open to keep up the fuel pressure.
C. Injector Nozzles
Rigid pipes from the fuel-flow-divider connect to a nozzle in each cylinder inlet port. The nozzle has a jet. Fuel from the jet flows out under pressure into an air-bleed chamber. The air and the fuel mix in the air-bleed chamber and then goes into the inlet port.
Each nozzle is calibrated ± 2%. Nozzles of the same part number are interchangeable between cylinders and they are interchangeable between engines.
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