Fuel System
Carburetor, fuel delivery, and related components
Purpose
The fuel system delivers a controlled fuel-air mixture from the tanks to the cylinders in the correct proportion for combustion.
Main Components
- Fuel tanks — Store the fuel supply for the engine.
- Fuel lines — Carry fuel from the tanks to the carburetor.
- Fuel strainer / screens — Filter out contaminants before fuel reaches the carburetor. Regular inspection and draining of the sediment bowl is part of scheduled servicing.
- Carburetor — Meters fuel and mixes it with incoming air in the correct ratio for combustion.
- Mixture control — Lets the pilot adjust the fuel-air ratio to compensate for changes in altitude and air density. Leaning the mixture at altitude maintains efficient combustion and reduces fuel consumption.
Fuel is highly flammable. Keep open flames and sparking tools away from the fuel system during any inspection or servicing. Always relieve fuel system pressure and ensure the aircraft is properly grounded before disconnecting fuel lines.
2A.2 Fuel Drains and Strainer Service
Source: D971-13, Section 2, paragraphs 2-19 and 2-20, and figure 2-4 (Sheet 2).
- Drain locations — Fuel tanks, fuel line forward of the ON-OFF valve, fuel strainer, and carburetor
- Strainer drain valve — An integral part of the fuel strainer assembly. Its control is located adjacent to the oil dipstick; access is through the oil dipstick access door
- Daily, before the first flight — Drain off any water and sediment from the fuel strainer and, where quick-drain valves are installed, from the fuel tank sump drains
- Fuel sampling — Place the cup up to the valve and depress the valve with the rod protruding from the cup
If water is found in the fuel strainer during daily inspection, there is a possibility that the wing tank sumps or the fuel line also contain water. Remove all fuel drain plugs and drain all water from the system — do not treat a wet strainer as an isolated finding.
2A.2a Carburetor Drain Plug Inspection
Source: D971-13, Section 2, paragraph 2-20.
Performed at each 100-hour inspection and any time water in the fuel is suspected, to prevent thread sealant contamination in the carburetor float chamber.
- a. With the fuel valve OFF, remove the carburetor drain plug and clean off any sealant present on the end of the plug or in the threads on the plug.
- b. Inspect the drain plug hole in the carburetor and remove any sealant remaining in the hole.
- c. Turn the fuel valve ON to flush the float chamber and drain plug chamber while probing the drain plug hole, to ascertain that all residue of sealant material is dislodged and washed out of the chamber. The flushing operation should last 15 to 30 seconds.
- d. Perform a second flushing, retaining the drained fuel for inspection to ensure no sealant particles are present.
- e. Install the drain plug in the carburetor 1½ to 2 turns, apply sealant to the drain plug threads (NS-40 / RAS-4 or equivalent), then tighten and safety the drain plug.
- f. Turn the fuel valve ON and inspect for evidence of fuel leakage.
2A.3 Approved Fuels
Source: D971-13, Section 2, figure 2-4 (Servicing, Sheet 1).
- Minimum grade — 80/87 Aviation Grade
- Alternate 1 — 100/130 Low Lead Avgas, with lead content limited to a maximum of 2 cc tetraethyllead per gallon
- Alternate 2 — 100/130 Higher Lead Avgas, with lead content limited to a maximum of 4.6 cc tetraethyllead per gallon
Compliance with the conditions stated in Continental aircraft engine Service Bulletins M74-6 and M75-2, and supplements or revisions thereto, is recommended when using an alternate fuel. Higher-lead fuels increase lead deposits on spark plugs and in the combustion chamber; the plug servicing and rotation schedule in the Ignition System section becomes more important when they are used.
Fuel tanks should be filled immediately after flight to lessen moisture condensation. Fuel drains are located in the fuel tanks, fuel line, fuel strainer, and carburetor. If water is found in the fuel strainer during daily inspection, the wing tank sumps or fuel line may also contain water — remove all fuel drain plugs and drain all water from the system.