Fuel metering

Seven procedures. The last three are performed on a running engine and each can destroy it.

Procedure 0 — Before opening any fuel component (R3, R4, R5)

1. Fuel off, and the system depressurized per the manual.
2. Ignition off and magnetos grounded (PP-H Procedure 0).
3. Containment ready. Whatever you remove contains fuel (PP-I Concepts §4).
4. Ventilation, no ignition sources, extinguisher present.
5. Cap every port as it opens.

Check — assume it contains fuel, including on the bench. Carry it upright and put it on a tray.

Procedure 1 — Remove and install a float carburetor (S9)

1. Procedure 0.
2. Note the linkage positions and settings before disconnecting — mixture and throttle rigging is easy to lose and awkward to recover.
3. Disconnect fuel, linkages and drains, capping everything.
4. Remove, keeping it upright.
5. Install with new gaskets, correct orientation, torqued evenly per the manual.
6. Reconnect and rig the linkages: full travel both ways, correct stops, no binding, and idle cut-off actually cutting off.
7. Leak check with fuel on, before starting.

Check — operate throttle and mixture through full travel and confirm they reach both stops at the carburetor, not just at the cockpit control. A control that reaches its cockpit stop without reaching the carburetor stop is not rigged.

Procedure 2 — Inspect the needle, seat and float level (S8)

1. Disassemble the bowl per the manual.
2. Inspect the needle and seat for wear, grooving and debris — a worn needle floods.
3. Check the float for fuel inside it, damage or distortion. A float with fuel in it sinks and the engine floods.
4. Measure the float level by the manual's method and dimension.
5. Adjust only by the specified means.

Check — shake the float and listen. Fuel inside is audible, and a float that has taken on fuel looks perfect.

Procedure 3 — Remove and install a metering jet (S5)

1. Procedure 0.
2. Confirm the jet part number against the engine data. Jets are calibrated orifices and a similar-looking jet is a different mixture.
3. Remove without damaging the orifice — correct tool, no drills, no wire.
4. Inspect for erosion or partial blockage against a light.
5. Install to the specified torque; over-tightening distorts the orifice.

Check — read the jet's marking and confirm it against the parts data. The orifice size is the calibration, and nothing about a wrong jet is visible once fitted.

Procedure 4 — Remove and install an engine-driven fuel pump (S16)

1. Procedure 0.
2. Note drive coupling type and condition, and the orientation.
3. Remove, cap lines and the pad.
4. Fit new gasket and seals, confirm the drive coupling engages, torque per the manual.
5. Prime per the manual.
6. Check output pressure against specification with the engine running (Procedure 7).

Check — confirm the drive engages before torquing down. A pump fitted with the coupling misaligned turns nothing and looks correctly installed.

Procedure 5 — Service a fuel filter or strainer (S27, S19)

1. Procedure 0.
2. Remove and inspect the contents: water, sediment, rubber, sealant, metal, microbial growth (AF-J Procedures Procedure 3).
3. Clean or replace per the manual, with new gaskets.
4. Reassemble, torque, leak check with the system pressurized, and safety as required.

Check — look at what came out before you wash it away. It is direct evidence about the tanks and lines upstream.

Procedure 6 — Determine a nozzle spray pattern (S30)

1. Remove nozzles per the manual, keeping them in cylinder order.
2. Inspect for carbon, erosion, and blocked or distorted orifices.
3. Where a test rig is available, check the spray pattern: an even, correctly shaped cone with no streaking or splitting.
4. Compare nozzles against each other as well as against the specification.

Check — compare the set. One nozzle differing from the others points at that cylinder, and on a turbine at the liner opposite it (PP-B §3).

Procedure 7 — The adjustments (S7, S10, S23, R1, R2)

On a running engine. Read PP-I Concepts §1 first.

1. Everything in GEN-F Procedures Procedure 7 — checklist, area brief, clear call, nobody in the propeller arc.
2. Get the target figures from the engine manufacturer for this engine in this installation.
3. Warm the engine to the specified temperature. Adjustments made cold are wrong.
4. Note ambient conditions and correct where the procedure requires it (GEN-J Procedures Procedure 3).
5. Adjust in small increments, allowing the engine to stabilize between changes.
6. For idle mixture, use the specified method — typically a small RPM rise as the mixture is pulled to cut-off, of the magnitude the manufacturer states. More rise is not better; it means too rich.
7. Record before and after readings, and what you changed.
8. Shut down and re-check anything that must be checked static.

Check — state the target figure and its source before you touch the adjustment. If the answer is "until it runs nicely", stop — that is how an engine is set lean enough to detonate (PP-I Concepts §1).

Failure — adjusting by ear or by feel. It produces an engine that runs beautifully on the ground and destroys a piston over the next fifty hours.

The checks, collected

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