1) Core theory (short)
- Purpose: the distributor times and routes high-voltage sparks to each cylinder at the correct crank angle. It provides base (static) timing plus dynamic advance via centrifugal (mechanical) and vacuum systems so ignition occurs earlier as rpm/load require.
- Main parts: cap, rotor, ignition pickup/points (or electronic trigger), condenser/ignition module, distributor shaft & drive gear, centrifugal weights/springs, vacuum advance canister, bushings/bearings, seals. Wear or failure in any changes point of spark or prevents spark delivery.
- Effects of faults: worn shaft/bushings = erratic timing/shaft wander; bad cap/rotor/cracks = misfires or cross-fire; bad pickup/module = no spark or random cuts; stuck/weak advance = poor power or detonation; vacuum advance leak = incorrect part-load timing.

2) Symptoms and ordered tests (diagnose before removal)
1. Note symptoms: rough idle, miss under load, hard start, pinging, poor fuel economy, backfire, no-start.
2. Visual: inspect cap for carbon tracking, cracks, corrosion on terminals; rotor for burning; plug wires for damage.
3. Static test: set engine at TDC compression for #1; check rotor points to cap terminal for #1. If not, suspect slipped distributor or incorrect installation.
4. Shaft play: measure axial & radial play (feel by hand, lean on shaft, or use dial indicator). Excessive play -> timing wander.
5. Cap continuity: check resistance/insulation between terminals; look for shorts to cap body.
6. Pickup/coil: measure pickup coil resistance and primary/secondary of ignition coil to spec; check module for intermittent faults with oscilloscope or substitution.
7. Advance check: with engine running, use timing light to verify mechanical advance curve with rpm; apply vacuum to vacuum canister and verify timing shifts.
8. Leakage: check vacuum advance diaphragm for vacuum hold (apply hand vacuum).

3) Removal (in order, preserve timing references)
1. Bring engine to TDC compression on #1. Confirm by rotating crank and observing piston/cam marks.
2. Mark distributor housing to engine block and mark rotor position relative to housing (draw scribe lines or tag). These marks ensure you can reinstall to same phasing.
3. Disconnect battery negative.
4. Label and disconnect plug wires from cap and any electrical connectors (vacuum line, pickup connector).
5. Remove distributor cap screws without disturbing rotor if possible; lift cap and leave wires attached to cap if it’s easy to move as an assembly.
6. Note rotor orientation again; remove rotor retaining screw if present and lift rotor.
7. Loosen distributor clamp fastener(s) and lift distributor straight up. If it resists, turn slightly back toward cam rotation a few degrees while pulling up to free the drive dog.
8. Inspect drive gear and mating gear on cam/crank for wear.

4) Inspection & overhaul (in order, theory tied to inspections)
1. Shaft/bushings: inspect for scoring, ovality. Theory: worn bushings let shaft tilt/rotate off center, changing rotor-to-cap timing and causing erratic advance. Repair: replace bushings or full distributor unit; press-fit new bushings and check concentricity.
2. Cap & rotor: clean or replace. Theory: carbon tracks or pitted terminals provide leakage paths, weak or misrouted sparks → misfires. Repair: replace cap/rotor, ensure terminals clean and correct orientation.
3. Pickup/points & module: inspect points for pitting; measure pickup coil resistance. Theory: erratic or no trigger signal causes missing/weak spark. Repair: replace points/condensor or electronic pickup/module.
4. Bearings/grease & seals: renew end-play shims/o-rings. Theory: axial play changes timing and seal failure allows oil ingress causing shorting. Repair: replace seals and set endplay per spec.
5. Advance mechanism: clean pivots and springs; check weights for free movement and springs for correct tension. Theory: sticky or weak springs change advance curve → loss of mid/upper rpm power or detonation. Repair: free pivots, replace springs or entire advance assembly.
6. Vacuum advance: check diaphragm for leaks; replace if any. Theory: vacuum advance wrong = incorrect part-load timing causing poor driveability and fuel economy.
7. Electrical: replace worn wiring, connectors, and apply dielectric where needed.

5) Reassembly & refit (in order)
1. If bushings replaced, ensure shaft rotates smoothly with minimal play; reinstall rotor and cap.
2. Lower distributor back into engine so drive gear meshes. Align rotor to the mark you made so the rotor points to the #1 terminal location at the moment the distributor seats.
3. Seat fully and secure clamp lightly — do not fully torque yet (so you can rotate for timing).
4. Reconnect pickup/ignition connectors and vacuum line.
5. Reinstall cap and plug wires to their correct firing order positions.

6) Timing set and advance verification (ordered)
1. Reconnect battery. Start engine and allow warm-up to normal operating temperature.
2. Use a timing light on #1 spark plug lead. With idle set to spec, rotate distributor body until base (static) timing matches the factory degree mark. Tighten distributor clamp to final torque spec.
3. Rev engine and watch timing light to confirm mechanical advance increases timing smoothly to specified maximum at specified rpm. If not, recheck springs/weights.
4. Apply vacuum to vacuum advance with hand pump (while engine idling or at specified rpm per manual) and confirm timing moves to the specified additional degrees. Replace diaphragm if no movement or leakage occurs.
5. Road test under load: verify no pinging, no misfire, and proper power.

7) How each repair action fixes specific faults (concise)
- Replacing cap/rotor removes carbon tracking and burned terminals → restores correct spark insulation and routing → fixes misfires and cross-firing.
- Replacing worn bushings/shaft restores concentricity and reduces endplay → stabilizes rotor position relative to cap → prevents timing variation and rough idle/erratic misfire.
- Servicing/renewing pickup or ignition module restores clean trigger signal → restores consistent spark timing and prevents intermittent/no-starts.
- Cleaning/freeing or replacing mechanical advance (weights/springs) restores correct rpm-dependent advance curve → restores power at higher rpm and prevents detonation when over-advanced or sluggish performance when under-advanced.
- Fixing vacuum advance diaphragm/line restores part-load timing correction → improves throttle response, driveability, and fuel economy.
- Reinstalling with correct phasing and setting base timing ensures spark occurs at correct crank angle; combined with correct advance curve produces optimal combustion timing across loads and rpm.

8) Verification checklist (final, ordered)
1. No visible arcing or carbon tracks on cap after a short run.
2. Shaft play within allowable limits.
3. Static timing at spec.
4. Mechanical advance reaches specified degrees smoothly.
5. Vacuum advance responds and holds vacuum.
6. No misfire, smooth idle, good throttle response, correct fuel consumption.

Do not deviate from manufacturer torque and tolerance specs when available.
rteeqp73

From the Archives: Hino FG Series Old footage of the Hino FG Series truck.

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