Tune-Up - Major

Complete major tune-up service including ignition system components, carburetor adjustment, valve adjustment, and timing optimization for the 390ci V8 engine.

Warnings

Tools required

Parts

• Spark plugs (Champion N12Y or equivalent) × 8 — N12Y
• Distributor cap × 1 — Use OEM specification
• Distributor rotor × 1 — Use OEM specification
• Ignition points set × 1 — Use OEM specification
• Condenser × 1 — Use OEM specification
• Spark plug wires (8mm set) × 1 — Use OEM specification
• Distributor cap × 1 — Use OEM specification
• PCV valve × 1 — Use OEM specification
• Air filter element × 1 — Use OEM specification
• Fuel filter × 1 — Use OEM specification
• Valve cover gaskets × 2 — Use OEM specification
• Carburetor gasket kit × 1 — Use OEM specification

Preparation

1. Ensure engine is completely cold
2. Disconnect negative battery cable
3. Remove air cleaner assembly and set aside
4. Clean engine bay area around valve covers and distributor to prevent debris entry
5. Mark spark plug wire positions with tape and numbered labels
6. Verify you have correct replacement parts for 1970 AMC 390ci V8

Procedure

1. 1

   Remove and inspect spark plugs
   Remove spark plug wires one at a time, noting their cylinder positions. Use spark plug socket to remove all eight spark plugs. Inspect old plugs for fouling, oil deposits, or abnormal wear patterns which may indicate engine problems. Check plug gap on new plugs and adjust to 0.035 inches. Apply anti-seize compound to spark plug threads before installation.
2. 2

   Perform compression test
   With all spark plugs removed, install compression tester in cylinder #1. Have assistant crank engine for 5-6 compression strokes while holding throttle wide open. Record compression reading. Repeat for all eight cylinders. Normal compression should be 140-160 psi with no more than 10% variation between cylinders. Low compression indicates potential valve or ring issues.
3. 3

   Install new spark plugs
   Thread new spark plugs in by hand to avoid cross-threading. Tighten spark plugs using torque wrench. Since no specific torque value is provided in database, use 25-30 ft-lbs for tapered seat plugs on this application. Do not over-tighten as this may damage aluminum heads.
4. 4

   Remove distributor cap and inspect components
   Remove distributor cap hold-down screws and lift cap clear with wires attached. Inspect cap interior for carbon tracking, cracks, or burned contacts. Examine rotor for burning or wear. Mark distributor body position relative to engine block with paint pen before any adjustments. Note rotor position for reference during reassembly.
5. 5

   Replace points and condenser
   Remove distributor rotor by pulling straight up. Disconnect point wire and condenser wire from distributor terminal. Remove point set retaining screw and lift out old points. Remove condenser mounting screw and old condenser. Install new condenser and new point set. Ensure point contact surfaces are clean and properly aligned. Do not touch point surfaces with fingers as oil contamination will cause premature failure.
6. 6

   Adjust point gap and dwell
   Rotate engine by hand until rubbing block is on high point of distributor cam lobe. Using feeler gauge, set point gap to 0.016-0.020 inches. Tighten point retaining screw. Reconnect battery and attach dwell meter. Start engine and adjust points for 29-31 degrees dwell angle using distributor adjustment window. This setting is critical for proper ignition timing and performance.
7. 7

   Install new distributor cap, rotor, and wires
   Install new rotor ensuring it seats fully on distributor shaft. Install new distributor cap and secure with hold-down screws. Install new spark plug wires one at a time, following firing order 1-8-4-3-6-5-7-2. Route wires away from exhaust manifolds and moving parts. Ensure wires seat firmly in distributor cap towers and on spark plugs.
8. 8

   Remove valve covers and adjust valves
   Remove valve cover bolts and carefully lift off valve covers. Clean gasket surfaces thoroughly with scraper and solvent. Rotate engine to TDC compression stroke for cylinder #1 (both valves closed, rotor pointing to #1). Adjust intake and exhaust valves for cylinders in firing order sequence using two-turn method. Set valve lash to 0.012 inches cold for both intake and exhaust. Follow firing order pattern, rotating crankshaft 90 degrees between adjustments.
9. 9

   Install valve covers with new gaskets
   Position new valve cover gaskets on valve covers. Carefully set valve covers in place, ensuring gaskets remain properly seated. Install valve cover bolts and tighten in progressive pattern from center outward. Tighten evenly to prevent gasket distortion and oil leaks. Use moderate torque only as over-tightening will crush gasket.
10. 10

    Replace PCV valve and air filter
    Locate PCV valve in valve cover or intake manifold. Remove old PCV valve and install new valve, ensuring proper seating. Replace air filter element in air cleaner housing. Inspect air cleaner housing gasket and replace if damaged.
11. 11

    Replace fuel filter
    Locate inline fuel filter between fuel pump and carburetor. Place shop towel around filter to catch fuel spillage. Using two wrenches, disconnect fuel lines from filter. Note flow direction arrow on old filter. Install new filter with arrow pointing toward carburetor. Tighten fittings securely but do not over-tighten to prevent line damage.
12. 12

    Clean and adjust carburetor
    With engine at normal operating temperature, remove carburetor air cleaner. Clean carburetor exterior and inspect for vacuum leaks. Set idle mixture screws to base setting of 1.5 turns out from lightly seated. Start engine and allow to reach normal operating temperature. Adjust idle speed to 600 RPM in neutral. Adjust idle mixture screws equally for highest vacuum reading and smoothest idle. Reset idle speed if necessary.
13. 13

    Set ignition timing
    Connect timing light to #1 spark plug wire. Locate timing marks on harmonic balancer and timing tab. Start engine and allow to reach normal operating temperature. Disconnect and plug vacuum advance line at distributor. Aim timing light at timing marks while engine idles. Loosen distributor hold-down bolt. Rotate distributor to set initial timing to 5 degrees BTDC (factory specification for 1970 390 Machine). Tighten distributor hold-down bolt and recheck timing. Reconnect vacuum advance line.
14. 14

    Final carburetor adjustment
    With timing set and engine at operating temperature, perform final idle and mixture adjustments. Adjust idle mixture screws for smoothest idle and best vacuum reading. Set final idle speed to 650 RPM in neutral with all accessories off. Install air cleaner assembly and verify idle speed remains stable.

Reassembly

1. Verify all vacuum lines are properly connected and not kinked
2. Ensure spark plug wires are routed away from exhaust manifolds
3. Double-check that distributor vacuum advance line is reconnected
4. Confirm air cleaner assembly is properly sealed to carburetor
5. Reconnect negative battery cable

Verification

• Start engine and verify smooth idle at 650 RPM
• Check for vacuum leaks by listening for hissing sounds around carburetor and intake manifold
• Verify no oil leaks around valve covers after 15 minutes of operation
• Test drive vehicle to confirm smooth acceleration and no hesitation
• Recheck ignition timing after test drive to ensure distributor has not shifted
• Monitor engine temperature to ensure proper cooling system operation
• Verify no fuel leaks at new fuel filter connections after driving

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