Drivability Diagnosis

Systematic diagnosis of drivability issues on a 1970 AMC Rebel Machine with 390ci V8, including inspection of ignition, fuel, compression, and carburetor systems to identify the root cause of poor performance, stalling, hesitation, or rough running.

Warnings

Tools required

Parts

• Spark plugs (if fouled) × 8 — Champion N-10Y or equivalent
• Points and condenser (if worn) × 1 — Use OEM specification
• Fuel filter × 1 — Use OEM specification
• Air filter element × 1 — Use OEM specification

Preparation

1. Park vehicle on level surface and apply parking brake
2. Allow engine to cool completely if recently operated
3. Ensure fuel tank has adequate fuel (at least 1/4 tank) for testing
4. Gather all diagnostic tools and have notepad ready to record readings
5. Review any customer complaints or symptoms to guide diagnosis

Procedure

1. 1

   Visual inspection of ignition components
   Open hood and inspect all visible ignition components. Check distributor cap for cracks, carbon tracking, or moisture. Remove cap and inspect rotor for burn marks or excessive wear. Examine spark plug wires for cracking, hardening, or damage. Check wire routing to ensure proper firing order (1-8-4-3-6-5-7-2). Verify all wires are fully seated on distributor cap and spark plugs. Inspect coil for cracks or oil leaks.
2. 2

   Check spark plug condition
   Remove all eight spark plugs and lay them out in firing order. Examine each plug for fouling, oil deposits, carbon buildup, or electrode wear. Check gap on each plug (should be 0.035 inches for 390ci V8). Note any cylinders with abnormal plug appearance as this indicates cylinder-specific issues. Compare plug heat range to specification (N-10Y or equivalent). Replace plugs if fouled, worn, or incorrect heat range.
3. 3

   Perform compression test
   With spark plugs removed, disable ignition by disconnecting coil wire and grounding it. Block throttle and choke fully open. Install compression tester in cylinder #1. Use remote starter or have assistant crank engine for 4-6 compression strokes. Record reading. Repeat for all eight cylinders. Normal compression should be 140-160 psi with no more than 10% variation between cylinders. If any cylinder reads low, perform wet test by adding tablespoon of oil and retesting. If compression increases significantly, piston rings are worn; if no change, suspect valve or head gasket issues.
4. 4

   Inspect and test distributor points
   Remove distributor cap and rotor. Examine points for pitting, burning, or excessive wear. Check point gap with feeler gauge (should be 0.016 inches). Verify points open and close smoothly as engine is rotated by hand. Check condenser wire connection is tight. Connect dwell meter and start engine. Check dwell angle at idle (should be 29-31 degrees for 8-cylinder). If dwell is incorrect or points are badly worn, replace points and condenser as a set and reset dwell.
5. 5

   Verify ignition timing
   Connect timing light to cylinder #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 hose at distributor. Set idle to specified RPM (typically 650-700 RPM in neutral). Aim timing light at timing marks. Initial timing should be 5 degrees BTDC for 1970 390ci. If timing is incorrect, loosen distributor hold-down clamp and rotate distributor to adjust. Retighten hold-down and recheck. Reconnect vacuum advance and verify advance occurs when RPM is increased.
6. 6

   Test fuel system pressure and delivery
   Install fuel pressure gauge in line between fuel pump and carburetor. Start engine and observe fuel pressure at idle. Pressure should be 4-6 psi for the Carter AFB or Edelbrock carburetor typically fitted to Rebel Machine. If pressure is low, check for weak fuel pump, clogged fuel filter, or restricted fuel line. If pressure is excessive, check for incorrect fuel pump or stuck pressure regulator. Observe pressure gauge while revving engine to ensure pressure remains stable and pump can meet demand.
7. 7

   Perform vacuum leak inspection
   With engine at operating temperature and idling, connect vacuum gauge to manifold vacuum port. Reading should be steady 16-21 inches Hg at idle. Fluctuating needle indicates valve or ignition issues. Low steady reading suggests vacuum leak or incorrect timing. Slowly increase engine speed while watching gauge; vacuum should drop slightly then stabilize. Inspect all vacuum hoses for cracks, splits, or disconnection. Check carburetor base gasket, intake manifold gaskets, and PCV system for leaks. Use carburetor cleaner spray around suspected leak areas while monitoring idle speed changes.
8. 8

   Inspect carburetor operation and adjustment
   With engine at operating temperature, check idle mixture screws (typically 1.5-2 turns out from lightly seated). Adjust each screw for smoothest idle and highest vacuum reading, then back out 1/4 turn. Check accelerator pump operation by working throttle linkage by hand and observing pump discharge into carburetor throat. Verify choke fully opens when engine is warm and fully closes when cold. Check for fuel leaks around carburetor gaskets and fittings. Inspect air cleaner and element for restriction. On dual-quad setups (if equipped), verify proper synchronization between carburetors.
9. 9

   Check PCV and crankcase ventilation system
   Locate PCV valve (typically in valve cover or intake manifold). Remove PCV valve and shake it; should hear rattle indicating valve is not stuck. Test by running engine at idle, removing PCV valve, and placing thumb over end. Should feel strong vacuum and engine speed should drop slightly. If no vacuum or valve doesn't rattle, replace PCV valve. Check all crankcase ventilation hoses for cracks or blockage. Verify breather cap or oil filler cap has proper ventilation.
10. 10

    Test drive and monitor symptoms
    Perform controlled test drive to replicate customer complaint. Note specific conditions when problem occurs (cold/hot, acceleration/deceleration, specific speed, etc.). Monitor engine temperature gauge. Test acceleration from stop, highway cruising, and part-throttle response. Listen for detonation (spark knock), backfiring, or hesitation. Note any flat spots in acceleration. Return and recheck vacuum and timing at operating temperature. Compare findings to initial symptoms.
11. 11

    Analyze findings and identify root cause
    Review all recorded readings and observations. Cross-reference compression test results with spark plug condition to identify cylinder-specific issues. Compare vacuum readings, timing, and fuel pressure to specifications. Correlate any vacuum leaks, ignition problems, or fuel delivery issues with reported symptoms. Determine if problem is ignition-related (timing, points, plugs), fuel-related (pump, carburetor, filter), mechanical (compression, valves), or combination. Document specific failed components or adjustments needed.

Reassembly

1. Reinstall any components removed during diagnosis (spark plugs, distributor cap, air cleaner)
2. Ensure all vacuum lines are properly connected and routed
3. Verify all electrical connections are secure, especially coil wire and distributor connections
4. Double-check that all test equipment has been removed from vehicle
5. Reset idle speed and mixture if adjustments were made during diagnosis

Verification

• Start engine and verify smooth idle with no hesitation, surging, or rough running
• Recheck ignition timing with timing light to confirm specification (5 degrees BTDC)
• Verify vacuum gauge shows steady 16-21 inches Hg at idle with no fluctuation
• Confirm fuel pressure remains stable at 4-6 psi during idle and acceleration
• Perform brief test drive to verify symptom has been identified and can be corrected with recommended repairs
• Document all findings, test results, and recommended repairs for customer review

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