Jim Smart
August 13, 2010

If there's spark and fuel, here are other items to check.
1: Does the engine crank normally or does it crank unevenly like a spark plug has been removed?

2: If it cranks unevenly, check compression. Compression should be close across the board. Unusually low compression on one or more cylinder indicates a serious mechanical problem such as failed timing set, broken camshaft, blown cylinder head gasket, cracked cylinder head or block, or damaged piston.

3: If there's a failed timing set, you will hear the tell-tale clicking of valve-to-piston contact along with an unfamiliar cranking pattern. Stop cranking immediately to prevent engine damage. Sometimes, there's also backfiring through the intake or exhaust system.

Engine Runs Rough

  • Carburetor choke sticking (flooding with rich mixture).
  • Idle-air mixture bleed(s) plugged or out of adjustment.
  • Intake manifold vacuum leak-gasket, carburetor spacer, vacuum hose.

1: Manifold vacuum should be 15-22 inches at idle.

2: Spray carburetor cleaner around suspected vacuum leak area. If engine surges, you've found the vacuum leak. If engine operation does not change, keep searching.

3: Vacuum leaks at carburetor base and spacer are typically due to warpage or defective gaskets. Check surfaces with a straight edge. A machine shop can mill these surfaces to get them true.

4: Flaws in carburetor castings can cause vacuum leaks.

5: Throttle shaft vacuum leak - worn shaft and bushings.

6: Vacuum leak at automatic choke.

7: Manifold gasket leakage. If underneath, engine oil is drawn into intake ports, which will cause blue or white exhaust smoke. This typically happens with engine rebuilds where heads and block decks have been milled, which creates manifold vacuum leaks.

8: A faulty PCV (or hose) valve can cause an irregular vacuum leak and rough operation.

9: Ruptured vacuum modulator at transmission (C4, C6, and FMX). Not only causes a vacuum leak, but fluid loss from the transmission because fluid is drawn into the engine intake and burned, also causing smoke from the exhaust.

Engine Stalls

  • Idle Speed set too low.
  • Vacuum leak (see previous).
  • Ignition point or PerTronix Ignitor gap improperly set.
  • Distributor breaker plate ground wire not installed.
  • Ignition timing too late.
  • Sticking float or float level set too high (flooding).
  • Valve adjustment too tight.
  • Vapor lock (hot weather).
  • Frozen fuel line.
  • Excessive water in fuel or other form of fuel contamination.
  • Old fuel that's been sitting (will smell like varnish).

Engine Misfires

  • Faulty spark plugs or spark plug fouling.
  • Ignition wire insulation breakdown and crossfire.
  • Weak battery.
  • Improper ignition point gap.
  • Weak distributor ground (check breaker point ground strap).
  • Faulty ignition coil.
  • Cracked distributor cap or rotor.
  • Poor distributor rotor continuity.
  • Wet ignition system.
  • Intermittent short in primary or secondary ignition wiring.
  • Blown head gasket between cylinders (do a compression check).
  • Fuel mixture extremely lean.

Poor Performance Under Acceleration

  • Ignition timing too far advanced (pinging/pre-ignition).
  • Ignition timing too late (runs like a slug and exhaust is loud).
  • Improper centrifugal advance curve.
  • Aggressive vacuum advance curve.
  • Point bounce.
  • Faulty ignition coil.
  • Defective ignition wires.
  • Throttle linkage improperly adjusted (not opening all the way).
  • Secondaries not opening on four-barrel carburetor.

Smoke From Tailpipe

  • Under acceleration, mixture too rich or excessive piston ring wear.
  • When first started, excessively worn valve guides and/or valve seals.
  • Steady smoke at idle means excessive piston to cylinder wall clearance and worn piston rings.

  • Black Smoke: Rich fuel mixture.
  • White Smoke: Engine oil or automatic transmission fluid via ruptured vacuum modulator diaphragm.
  • Blue Smoke: Engine oil.

Spark Knock (Pinging, Detonation)

  • Ignition timing too far advanced.
  • Anything beyond 36-38 degrees BTDC total timing will do engine damage under hard acceleration.
  • Fuel fixture too lean.
  • Fuel octane rating too low for compression ratio.
  • Hot spots in combustion chambers (carbon, sharp edges, spark plug heat range is too high).

Flat Spot or Hesitation

  • Faulty carburetor accelerator pump or pump adjustment
  • Fuel quality poor (low volatility from poor refinement or age)
  • Hot fuel due to hot weather
  • Late ignition timing