Under the Bonnet: Fall 1991

Cleaner Running

Emission control, once dirty words to many automobile enthusiasts, is now accepted as a necessary fact of life. Even those of us who drive pre-pollution control cars can do our part to minimize our cars’ adverse effects on the air we breathe, and, at the same time, have our engines run better and last longer. It’s not magic that is needed to accomplish this, but rather a thorough and accurate tune-up, followed by simple by-the-book maintenance. On cars equipped with emission controls, if everything is working properly, and is in proper condition, the car will pass an emissions test. If it does not pass, something on the car isn’t functioning properly.

All cars work better and pollute less when brakes are not dragging, wheel bearings are properly adjusted and lubricated and when driven sensibly. Even wheel alignment makes its contribution. The following outline should serve as a general guide to cleaner, better driving; you’ll find complete instructions and details in the workshop manual appropriate for your particular car.


  • Distributor
    • In good mechanical condition. (Rotates freely without shaft wobble, advance mechanisms working correctly.)
    • Adjusted correctly (points and timing).
    • In good electrical condition (Cap, rotor, points, low tension lead, insulation).
  • Coil & Wires
    • Coil output (best checked with an oscilloscope).
    • Wires of correct type and in good condition.
    • Good, solid electrical connections.
    • Correct coil polarity.
  • Spark Plugs
    • Correct type: reach, heat range. (Reach is the length of the threaded shank; heat range refers to the spark plug’s ability to transfer heat—not how hot the spark is.)
    • Clean and correctly gapped.
    • Sealing washer in good condition.


  • Good mechanical condition. (Clean; linkages free, but without excess looseness; throttle shafts unworn; fuel, vent and over flow fittings and connections tight and unobstructed; no vacuum leaks.)
  • Properly adjusted. (Mixture, float height, slow idle, fast idle, choke, synchronization of multiple carbs.)
  • Clean and unrestricted air filter. (A dirty/clogged air filter will dramatically enrich your air fuel mixture.)
  • Correct amount and type of oil in SU and Zenith-Stromberg dashpots. (Generally, the heavier the oil, the richer the mixture on acceleration.)


An important factor is equal power output from each cylinder. This is largely governed by equal compression in all cylinders (assuming proper carburetion and ignition).

  • Valves (clearance, seating, timing).
  • Piston rings.
  • Head gasket.
  • Valve guides (not worn).
  • Pushrods (not bent, correct length).
  • Rocker arms and shaft (not worn).

Venting of assemblies is vital to their proper operation. A plugged differential vent, for example, can cause oil to be flown out past the oil seals—dangerous to the differential, a mess on your car and polluting to the environment.


  • Engine (crankcase breather).
  • Carbs (dashpots, overflow).
  • Gearbox and differential.

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