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Ignition System Service page 2

   The third diagram I have included illustrates what is meant by distributor “dwell” or “cam” angle.  Dwell is the number of degrees of breaker cam rotation that the contact points are closed.  The dwell number increases inversely to the point gap, a larger gap results in a smaller dwell reading and vice versa.  The other thing to remember when adjusting the dwell is its effect on timing.  Dwell changes ignition timing, but ignition timing does not change dwell.   
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You should always make your final point adjustment with a dwell meter, a feeler gauge is good enough to get the engine started, but you need to check and adjust the points to get the proper dwell reading.  Since the engine is running when reading the dwell, you are seeing an average including each lobe of the breaker cam.  If the dwell reading is unstable, it is likely that the upper bushing in the distributor housing is worn out and allowing the shaft to wobble.  The only fix is to rebuild or replace the distributor.  A good tune-up man could find this condition in a few moments with an oscilloscope.  The “scope” shows each lobe of the breaker cam individually and when these readings are stacked vertically on the screen you can actually see the lines curve due to the wobble of the shaft.  A distributor machine like the one that Dave Luken brought to Amana last year will also find this condition; it also shows each lobe individually to the operator.
 If you are careful and always achieve the same dwell reading during your tune-up, you will find that the base ignition timing remains where you set it last, excepting any change with the Gaselector.
  You need to visually inspect the distributor cap and rotor for excessive wear of the contact terminals or corrosion.  It is not uncommon for the cap to get oily from vapors exiting the oil filler tube.  Electrical contact cleaner is the best product to use when cleaning the distributor cap, rotor or any other electrical components, although good quality “brake” cleaner is also suitable.  Neither of these leave any residue like mineral spirits or aerosol carburetor cleaner would.
  We are all familiar with the oil vapors our cars disburse from the oil filler and road draft tubes.  All of the external surfaces of ignition system components should be kept clean of this residue.  You should carefully wipe off the plugs and wires from time to time, and spray the distributor cap and top of the ignition coil with contact cleaner.  I do not recommend spraying anything on these components, clean and dry is best.  In an emergency, certainly use WD-40 or other similar product to displace moisture so you can get your car running again, but you should fix it properly at the conclusion of your trip to prevent further troubles.
  It appears that Pontiac did not install synthetic rubber boots on the spark plugs until the early fifties, but they were available at the parts counter to be retro-fitted by the owner and I recommend using them.  It is also beneficial to use a small amount of silicone dielectric grease on each end of the plug wires.  It will seal out moisture and keep the wires from becoming stuck to either the spark plug or in the distributor cap.  Silicone dielectric grease is a very useful product for any electrical connections where moisture or corrosion could be a problem.  
    Now that the distributor is cleaned, lubricated and the contact point adjustment has been set and confirmed with your dwell meter, the final adjustment for our ignition system service is setting the timing.  The ‘48 and older models have the timing marks on the flywheel, as seen through an access hole on the driver’s side near the starter.  On ‘49 and newer cars the timing marks are on the harmonic balancer at the front of the engine.
  The majority of Pontiac engines have a “Gaselector” as shown.  This allows for the final timing adjustment to be made under road-test conditions.  Clean off the degree scale and set the pointer at 0 (zero) degrees by loosening the bolt on the Gaselector arm; then tighten the bolt again.
  You should also clean the timing marks on the flywheel or harmonic balancer before attempting to use your timing light.  A crayon or chalk is particularly helpful to highlight the timing marks, especially those on the flywheel.  The initial timing adjustment is made by loosening the clamp around the base of the distributor housing, not the Gaselector.  
Ignition System Service