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Ignition System Service page 3   
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    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.
  The photograph above identifies the timing marks on ‘49 and newer engines.  UDC is upper dead center, now called top dead center (TDC).  The remaining marks are 3 and 6 degrees before TDC.
  The next photograph shows flywheel marks from the 1940 Pontiac Service Manual.  The line at the left is TDC, the next line is 2 degrees before TDC and the final line is 6 degrees before TDC.
  I run my cars at 6 degrees before TDC and even then have found they will accept additional advance with the Gaselector before “pinging” on acceleration.  Pontiac recommended the timing be advanced with the Gaselector to “borderline detonation”, which is barely audible “pinging” when the accelerator is fully depressed between 20 and 30mph on a level road.   
    This photo illustrates the proper orientation of the distributor in the engine; the vacuum advance unit should be at about 8 o’clock.  Note that the vacuum tube from the carburetor extends down from the advance unit, this is done by design!  There is always the possibility of gasoline getting down the tube into the advance unit, so they are positioned to allow any gas to drain out and prevent the diaphragm from being compromised.  It is a good idea when doing your tune-up to remove the tube and allow it to drain.  Worn engines with heavy “blow-by” will typically force engine oil into the vacuum advance unit, as well.
  A good tune-up will pay you huge dividends with regard to economy and reliability of your Pontiac!
Ignition System Service