Classic/Antique Car Repair: engine timing, yellow paint, atdc

Question
Dick,

I've been working on cars for 20 yrs and today I had to question a very simple thing I've always taken for granted.....how to read a timing cover gauge!  I'm working on a 67 Lemans with a 326 and have the original engine.  Timing is supposed to be set at 6 degrees BTC.  I've always translated that to mean -6 on the cover....however I noted that the timing cover has 3 numbers on it....+6, 0 and -4....I tried getting it to -6 which would be 2 degrees off the cover gauge, car started running real rough so I swung it back and hit the +6 where it smoothed right out...I got to thinking about why Pontiac would bother with + numbers on a cover when they're always BTDC instead of ATDC and why wouldn't they put a mark on the cover for what that engine called out for.  Can you confirm I'm a moron and have been timing my engines to the wrong side of TDC for years!?  My knowledge of math has betrayed me....a negative number comes BEFORE zero, a positive number comes AFTER zero so 6 degrees before TDC (0) should be -6 !!!

Thanks in advance for wasting time on a stupid question!
Dennis

Answer
The easy way to think about this is that the engine rotates clockwise when viewed from the front of the engine, so timing it to a BTC setting means the spark is going to occur BEFORE the top dead center line is reached.  I suppose they use the + sign to indicate advanced timing, so 6 BTC would be +6 on the dial.   
The "-" mark is probably there for setting valve timing.

On my cars, I always put a very clear yellow paint dab at the desired setting, then I can see it easily when setting up an engine.

Be sure to check the point dwell first - it affects the timing setting, but not vice-versa.

Another way to set timing is to do it by ear - advance the timing until you just barely hear "ping" on moderate acceleration in high gear at about 30 MPH.  Have the same gas in it that you are going to run with. This is the optimum setting, and may or may not be the same as what the factory specified, due to buildup of carbon in the combustion chamber, variations in altitude, gas octane etc.

Dick