ORIGINAL: freebird1
It is on the way down(way after TDC). The sencor is one that is adjustable. The fly wheel has marks to show TDC. I have already taken the plug out to make sure it was at TDC. I guess my question is should fireing be at TDC or shortly after. THANKS!!!!
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The reason that it is called spark advance is because the spark fires before the piston reaches TDC. This is done because it takes a while for the mixture to combust entirely. Firing as the piston is going down is of no benefit at all. All of this assumes that you are looking at the front of the engine and the crankshaft is being turned counter clockwise. Good luck.
Ed Cregger