Ignition timing, in a spark ignition internal combustion engine (ICE).
Advance or retard the ignition timing by rotating the distributor in small increments. This is a "Timing by ear" method of adjustment because you set the timing by sound. In a vast majority of cases, the spark should fire at a certain angle before top dead center (BTDC) because fuel does not completely burn the instant the spark fires, the combustion gasses take a period of time to expand.
At higher altitudes, more initial advance is required to ignite the thinner air fuel mixture.
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