Ignition Adjustment
...also called Spark Timing, Advance Setting, etc.
The idea is to get the spark to ignite the fuel-air mixture
at the right time so that the resultant combustion imparts
the maximum possible energy to the piston at any engine speed.
Adjusting the Spark Timing
The timing of the spark, like that of most things in life,
is of utmost importance.
Recall that the combustion-front formed at the spark-plug
is moving down while the piston is moving up; and ideally,
they meet at TDC and the piston is thrust down to BDC,
by the combustion-front in the Power-Stroke.
* What happens if the spark takes place too early ?
The combustion-front hits the piston yet to pass the TDC.
This "head-on" collision pushes the crankshaft
in a direction opposite to that intended,
which is nothing but the dreaded "back-kick" of the Bullet.
In less extreme cases, the engine will run,
but with a pinging sound from the collisions,
and less power due to losses in the said collisions.
Prolonged running in this condition can damage the piston,
extensively, and so is to be avoided, scruplously.
* What happens if the spark takes place too late ?
If the spark is too late, the combustion-front hits the piston
when well past the TDC, already far and accelerating rapidly away.
The transfer of energy to the piston is less,
and the mixture may be still burning when the exhaust-valve opens
discharging it into the silencer.
Maybe overheating & oxidising the exhaust-bend to blue colour.
Prolonged running in this condition overheats the entire engine,
ultimately leading to seizure in extreme cases.