Honda 125cc - Ignition Timing Adjustment?

Two bikes, same make but different year model, climb a mountainous road,

a) BIKE-A (Honda) - 125cc, Single Cylinder, Capacitor Discharge Ignition (CDI)
b) BIKE-B (Honda) - 125cc, Single Cylinder, Point Ignition

After running for say about 20 minutes, BIKE-A looses its power whereas, BIKE-B maintains its power/speed.

1) Why does BIKE-A looses its power?

2) Does BIKE-A needs retardation for the Ignition Timing?

3) If yes, how do we know, how much retardation is required?
Added (1). The scenario is just an example. It always happens this way.

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