Honda CRF250F Owner's Manual Installation And Operation page 145

Use unleaded petrol with a research octane
number of 91 or higher.
20
If ''spark knock'' or ''pinking'' occurs at a
steady engine speed under normal load,
change brands of petrol. If spark knock or
pinking persists, consult your dealer.
Failure to do so is considered misuse, and
damage caused by misuse is not covered by
Honda's Limited Warranty.
Occasionally you may experience light
spark knock while operating under heavy
loads. This is no cause for concern, it simply
means your engine is operating efficiently.