Honda EM650 Owner's Manual page 24

Occasionally you may hear light "spark knock" or "pinging" (metallic rapping
noise) while operating under heavy loads. This is no cause for concern.
If spark knock or pinging occurs at a steady engine speed, under normal load,
change brands of gasoline. If spark knock or pinging persists, see an
authorized Honda generator dealer.
[ NOTICE 1 Running the engine with persistent spark knock or pinging
can cause engine damage.
Running the engine with persistent spark knock or pinging is misuse, and the
Distributor's Limited Warranty does not cover parts damaged by misuse.
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