Honda Automobiles HRM215 Harmony Owner's Manual page 29

Fuel Recommendations
Use gasoline wi.th a pump octane rating of 86 or higher.
We recommend
unleaded fuel because it produces fewer engine
and spark plug deposits and extends exhaust system life.
Never use stale or contaminated gasoline or an oil/gasoline
mixture.
Avoid getting dirt or water in the fuel tank.
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 lawn mower dealer.
1 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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