Honda WMP20X Owner's Manual page 18

PRE-OPERATION CHECKS
Fuel Recommendation
We recommend unleaded gasoline because it produces
fewer engine and spark plug deposits and extends exhaust
system life.
Never use stale or contaminated gasoline or 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 water
pump dealer.
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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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