Honda HP400 Owner's Manual page 13

Fuel Recommendations
Use unleaded gasoline with a pump octane rating of 86 or higher.
This engine is designed to operate on unleaded gasoline. Unleaded gasoline
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 a 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
Joad, change brands of gasoline. If spark knock or pinging persists, see an
authorized Honda servicing 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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