Honda F501 Owner's Manual page 12

Gasoline Recommendation
Pump octane rating: 86 or higher
We recommend unleaded gasoline because it produces fewer engine and
spark plug deposits and extends the exhaust system life.
If "spark knock" (metallic rapping noise) or persistent "pinging" occurs at
a steady engine speed under normal load, change brands of gasoline. If
spark knock or pinging persists, see an authorized Honda tiller dealer.
1 IMPORTANT NoT'CE 1 Running the engine with perisistent spark knock or
pinging can cause engine damage.
Running the engine with persistent spark knock or pinging is considered
misuse, and the Distributor's Limited Warranty does not cover parts
damaged by misuse.
Occasionally you may hear light spark knock while operating under heavy
loads. This is no cause for concern. It simply means your engine is
operating efficiently.
Never use stale or contaminated gasoline or an oil/gasoline mixture. Avoid
getting dirt or water in the gas tank.
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