Honda HS621 Owner's Manual page 13

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 snowthrower 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.
Oxygenated Fuels
Some conventional gasolines are being blended with alcohol or an ether
compound. These gasolines are collectively referred to as oxygenated
fuels. To meet clean air standards, some areas of the United States and
Canada use oxygenated fuels to help reduce emissions,
If you use an oxygenated fuel, be sure it is unleaded
and meets the
minimum octane rating requirement.
Before using an oxygenated fuel, try to confirm the fuel's contents. Some
states/provinces require this information to be posted on the pump.
The following are the EPA-approved percentages of oxygenates:
ETHANOL (ethyl or grain alcohol)
You may use gasoline containing up to 10% ethanol by volume. Gasoline
containing ethanol may be marketed under the name "Gasohol".
MTBE (methyl tertiary butyl ether)
You may use gasoline containing up to 15% MTBE by volume.
METHANOL (methyl or wood alcohol1
You may use gasoline containing up to 5% methanol by volume as long as
it also contains cosolvents and corrosion inhibitors to protect the fuel
system.
Gasoline containing more than 5% methanol by volume may cause starting
and/or performance problems. It may also damage metal, rubber, and
plastic parts of your fuel system.
lf you notice any undesirable operating symptoms, try another service
station or switch to another brand of gasoline.
Fuel system damage or performance problems resulting from the use of an
oxygenated fuel containing more than the percentages of oxygenates
given above are not covered under warranty.
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