O.S. Engines FS-91 II FI - Manual (Page 3)

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This engine was designed for model aircraft. Do not attempt to use it for
any other purpose.
Mount the engine in your model securely, following the manufacturers'
recommendations, using appropriate screws and locknuts.
Be sure to use the silencer (muffler) supplied with the engine. Frequent
exposure to an open exhaust may eventually impair your hearing.
Such noise is also likely to cause annoyance to others over a wide area.
Install a top-quality propeller of the diameter and pitch specified for the
engine and aircraft. Locate the propeller on the shaft so that the curved
face of the blades faces forward-i.e. in the direction of flight. Firmly tighten
the propeller nut, using the correct size wrench.
Always check the tightness of the propeller nut and retighten it, if
necessary, before restarting the engine, particularly in the case of four-
stroke-cycle engines. A safety locknut assembly is provided. Always use it.
This will prevent the propeller from flying off in the event of a "backfire",
even if it loosens.
If you install a spinner, make sure that it is a precision made product and
that the slots for the propeller blades do not cut into the blade roots and
weaken them.
Discard any propeller which has become split, cracked, nicked or
otherwise rendered unsafe. Never attempt to repair such a propeller:
destroy it. Do not modify a propeller in any way, unless you are highly
experienced in tuning propellers for specialized competition work such as
pylon-racing.
Use an electric starter for this engine. The wearing of safety glasses is also
strongly recommended.
Take care that the glow plug clip or battery leads do not come into contact
with the propeller.
Also check the linkage to the throttle arm. A disconnected linkage could
also foul the propeller.