Mercury 135 Optimax User Manual page 24

INSTALLATION
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If changing conditions cause the RPM to drop below the recommended range, such as warmer, more humid
weather, operation at higher elevations, increased boat load, or a dirty boat bottom/gearcase, a propeller change
or cleaning may be required to maintain performance and ensure the outboards durability.
Check full‑throttle RPM, using an accurate tachometer, with the engine trimmed out to a balanced‑steering
condition (steering effort equal in both directions) without causing the propeller to break loose.
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