1988 Johnson Evinrude "CC" 9.9 thru 30 Service Manual, P/N 507660, Page 52Get this manual
Weather, Altitude and Engine Performance The horsepower ratings of all engines are those that would be obtained under standard conditionsHowever, changes in weather and altitude will affect the performance of any outboardSome factors that will decrease the output of an engine areLow barometric pressure High temperature High humidity High altitude
Performance can decrease noticeably when an engine is run on hot, humid daySometimes the weather or altitude alone can cause the engine RPM to drop below the recommended operating range In such case, lower pitch propeller can be used to bring RPM back into the operating rangeIn any case, top boat speed will not be as high as on cooldry dayA loss of engine performance can occur if an engine has not been properly modified for high altitude operationIf you service high altitude engines, consider ordering Information and Decal Kit, PI 393533, from your distributorTo avoid permanent powerhead damage, be sure that an engine modified for high altitude operation is properly identified and returned to original calibration if operated at or near sea level