Question about high air temperatures and humidity on Johnson outboard/70 Horse J70ELC

[Big Bubba]

Happy Fourth of July weekend to all,
I have a question that might be good to put in the "Frequently asked questions section" of IBoats. Anyway, I took my pontoon boat up to Lake Jocassee in South Carolina and had a Great time with my family, a Air Force buddy that just got back from Afghanistan. Anyway, to make a long story short, does high air temperatures, say 95 degrees, and high humidity make your outboards run sluggish? Why I am saying this is yesterday toward the end of the day at 3 o;clock in the afternoon my outboard seemed to be a little weaker on running but never stalled out and quit and at wide open throttle was running at 5100 to 5200 RPM's at 16 mph earlier that day I was turning 5400 RPM's at 18 to 19 mph. What do you think my outboard seem to run fine otherwise. Thank you, Bob

 

[James R]

Re: Question about high air temperatures and humidity on Johnson outboard/70 Horse J7

Never known the temp' and humidity to change performance. When were the carbs last rebuilt and do you have a water trap filter in the fuel line. Loss of RPM is often an indicator of one cylinder not performing well.

 

[Big Bubba]

Re: Question about high air temperatures and humidity on Johnson outboard/70 Horse J7

Happy Fourth of July to all,
I just got through reading my OMC service manual for my motor and regarding the question if High ambient air temperatures, and high humidity affect performance of outboards, it definitely does. This is straight out of the OMC Service manual, 2 thru V-6 models dated 1982 and service manual part number 393765. states as follows on page 1-77: "The horsepower ratings of all motors are those that would be obtained under standard conditions. However, weather and altitude will affect the performance of any outboard motor. Some factors that will decrease the output of the motor are:
- Low barometric pressure
- High temperatures
- High humidity
- High altitude
"When running an engine on a Hot humid summer day, the performance of a motor can be decreased noticeably. Sometimes the weather or altitude alone can cause the engine rpm to drop below the recommended operating range. In such a case a lower pitch propellar can be used to bring rpm back into operating range, but in any case, top boat speed will not be as high as on a cool dry day." So today, since Lake Bowen in South Carolina is having a fireworks display on the water tonight I decided to put my boat in and keep it over at my mom's dock since she lives on the water. Today with just me inside the pontoon boat and on my way from the public boat launch to my mom's dock, I was able to get it up to 5,600 RPM's at 21 MPH and she seemed to be running strong.