Hello everyone. I need help on my 75 mercury 115 hp outboard engine.
The engine runs perfectly on neutral and on rear if the propeller is out of the water. However, if the propeller is in the water then the engine run properly only in neutral mode. I shift it in rear, then the engine lost power and runs very weak. It only push my 16 feet boat at about 3 miles per hour. I do not know what is the problems. Please help. Thank you.
So the engine reves up when you have it on the trailer, but when you put it in gear in the water and advance the throttle it just struggles and does not push the boat....that is either compression or spark you are missing. Check to make sure you have spark on all six, then get a compression testing from an auto parts store, warm her up, them test compression. They should +/- 15% of each other.
It reves up on land b/c there is no load on the motor, in the water if your missing a few cylinders it make a huge difference.