i had same problem with a 115 merc on my sled,allthough i used mine all the time. talked to 5 different marine techs and got 10 different answers to what caused it. was the #3 piston. i was lucky i rebuilt it myself and put a size larger jet in that cylinder to help #3 run cooler.not sure if it was the right thing but it has run great with no problems ever since.ask your marine guy about that.