here is another quirk I found with my 5 hp Briggs air cooled motor. last week I ran about 25 miles up and down the river before it started to happen, but cruising along at full throttle the engine suddenly revved a bit more and the boat slowed. Seemed very much like the prop was slipping. So before my next trip (last trip I took the boat on) I tightened the bolt down and it was in fact a tad loose. So on my last trip I was headed back to the ramp after a day fishing, and yet again my motor acted up. I could not run at full throttle without something slipping up pretty badly. Pulled the boat out and checked the prop - bolt had not loosened much if at all, but I double tightened it down as tight as I possibly could just in case. have not run it since then. Wondering what might be wrong if it's not the prop slipping? Does the nut in the prop have to be torqued down very tight or could something else be slipping up?