I have a 97' Yamaha 90hp on a 17' Skeeter bass boat. I just purchased it from a dealer and it has been running great for the 4 or 5 times I've had it out. Yesterday, while running down the boat lane trimmed up at WOT, I trimmed down to make a sharp turn but did not throttle down. The motor made an erratic knocking sound like someone was pecking on the lower unit with a hammer lightly. I could feel it in the throttle handle but didn't notice an RPM change. I stopped and then powered up again to WOT, trimmed up again to max RPM (5200) and when I trimmed down I heard the same noise only a little louder, still no RPM change but I felt the boat hesitate which coincided with the knocking noises. I had someone positioned at the motor to see where the sound was from and he said it definitely sounded like it was coming from the lower unit area. The sound is like a knock or maybe what I think backfiring thru the carbs. would sound like. Not pinging. But a muffled solid knocking noise. I pulled the spark plugs and looked at the top of pistons and all looks good. Any suggestions before it goes to shop?
