Hi all! I have a 2000 XR175 Sportjet, I was running up the Sky (been running for about half and hour) when the motor started dropping rpm's quickly and knocking, I shut it off. I jumped on the trolling motor and got to a safe spot. Started it up and it was knocking but couldn't tell what it was, shut it off and cleared the pump via the stompgrate. Started it up again and it was fine. Jumped back up on plane, about 15 seconds later, lost power again and this time it froze.

The mechanic said I had lost a piston due to detonation. The rest of the motor is in great shape, pistons and all. The shoulder of the piston about a quarter (a little less than a quarter) of the way around was gone about a quarter of an inch down. Spit the rings, etc.

I am curious if anyone has had similar problems or any input on this event. The boat is up and running again now, new head, bored one cylinder and new piston. They said it was due to bad gas.

The boat was run only 5 times or so a year for the last 2 years and I always kept the tank full (to keep condensation down). Would add gas when it got down to half a tank. Has a 50 gallon tank. Anyone else experience this or is it possibly something else? Thanks for the help.

Doubletake

5x per year for 2 years is really low usage. Since you kept the tank full, you may have retained some really old gas in there, and even with the tank full, the various compounds in the gas will react chemically over time and go sour/stale/bad. Some of these compounds may have a lower flash point than normal, and so you get detonation.

In the future, run the tank close to empty at least once every year, like in the spring and in the fall, then over the winter keep it topped off and add stabilizer. If you just don't run the boat enough to do this, drain the tank into your truck every spring and fall, and refill with fresh fuel.

BTW, a fuel/water separator doesn't help matters if the fuel has broken down due to age.

Regards, and sorry about your repair bill

Finegrain in Woodinville

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.