720 motor blown

[swill777]

So I had this going on another forum.
and Timmyboy has been helping me out a great deal too.

I took the head off to check it out, and plan on getting this motor out this week. Still trying to figure out exactly how this happened. I don't know if my pressure gauge is accurate, but on the other motor, both cyls show 125psi, and the other one on this blown motor showed 125psi as well before I pulled the head. I plan to dig in soon to find out what went down. Someone told me detonation could have blown the hole in this piston... perhaps from running lean at some point.

Obviously pieces of the piston fell down into the bottom end of the engine.. so there is no telling how damaged it is.

Here are the carnage pics:

 

[PWCdoc]

Looks like your crank let go - not detonation or a lean condition. Go ahead and plan on a complete rebuild.

 

[PWCdoc]

Looks like your crank let go - not detonation or a lean condition. Go ahead and plan on a complete rebuild.

 

[timmyboy76]

looks like the needle bearing gave way...

 

[swill777]

So the motor is out. When I turn the PTO wheel, the pistons go up and down like normal, and don't feel loose or "broken" in any way. I also get a suction type sound from the intake after each cycle.
The cylinder wall of the "bad" cylinder even looks ok.
What's the deal? Can I just replace this one piston?

I'm going to take the exhaust off, then the intake and rotary valve cover to take a look at that. I assume the piston is still connected properly to the connecting rod if everything is turning over ok.

 

[kcr357]

Im guessing the one piston fragged from being old and worn out, then a small piece made its way out the exhaust port, then got sucked into the other cylinder and made the marks in the other cylinder. How many hours were on that engine?

 

[Johawk]

I would love to know what happened. It looks exactly like the 650 that came out of my speedster. Machine shop tells me the cylinders have been bored 1mm already and now Im heading for 2mm. Something keeps burning pistons in my boat... whether its fuel starvation or an oiling issue. The second ring on both pistons were welded in the ring land. Looks as if we are both in the market for a replacement cylinder head. :cheers:

 

[timmyboy76]

2mm...

I would love to know what happened. It looks exactly like the 650 that came out of my speedster. Machine shop tells me the cylinders have been bored 1mm already and now Im heading for 2mm. Something keeps burning pistons in my boat... whether its fuel starvation or an oiling issue. The second ring on both pistons were welded in the ring land. Looks as if we are both in the market for a replacement cylinder head. :cheers:

Damn, thats goin BIG...if not modding/upgrade'n the rods..etc. You got, std, .25, .50, .75, then 1mm. Might need to check another mechanic, if try'n to keep stock. You definetly need to rejet for something that drastic.

 

[kcr357]

How was it running before you noticed a problem? Any noise before it happened? After?

 

[timmyboy76]

rv area..

Hey Swill...if carbs are off, guess it dont matter, but take RV plate off, and see if one of the brass nipples from inside the manifold, fell off and went for ride thru motor...

 

[swill777]

rotary cover

nah, they're still on there.
motor was a bit noisy and ran fine, until I simply backed the throttle down and turned the boat off for an hour while we stopped somewhere. It was the weirdest thing. Although, my Wife says she heard a "noise" about 15 mins before it died. So I guess she has good hearing. Check pics.

 

[ski-d00]

here's my opinion...

it looks like one of your piston rings failed and came apart, look carefully at them and I thnk you'll see it's not all there. All those marks in the head are from a piece of a ring bouncing around, the rings are really hard steel in comparison to the soft aluminum of the head and piston.
why did the ring fail? not sure, could be fatigue, could have snagged a port, etc..
rv surface doesn't look too bad, seen worse and it can easily be remachined. when large grooves connect one opening to the other, that's bad.

I would say a complete rebuild would be a good idea and you'll probably find pieces of the ring in the bottom of the cases.

 

[robin savell lloyd]

it looks like one of your piston rings failed and came apart, look carefully at them and I thnk you'll see it's not all there. All those marks in the head are from a piece of a ring bouncing around, the rings are really hard steel in comparison to the soft aluminum of the head and piston.
why did the ring fail? not sure, could be fatigue, could have snagged a port, etc..
rv surface doesn't look too bad, seen worse and it can easily be remachined. when large grooves connect one opening to the other, that's bad.

I would say a complete rebuild would be a good idea and you'll probably find pieces of the ring in the bottom of the cases.

Yes I do believe that you are correct good eye. :agree:

 

[swill777]

yeah a piece of the ring definitely came off... i was just trying to figure out if this was from running too lean, water getting it it, etc... i bought it used after 10 years of who knows what. my new core came in today so I'm in the process of replacing the motor with a sbt rebuild... i also got 2 mikuni oem carb rebuild kits.

 

[robin savell lloyd]

yeah a piece of the ring definitely came off... i was just trying to figure out if this was from running too lean, water getting it it, etc... i bought it used after 10 years of who knows what. my new core came in today so I'm in the process of replacing the motor with a sbt rebuild... i also got 2 mikuni oem carb rebuild kits.

You sound like you will be in good shape.

 

[seadoodude1]

Looks like a lean burn to me the way the piston is eaten/melted away around the edges leading to ring failure. A lean burn is hot enough to melt the aluminum pistons. First thing to check are the carbs. Have they ever been rebuilt? Could also be an air leak into crankcase from a hole in the pulse line or RAVE valve vacuum line.

 

[swill777]

I bought the boat used after 10 years of who knows ?
I cleaned the carbs a few months ago, and the motor seemed to be running fine. I got the mikuni rebuild kits now, and they are both getting rebuilt.
I don't have rave valves on this 720 motor, but I will check to make sure the pulse line doesn't have a leak. Thanks.

 

[swill777]

more pics

here are some more pics...