787 Piston Cratered - What happened?

[HawkEye Media]

I have a '96 XP that I purchased in January for a project. It had an SBT motor, but the owner hadn't finished installing it yet. So I picked it up for a project ski. I finished setting up and installing the motor and I've got about 15 hours on it. It's been running great. I had also overhauled the carbs and had them off three times tweaking the pop off pressure.

My son was riding it and putted back to the beach and said "it's not running right". Hah, no kidding. I pulled it home and discovered the mag cylinder had no compression at all. So I pulled the engine back out and this is what I found:



It's obvious a piece of the ring broke free and beat the top of the piston up, but I'm trying to understand why. My feeling is this cylinder ran lean, but the carbs and fuel system are clean. I replaced all of the fuel lines and cleaned the tank and fuel valve before putting the engine in.

I think I'm going to swap it with another SBT block, but I don't want this to happen again!

So my question is, what went wrong?

Regards,
Mark

 

[Robj]

My guess is that was due to a lean condition or an air leak. What did the plugs look like?

 

[HawkEye Media]

The spark plug on the right is from the mag cylinder and is darker than the pto plug. They've been about the same color up until this happened. I pull them out and spray some tri-flow oil in the cylinders for rust prevention when I'm storing the skis. The compression in both cylinders had been 150.




When I removed the carbs, all of the hardware was tight and the gaskets were in place.

 

[Matt Braley]

My guess is that was due to a lean condition or an air leak. What did the plugs look like?

Yeah, what he said. Poor fuel quality and the timing being off can also cause the detonation damage you have.

 

[raider352]

you said you pulled the carbs 3 times... did you happen to replace the base gasket for the carbs at all? just wondering.

 

[mikidymac]

To me it does not look lean. The piston and plug show signs of sufficient oil but if you look where the ring broke it is in direct line with the exhaust port. It is possible that the exhaust port was not chamfered enough or the RAVE valve was not clearanced enough. Looks like you snagged a ring to me.

 

[KyleB]

To me it does not look lean. The piston and plug show signs of sufficient oil but if you look where the ring broke it is in direct line with the exhaust port. It is possible that the exhaust port was not chamfered enough or the RAVE valve was not clearanced enough. Looks like you snagged a ring to me.

That's what I was thinking also. Check the RAVE and see if you see evidence of where the ring snagged it. If not then look at the exhaust chamfer.:cheers:

Kyle

 

[Matt Braley]

Yeah, I'm kinda feeling the Noid's point also after seeing those plugs. They don't have the white, lean look. Sometimes part of the plug will be ate up too. Mark seems like he is pretty with it too, so that helps rule out newb fault possibilities.

 

[skewba98z28]

I have a '96 XP that I purchased in January for a project. It had an SBT motor, but the owner hadn't finished installing it yet. So I picked it up for a project ski. I finished setting up and installing the motor and I've got about 15 hours on it. It's been running great. I had also overhauled the carbs and had them off three times tweaking the pop off pressure.

My son was riding it and putted back to the beach and said "it's not running right". Hah, no kidding. I pulled it home and discovered the mag cylinder had no compression at all. So I pulled the engine back out and this is what I found:

View attachment 13022

It's obvious a piece of the ring broke free and beat the top of the piston up, but I'm trying to understand why. My feeling is this cylinder ran lean, but the carbs and fuel system are clean. I replaced all of the fuel lines and cleaned the tank and fuel valve before putting the engine in.

I think I'm going to swap it with another SBT block, but I don't want this to happen again!

So my question is, what went wrong?

Regards,
Mark

It might still be under warranty but you did pull the head...... Call them with the number that is on the motor and you might be in luck. If it is under warranty explain to them you did pull the head to see what went wrong.

 

[Dennis]

I don't think it was from being lean either. The piston edge would have more of a smooth "melted" look. I'm thinking RAVE valve issue.

 

[s14unimog]

and he got 15hours of run time out of it?? Surely a RAVE valve clearance issue would have shown its self right away. Just a thought.

 

[Dennis]

I agree. Did he look to see if the valve broke? Is there a corner of the valve missing? I've seen both situations before. :cheers:

 

[riding high]

The spark plug on the right is from the mag cylinder and is darker than the pto plug. They've been about the same color up until this happened. I pull them out and spray some tri-flow oil in the cylinders for rust prevention when I'm storing the skis. The compression in both cylinders had been 150.

View attachment 13023
View attachment 13024

When I removed the carbs, all of the hardware was tight and the gaskets were in place.

I just finished doing a top end on a 787 that had the exact same indications. I am guessing that the plug in the bad cylinder was still getting oil as it was dying and therefore appears that it was running rich. I would not be suprised to see some damage to the rave valve. I had damage to the rave also, but when I checked it there was no way that the rave could have got into the ring. Since your engine is fairly new I suspect that any damage to the rave was cause by the avalanche. I wouldn't be a bit surprized if you don't find a fuel delivery problem to that cylinder.

 

[kobiskytony]

To me it does not look lean. The piston and plug show signs of sufficient oil but if you look where the ring broke it is in direct line with the exhaust port. It is possible that the exhaust port was not chamfered enough or the RAVE valve was not clearanced enough. Looks like you snagged a ring to me.

Had same problem, w/97 seadoo GSX, found i had put my rave valve blades in improperly, should be some markings( top),on the blade for direction,replaced the piston& rings,put the blade in with the top marking up, no more problem for me. Hope this helped.