This post might sound like a bash, but I'm not, just asking and throwing out ideas so please don't take any of my comments/questions the wrong way
All the sand................normal in every 787 I see around me. Wouldn't worry me in the least. If you really want to be sure, how much was in the back side of the cylinder near the drains? If the drains were blocked then maybe I'd agree. You'd have to pack it pretty good to overheat that cylinder but that little bit of mung is nothing. How do you correct it? Shut off and coast into the shore and push out to about waist deep before you start. You can't run these right up to the shoreline.
32:1 is wrong for that engine, unless you're still breaking in. 40:1 is where you want to be. A seadoo at WOT is running the oil injection pump at 40:1
When you rebuilt the engine, did you align the oil pump tick marks? Possibly you were off and putting too much oil into it at idle.
Was the oil pump still using the original white oil pump drive not the later black style. Or did you use the SBT one and not the OEM black one?
I'll assume that you didn't pressure test the engine. While you have the cylinders off, take a straight edge and go across the top of the top case where the cylinder bolt holes are. See if you can slip a piece of paper under the straight edge between the bolt holes, if so that's not good. 787's will pull the threads up creating rise in the surface, I've had several pass thru my hands like that. I also noticed there is no sealant on your base gasket, dry is usually fine but I just wipe a little on the base gasket to ensure a good tight seal, did you put sealant between the cases before you reassembled it? I'm also not seeing a new gasket on the mag housing to case, what about the crank seals at the end of the engine, are they old or new, assuming old since you didn't remove the PTO in your rebuild thread?
I battled the same scenario you are. MAG piston on my 787 just burning down. Pressure test revealed an air leak. But mine was between the case halves. Engine would last 6 hours. You need to use Threebond 1211 (or the like---not starting a debate on what's best). Yamabond or Hondabond are equivalents, just not RTV. Oh, and I switched to premix cause I was sold it was injection issues. Yeah, I'm running that oil pump on one of my skis for the last 5 years. So, it wasn't the pump. I also had a shop look at it and another engine builder and I was the one who found the air leak. I was done paying guys to work on my boat.
I'm with Mik, you have fuel issues or an air leak. Looks like it's just your MAG piston that failed. Is the PTO piston still in good shape? If so, how can the oil pump the correct oil to one piston but not the other, unless you have a broke line or clogged injector tip (doubtful)? If that was short on oil that rod would be dry and discolored and the crank would sound like a bag of marbles.
With all the sealant, someone else was already in there before you. Did you scrape all that off and reseal it? Also looks like someone used an air angle grinder with a cookie cutter pad on it to clean the cylinder surface on the case which I don't agree with, it will round the edges over and possibly create a low spot in the surface. Doesn't take much on that cheap aluminum Seadoo uses. You can see the swirl marks on the surface under the gasket.
Just looking at you build thread, one piston looks great the other meh, no good. Should be enough to confirm the oil pump is fine.
Burnt lower pipe----water not flowing from the Water control valve on the water box. Starving the pipe from water and torching the coupler to the water box
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