What would cause a 787 base gasket to blow out?

[Lt1350]

Hello. I borrowed my brothers 98 spx this past weekend with the 787 rotax motor. The motor has been freshly rebuilt with a weisco crank and pistons. I was told the motor only has about a tank of fuel thru it since the rebuild. I rode it a few times over the weekend and it ran like a top. The last time I had it out I noticed a couple inches of water in the hull when I brought it back in. Long story short, I strapped it on the trailer and backed it into the boat launch and fired it up to find the leak. It was pissing a small stream from between the crankcase and front cylinder, on the right hand side. It looks like it just pushed the gasket out a little bit and caused a leak. What would cause the gasket to do that? It does have a tdr water box. Could that have caused it to overheat or something? It ran great and the plugs look like they were both burning really nice. I have to replace the gasket now, but I'd like to know what may have caused that to happen so I can correct it. Thanks guys

 

[ryan199358]

Are you sure it's the gasket and not a cracked water jacket? I just saw a jug with a cracked water jacket, right at the bottom. It was from not winterizing it. I would look again.

 

[Lt1350]

Thanks for the reply! I just got home from work and went out and looked with a mirror and a flashlight and I did not see any cracks anywhere. And the base gasket is definitely torn and pushed out away from the cylinder. It's directly above the top right intake manifold bolt, on the front, mag side carb. Could the motor have gotten hot and pushed that gasket out? I believe it has a water restrictor as well as the after market water box. I was going to take a picture but there's no way you will be able to see it well until I get the pipe off. And I can't get to that until this week end.

Would it be possible for these motors to create excessive crankcase pressure some how?

Thanks for any other ideas guys.

 

[ryan199358]

You said its pissing water out right? I'm looking at the cooling system diagram in the shop manual, and the only water passage running through that area is the water jacket in the jug. If its really coming from the area around the base gasket, you should pull the jug. You might not be able to see the split when its still bolted on. If water was sitting in there during the winter, then froze and expanded, it could crack the jug. The water has the power to crack the case right open. When it split the case, it could have moved the gasket.

I would also do a compression test on the motor. Does it still run? It is possible that a metal fragment from the motor hit the gasket and the crank case and moved it. If the motor has good compression and runs well, we could rule that out.

My bet is that the jug is split.