Burned a Hole in My Piston!

[kepneriphone]

I ran into a problem this weekend with my 1994 SL 750. I took it up to my cottage in Canada and was running it for the first time since break-in and everything was fine until I suddenly lost power and was unable to get over 25-30 MPH.

I did a compression test and found that I had no compression in the middle cylinder (had 120 PSI across all 3 prior to this and the PTO and MAG were still firing at 120 after this happened). I popped off the cylinder head and saw the bad news...there was a hole burned into the piston as you can see in the attached pictures. There was also a sign of water on the cylinder head gasket.

It had come to my attention that we had forgot the gasket that goes between the exhaust manifold and the exhause pipe, so could this be the cause of the hole in the piston? But why would it only happen to the middle cylinder and not the other 2? Is there anything else that could have caused this? Is there a reason why the middle cylinder would have a hole and the other 2 cylinders are completely fine?

I am running pre-mix with the triple outlet fuel pump...no fuel lines appeared to be kinked and the carbs were rebuilt and needles all set to stock which gave me great results during break-in. The motor was out of the boat no more than 3 weeks ago to rebuild the entire top end and we performed a leak down test and we didn't seem to have any air leaks.

Obviously I need a new piston kit to fix this, but I need to address the problem with why the piston wasn't being cooled properly...any thoughts?

[ph2ocraft]

Assuming everything was done correctly, did you ever verify crank index?
One of the symptoms is unexplained holing of a cylinder.

[kepneriphone]

I believe we did check the crank index...I had very good help from Kevin (xlint), he helped me rebuild the entire thing in his driveway. I can't for the life of me think of what else could have caused this???

[SantaCruz163]

How does your fuel pump pulse line look?

I remember that's what caused my first hole-in-piston mod.

[xlint89]

Last minute scramble lead to the exh. pipe being installed without the gasket. We had to swap exh manifolds as 93 and 94 are different for cooling.

Never performed the leak down test after the carb were still installed and leaking around the choke and throttle shafts.

We did the "redneck" crank index and checked out OK. Not exact as a dial indicator and degree wheel.