Piston Melt Down 1996 GTX 787

[Shelato]

Where did I go wrong? The ski was given to me with the engine in pieces, heads and pistons off. I put it back together by the manual, it ran for a day and then I lost power. Drove it back to the dock, about 3 minutes. Found 150psi on one cylinder and zero on the back one. I pulled the jug and you can see what I found.

The piston is colored grey like a cast metal color and it melted. The head seemed to have water flow and wasn't scalding hot. I'm thinking it's a lack of oil but why only one cylinder? It's on the RAVE valve side, could that cause it? I've seen other pictures like this posted here but did not see a definitive answer.

I'm putting a 947 back together tomorrow and don't want to make the same mistake.

Thanks for your help!!!!!!!!


Dwight

 

[jhjesse]

I have experienced a similar situation. It looks like that cylinder "leaned out." in other words it was not getting enough fuel. I would check the settings on that carb and make sure the lsa was not screwed out too far.
Also I would run the first tank of fuel as premix after you finish putting Her back together just to insure good lubrication during break in. Good luck :cheers:

 

[Shelato]

Carb Settings

Thanks for the reply jesse. The low settings on the two carbs are 90 degrees to each other, but when I turn them in and count the turns out, they are both 1 1/4 turns. Should the dials be oriented exactly the same?

FYI, I rebuilt the carbs myself when I put it back together.

 

[jhjesse]

The other thing could be is that it was not getting oil in the small oil line going to that particular cylinder. When you rebuilt the engine did you have the oil pump off?
Whether or not you took the oil pump off, did you make sure the small oil lines running to the cylinders were full of oil.

This is why after a rebuild of any kind, It is recommended to premix the first tank of gas. Usually it is a 40:1 ratio, Keeps everything lubricated until all the lines get full of oil.

Just a thought!!! Someone else may have a better explanation.

 

[jhjesse]

Hey just had a thought. When you rebuilt it the first time was the cylinders bored out? Because from the look of that piston the damage is on the rave side.
If the cylinders were bored out more that 0.20mm the raves would have to be machines as well.

 

[chuckydoo]

Be carefull you may have a crank seal leak. If this is the case you will only fry it again. When i do top ends i always pressure test the crank case before I put in the ski.

 

[timmyboy76]

justa-thought

you made sure, the piston was installed correctly?

 

[Shelato]

Melt down

Thanks for all the input. I have the jugs off an XP at the moment. I'll be following all the adice as I put them back on tomorrow. I will definetly premix the first tank of gas!!!

The cylinder was not bored but the damage is in the area of the RAVE.

How do I "pressure test the crank seal"?