1996 XP blown motor

[J_loughman]

Picked up a 1996 XP with the 787 motor. Motor was new from SBT last year. Ran great for me 6 or 7 times on the river in the last week or so. Last time out the motor lost power so I stopped and when it idled down it died. Did a compression check and the rear cylinder is 150 and the front cylinder is 30. Motor turns over and there was no metal flakes on the spark plugs, no clunking, rattling, or unusual noises when it turns over either. What could be the possible issue here? Head gasket? Also the rave valve on the rear cylinder (the one with 150psi) is leaking oil and running down the block but it has been doing this for 2 rides now. Thanks in advance for any insight or advice.

 

[Spider4real]

Sounds like a lean condition over heated and scored the cylinder walls resulting in loss of compression. Only way to know for sure is pull the head off and inspect the pistons & cylinders. You might be able to see something if you pull the rave valves off and look in the cylinder. But at this point the cylinder is dead so a rebuild is required.

But you need to find out what went wrong so the problem can be fixed.

I would not worry about oil leaking from the rave valve. Mine did this pretty much the entire time I owned my boat. New gaskets seemed to fix the problem temperarily but it always came back. It frustrated me at first, but I just got so used to seeing it so I ignored it for the most part. SInce you seem to be in a top end rebuild territory you can clean the raves and put new gaskets and o-rings on while you are at it and see it that fixes the oil leak.

 

[J_loughman]

Sounds like a lean condition over heated and scored the cylinder walls resulting in loss of compression. Only way to know for sure is pull the head off and inspect the pistons & cylinders. You might be able to see something if you pull the rave valves off and look in the cylinder. But at this point the cylinder is dead so a rebuild is required.

But you need to find out what went wrong so the problem can be fixed.

I would not worry about oil leaking from the rave valve. Mine did this pretty much the entire time I owned my boat. New gaskets seemed to fix the problem temperarily but it always came back. It frustrated me at first, but I just got so used to seeing it so I ignored it for the most part. SInce you seem to be in a top end rebuild territory you can clean the raves and put new gaskets and o-rings on while you are at it and see it that fixes the oil leak.

I haven’t broken into it yet but if that’s the case what could cause this to happen. The carbs were rebuilt last year when the SBT motor was installed according to the previous owner.

 

[Spider4real]

Never trust the previous owner. To quote my favorite TV doctor. "everybody lies" - House MD. So unfortunately, we really have no idea what was (or was not) done.

The cause could be a number of things. Could be a carb issue. Could be an air leak in the case. Could be a oil injection issue such as a bad check valve, or broken oil line.

 

[mikidymac]

Call SBT and see if it is still in warranty. IF you open it the warranty is void.

 

[J_loughman]

Call SBT and see if it is still in warranty. IF you open it the warranty is void.

The warranty has expired unfortunately

 

[J_loughman]

Never trust the previous owner. To quote my favorite TV doctor. "everybody lies" - House MD. So unfortunately, we really have no idea what was (or was not) done.

The cause could be a number of things. Could be a carb issue. Could be an air leak in the case. Could be a oil injection issue such as a bad check valve, or broken oil line.

 

[J_loughman]

This was the condition of the cylinder that went down, obviously. Other one looks good. Both cylinders walls look good to all things considered. The rave valves had some pretty good carbon build up. What are the chances of the carbon causing one to stick closed and causing excessive heat?

 

[Spider4real]

That looks more like damage caused by something in the cylinder. Is the rave valve intack? If the cylinder was bored oversize and the rave was not properly filed down it could have made contact with the piston.

How did the spark plugs look?

Also what does the head look like?

 

[J_loughman]

That looks more like damage caused by something in the cylinder. Is the rave valve intack? If the cylinder was bored oversize and the rave was not properly filed down it could have made contact with the piston.

How did the spark plugs look?

Also what does the head look like?

The rave valve was completely intact. Doesn’t seem to have any chips taken out of it, no abrasions, gouges, etc. I believe this SBT motor was the stock bore. The spark plugs looked good, correct color, no chips, not bent or misshapen, no metal shavings on them. The head is destroyed though, the dome for the cylinder that was destroyed is all pitted. There are a few bits of broken off piston ring shoved into the top of the piston.

 

[mikidymac]

It is hard to say.
Something let go, could have been a ring or a crank/rod bearing.
The top of the piston looks like it still had fuel and oil but if you didn't do it yourself, I still suspect carbs and fuel system with these.

Keep in mind, this is a SBT reman so low quality parts and questionable tolerances. Not surprising it let go.

 

[J_loughman]

It is hard to say.
Something let go, could have been a ring or a crank/rod bearing.
The top of the piston looks like it still had fuel and oil but if you didn't do it yourself, I still suspect carbs and fuel system with these.

Keep in mind, this is a SBT reman so low quality parts and questionable tolerances. Not surprising it let go.

The rings definitely went. There’s broken off pieces of the rings shoved into the face of the piston. Probably tough to see in the picture I added above, but the edge of that piston is so beaten up that it is sort of beveled down around the edge to where you can see the top piston ring

 

[J_loughman]

Considering this is already an sbt motor. What would the tolerance for the ring gap be?

 

[mikidymac]

You really don't do ring gap on these like a car engine.
You measure the cylinder to piston clearance, it's in the service manual that can be viewed for free.
With that much damage you are going to have to bore oversized and new larger pistons and rings fitted. This not a slap in new parts and pray situation.