787 - I lost the PTO cylinder for the fourth time

[BoLongo]

How can this be happening??? I know… the answer is always buy OEM new or used.

This is the fourth time I’ve lost this PTO cylinder. I got my OEM rectifiers today and headed out to the lake. The boat ran great for about an hour and then the starboard engine felt like it was running on one cylinder. I checked the compression on the water and it was zero. I cried for about 5 minutes and headed back to the ramp on one engine.

I’m done with this motor it will be out of the boat this weekend. Tomorrow I’m going to look at a 1996 787 (motor only, no intake or exhaust) that has never been opened hopefully it’s good as the current owner thinks it is. I will be bringing my pressure testing kit and compression gauge.

This has been the most frustrating engine I have ever encountered.
The boat sat in a yard for a couple years and water had gotten into this motor and rusted the crank bearings. The PTO cylinder seized the first day I took it out to the lake because of a leaking crank seal.

I pulled both engines and replaced the seals. I also bought a rebuilt crank and counter balance shaft from SBT. Yes this is where I believe all my problems began.:facepalm:

The boat ran great for about 10 hours and then lost compression do to chipping and scoring on the same PTO cylinder.


I bought a set of used carburetors and a used oil pump. I bought another top end kit and bored the jugs. The engine ran good for about 30 hours and it seized again.


This time I bought a set of used OEM pistons jugs and head out of a good running Sea Doo. I honed the jugs and bought new rings. This time I got about ten hours and today the PTO piston chipped losing all compression.


The SBT crank had an issue with the squish gap with both jugs and heads. I had to use a 3-hole base gasket and I was just within the upper end of the spec.

I think the issue is with the crank but I can’t say for sure what is wrong with it. but the piston damage seams to be in the same area on the intake side. It looks to me like the top ring catches an edge on the intake ports.

On this engine I have changed to a different set of carburetors, different oil pump, different jugs, pistons and head. The exhaust system is clear and the cooling system seams to be working properly.

My biggest fear is that I replace the motor and it turns out to be something with ignition, fuel, cooling or exhaust.

I would love to hear your ideas?

 

[XP 787]

Dumb question but did you install the piston in the right direction? Arrow facing exhaust port.

 

[BoLongo]

There are no dumb questions at this point. Yes the pistons were installed correctly, this motor has had three different sets of pistons.

 

[racerxxx]

Have you pressure tested the engine?

 

[nickj718]

I would be doing a carb rebuild for sure. Strip, ultrasonic cleaner and genuine Mikuni parts

 

[BoLongo]

Yes I have block off plates and tested the engine each time I rebuilt the top-end to check the base gasket, I already knew the bottom end was air tight.

I do not believe the issue is related to a lean condition. I have EGT gauges which show all cylinders in the same temperature range when WOT. I also use my Laser Infrared Temp Gun to check the surface temperatures around the jugs and they read the same. I read my plugs every time I come back from the lake and they are all a dark brown.

I rebuilt the old set of carburetors twice using OEM rebuild kits and I rebuilt the current set using OEM rebuild kits.

I think I had an issue with the accelerator pump on my first rebuild, the boat did not come with check valves and I think I was sucking air back through the accelerator pump into the fuel line which was introducing air into the system but I now have check valves. I will go check the check valves to make sure they are still working.

 

[BoLongo]

I’m convinced the issue is in the rod to crank connection. Somehow the rod in putting pressure on the intake side of the piston creating friction that eventually destroys the piston, but I will never find out for sure because I just bought another engine.

I got a 1996 787 that has not been opened up with good compression and clean walls looking through the exhaust ports. The engine holds air and I got it for $300.00. I should be able to part out my old engine for at least that much. I should probably throw the old crank in the trash.

Busy weekend; pulling and replacing the engine.

I appreciate this Sea Doo community and the help y’all provide.:cheers:

 

[BoLongo]

New motor painted to match the 1999 motor, time to start the exorcism to remove the evil motor from my boat.:hurray:

 

[BoLongo]

New motor is in!
Engine aligned and oil pump tested and bled
Waiting on OEM exhaust gaskets. I also decided to weld up the JB Weld patches to stop water from leaking through the manufacturing plugs on the exhaust pipes.

 

[racerxxx]

What's the red thing?

 

[BoLongo]

It's the hydraulic pump for the trim tabs. I'm doing a write up on the trim tabs now, that i will post to my Resto thread.