Seized piston

[Eikari]

Bad day at the lake. The engine shut off by itself, the lanyard was still on. If i understand correctly, its due to lean running. It´s a 1996 GTX

It happened while i was towing a tube and also a spotter on the ski, total of 3. The ski didnt move so fast because it was just like it was freewheeling, speed maybe 25-30 km/hr but i could reach almost 7000 rpm and it was smoking a lot. This went on for about 2 minutes, similar situation has happened before and that was due to seaweed and debris getting caught in the drive train or impeller. I thought it would clear up by going reverse and forward again, but it didnt happen this time. And it died by itself.

The ski has worked fine before this happened, stock carb settings ( havent done the plug chop test though), rebuild engine from SBT, rebuild carbs, new raves and housings, new fuel lines, and everything new in the drive train

Clogged cooling line from seaweed? too lean settings on carbs?

Did a compression test, MAG side, 150 psi ; PTO side= 80 psi

Also took a picture of both cylinders and piston crown. Both crown look ok, some marks on pto cylinder. Maybe piston ring broken?

 

[Sportster-2001-951C-Stock]

I don't know what happened to your boat specifically as far as engine damage goes, but don't forget that if the impeller isn't hooking up for whatever reason then cooling water flow could be insufficient and the cylinders could quickly overheat and lead to seizure.

 

[Eikari]

Too much load on the engine and restrained cooling?

 

[Sportster-2001-951C-Stock]

Too much load on the engine and restrained cooling?

It sounded like from the description there wasn't much load on the engine, as if maybe the engine was uncoupled from the impeller. If the impeller isn't receiving full torque from the engine via the drive shaft or if water entry/exit are blocked from the pump, then the nozzle pressure would be low, thus the cooling water volume may have been insufficient.

I suppose it could've been a lean-seize, that's a possibility. Was the motor broken in properly as per SBT's instructions?

It looks like the oil film failed to lubricate the cylinder wall, judging by the photo. It looks like the oil decomposed(from excessive heat, presumably) and turned to varnish.

A lean mixture can cause excessive heat, so could a lack of adequate cooling water flow. A fully loaded engine tacking at 7000RPM would generate a lot of heat, and so it's important to have enough fuel and adequate cooling to keep the piston from overheating under those conditions. Lack of lubricant could be involved as well, or low quality fuel even. Was there pre-mix in the fuel, or was the oil injection pump the sole source of lubricant? If the pump was misadjusted, there could've been a lack of lubricant.

It's hard to say with any certainty, absent a bit more info. It does look like some kind of seizure occurred, based on the photos. And where are the honing marks, are they there and I'm not seeing them?

 

[Britrick]

Isn't it still under sbt warranty ?

 

[Eikari]

Actually i wiped the cylinders with a rag before i took the pictures, hence the lack of oil film. The engine was smoking a lot when this happened, and the oil injection was properly adjusted. The break in period was as according to SBT and it was followed from A to Z. There was no overheating alarm, but things happen fast in a 2 stroke, compared to the 4 stroke.

I´m not sure about the honing marks, can you elaborate more about that?

 

[Eikari]

Britrick, yes its still under warranty.

 

[Bills86e]

more than 8-10 psi difference in compression reading, ya need to top end her, and fix the cause of your piston going lean.

Bills86e.

 

[Sportster-2001-951C-Stock]

You have to remember too, a 2 stroke needs a bit of time after starting up in order to bring the cylinders up to temperature, the pistons expand faster than the cylinders do, can't just crank off a cold motor and run it WOT right away, need to give it a few minutes of warm up.

Like I said, I don't know the details of the conditions involved concerning this particular failure. Some folks might try to claim excessive piston speed was at fault under the conditions and that would be hard to argue against from the perspective that an engine cannot seize like this if the pistons aren't moving up and down in their bores.

It could simply be the cylinder/piston tolerances were incorrect. Seizures can occur both if there's too much, or too little tolerance. Perhaps a ring snagged on something, except I don't see any damage to the piston crown, or pock marks on the inside of the cylinder head.

 

[Sportster-2001-951C-Stock]

things happen fast in a 2 stroke, compared to the 4 stroke.

You're absolutely right about that!

Hone marks are a series of intentional cross-hatch micro score-marks made with a stone(s) at the end of machining the cylinder, so as to make for a surface that will help the piston rings lap into the cylinder walls and improve the ring sealing.

 

[Eikari]

Yes, the crowns look fine but i cant see how the rings are until i take off the cylinder. More pictures to follow later. At least i need a new piston kit.
And double checking the carbs, hi and low speed settings.
I always warm up the engine before going at any high speeds, am schooled in vessel engines.

 

[Eikari]

yes, the pics are not high quality, taken on my samsung phone. But i can see the honing marks.

 

[griz400]

you probably lost your warranty cause you took it apart ...:facepalm:

 

[Eikari]

Do i have to send the whole engine to Florida to avoid losing warranty?? I think it would be cheaper then to just a buy a new piston kit from osdparts then. I dont live in the States..

 

[Sportster-2001-951C-Stock]

you probably lost your warranty cause you took it apart ...:facepalm:

This is an excellent point, don't take any further apart until you receive some instruction from SBT!

 

[griz400]

here is their warranty http://www.shopsbt.com/w_policy.htm it says you cannot disassemble it, including the head .....

 

[Eikari]

The shipping cost is way to high to try to get a new piston from SBT under warranty. For a 65 usd piston kit from osdparts, it really is a no brainer for me what i am going to do, and also save a lot of time in the meanwhile.

 

[Sportster-2001-951C-Stock]

This photo is a pretty good example of what the honing marks look like prior to assembly:

http://imageshack.us/a/img855/7402/aq3k.jpg