Freshly Rebuilt Engine Gone, assistance in determining failure mode

[SpeChris734]

I was hoping for some assistance in determining the root cause of the failure of this motor.

Background
I purchased my ski back in 2017 (95 SJ) with a motor that was a little on the low side for compression, about 135psi in each hole. This motor was solid and ran for about 5 or 6 years until needle bearing failure destroyed the engine beyond repair. I decided to purchase a rebuilt motor from a seemingly reputable jet ski shop in Michigan, stand ups seemed to be the guys specialty he had about 40 personally owned. I mention the old motor because after cleaning i reused the intake system however, i did reset the carb to what i believe were factory settings and rebuilt the reeds for the new motor. When i got the new motor it initially did not have compression in the front cylinder (~30 front / 155 rear) took it back the the shop and the dude corrected the issue. I didn't ask at the time what the problem was. Afterwards the motor appeared to run fine, i went out maybe 3 or 4 times no apparent issues. The next time i took it out it would not start at the ramp, after checking compression it was back to (~30 front / 155 rear). Afterwards the ski sat for about 2 years.

Failure
I finally decided to get the ski going again because i purchased a house on lake erie . Today i checked compression again 30/155 same as two years ago so decided to disassemble thinking maybe there was a ring issue with the front cylinder. The engine had about 3-4hrs, cooling appeared OK from what i remember (no change in pisser volume after new motor installed), oil content 45-50:1, premium unleaded stabilized (no rec gas), plugs still looked brand new (only had a few hrs on them)

Rear Cylinder (155psi)
- a lot of carbon build up
- head gasket showing blow by
- seems to have experienced some heat but no failure

Front Cylinder (30psi)
- Rings welded to the piston
- a lot of scoring and contact/rub with cylinder wall

Basically I'm trying to understand root cause, my initial thought is a cold seize I'm a little hesitant though because of how much more damage was experienced by the front cylinder (my thought is damage would be more uniform). I'm undecided if i will rebuild this or purchase a new one. If the root cause possibly exists somewhere outside of the motor (carb/supporting systems) i'd like to understand that out so the issue doesn't repeat, skiing season is short in Michigan and i hope to not be rebuilding again in the middle of summer. Thanks in advance for the help/suggestions!

 

[wiikstromm]

I would say overheating/un even cooling

 

[kevbo]

One cylinder will always run hotter on single carb multi cylinder engines. With two strokes, you can never blindly follow others jetting advise. You must open the top screw 3 turns and see if power is reduced at full throttle. If not then install a larger jet. It is very difficult to hurt an engine tuned this rich. Plugs will turn dark very quickly. From this setting start closing the top screw just until you find best power. That’s how you tune the full throttle mixture. All this is assuming that other mistakes weren’t made like the piston fit being too tight or too much compression/ignition timing.

 

[WFO Speedracer]

You haven't provided enough information about what was done to make any real type of determination , personally I would be looking for an air leak , you have already pulled the engine apart so you can't check it now , but I would do a full teardown, crankshaft seal replacement and air leak test after reassembly , don't run it again without doing those things, what pistons were used and what piston to wall clearance were they run at ?

Scoring like that takes place because the piston becomes to big for the hole it is running in, at that point it acts like a brake and stops the engine , I doubt it's a cooling issue , either improper piston to wall clearance , an air leak or jetting that ways too lean , either of those will make a cylinder run hot enough to seize up like that , usually on a twin the rear cylinder runs hotter which is why I say air leak .

 

[Quinc]

Welcome to the world of powersports. Almost everyone is trying to scam you and sell you junk. My first thought is the guy you bought it from knew his engine was poop and when you called him out on it he threw some junk piston and rings in it. This time rebuild the engine yourself or get a top end kit from a reliable source like @JetManiac.

Also read through every thread here: https://www.x-h2o.com/forums/tech-faq.26/

And ask questions about any part of the process you are not sure about. =)

 

[WFO Speedracer]

I went back and read the first post a bit closer ,what happens a lot of times is people and even some shops out there fix the symptom but not the problem that made it fail in the first place .

I see it all the time in this business , if the front cylinder failed twice you can just about bet the front crank seal is leaking , usually but not always when you pull the flywheel you will see telltale signs of oil seepage or leakage at the crankshaft , there should be none as that is the seals job , other than that you have to do a pressure test with the engine fully assembled to find the leak and do it again after reassembly to verify that the leak is fixed when the new top end is installed.

There is no doubt in my mind on this one , a full teardown , seal replacement and crankshaft inspection is needed on this engine before even considering rebuilding the top end again.

Focus your attention on the bottom end and make sure it is good to go before fixing the top end situation.

 

[kevbo]

FWIW Kawasaki hot cylinder is usually the front, Yamaha is usually the rear. Air leaks are a possibility but they have to be more severe on a watercraft since our engines are always under heavy load. They tend to just push air out rather than draw it in. For comparison a bike with a leak goes very lean because it will coast with the throttle closed.

 

[WFO Speedracer]

The real issue is there are 25 -30 year old crankshaft seals still on some engines out there running, crankshaft seals are just rubber and they were never designed to last that long , the heat cycles and additives in the fuels and oils dry the runner out and they they don't seal , personally I like to change them out about every 5 years or so to be on the safe side , when these guys come in for a top end repair on an older machine , I refuse to do the job without replacing the crankshaft seals , it's better for them to eat the additional expense and have it done right than for me to eat the cost of the second repair when it goes horribly wrong .

One of the first and probably the most important lesson I was taught as a tech was it doesn't cost any more to go first class , do it right and do it once , do a halfassed repair job and you will pretty much be assured that you will get to do it again for free..

 

[Quinc]

The real issue is there are 25 -30 year old crankshaft seals still on some engines out there running, crankshaft seals are just rubber and they were never designed to last that long , the heat cycles and additives in the fuels and oils dry the runner out and they they don't seal , personally I like to change them out about every 5 years or so to be on the safe side , when these guys come in for a top end repair on an older machine , I refuse to do the job without replacing the crankshaft seals , it's better for them to eat the additional expense and have it done right than for me to eat the cost of the second repair when it goes horribly wrong .

One of the first and probably the most important lesson I was taught as a tech was it doesn't cost any more to go first class , do it right and do it once , do a halfassed repair job and you will pretty much be assured that you will get to do it again for free..

Do you pack between the crank seals with grease during assembly?

 

[kevbo]

I do.

 

[subnsxr]

Bet that crank bearing is shot too.

 

[WFO Speedracer]

Do you pack between the crank seals with grease during assembly?

Yes usually with the Bel Ray waterproof grease

 

[WFO Speedracer]

Bet that crank bearing is shot too.

You are probably right which is why I said teardown and inspection is mandatory on this one, unless you just want to assemble it with Velcro..

 

[waterfreak]

Complete tear down needed. Then , make sure you use a reputable machine shop to bore the cylinders. It's possible the person that did the work prior didn't check the bore properly and the cylinder could have been out of round.