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Discussion Starter · #1 ·
Hello, I rebuiltmy kawasaki stx 900 ( two stroke) and during break in this happened.. (see image) Looking at it I think the ring pinched and when it buckled it caught the exhaust manifold. What's your thoughts?
Hand
 

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My guess would be one of 2 scenerios.
Either like you, the ring hung in the exhaust port and broke the ring land. Or it was running hot/lean.

Did you pressure test the motor after rebuild to make sure you have no air leaks. If the front or rear crank seal is leaking it would lean the front or rear cyl.

What does the plug look like? Light or dark in color. Pics please.

Typical break in on an oil injected ski is usually, the injection plus the first tank mixed at around 50/1. Then after that tank just injection.
 

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Discussion Starter · #5 ·
It was the center cylinder that failed, the plug was pretty white, very little film, im assuming it was "washed" by the Un burned fuel after the compression went down. I did run it for a awhile(didn't immediately notice the drop in power). I can get some photos tomorrow though. The way the stx 900 works, is it injects the oil into the carb and mixes In the fuel, per individual carb. I know it's pumping oil, and I did. Split the carb and see the fuel in it was mixed. Unfortunately visual inspection can't tell me the ratio. Would the ring grabbing the exhaust port be caused by poor ring gap?

Thanks,
Tim
 

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I think your missing what I'm saying for break in. mix your fuel at 50/1. Like your running without injection. Plus you have the injection. This makes a very rich mix to help the beak in. Effectively double oiling for the first tank.

Usually a ring catching a port is due to a locating pin falling out and the ring spins and the end of the ring gets into the port. If the gap was very tight on assembly it could also cause it to possibly catch in the port as it heats and expands.
 

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Discussion Starter · #7 ·
Dave, first off I appreciate the help. Secondly I think i created a misunderstanding. I understand what you are saying in regards of the break in, oil pre mix. I also never heard this before, as such I didn't do it. Because of this what I was getting at is that for normal use I am not sure how to "check" the mix ratio. Due to the fact that rather then spraying oil into the intake like my 1100di does I can't see the oil going in.
 

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Discussion Starter · #8 ·
I think your missing what I'm saying for break in. mix your fuel at 50/1. Like your running without injection. Plus you have the injection. This makes a very rich mix to help the beak in. Effectively double oiling for the first tank.

Usually a ring catching a port is due to a locating pin falling out and the ring spins and the end of the ring gets into the port. If the gap was very tight on assembly it could also cause it to possibly catch in the port as it heats and expands.
I also want to add that the pins were both in the piston.
 

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Not sure if this pic from my manual helps, if not just disregard. As far as oil injection tool, i’ve used large calf type 2oz syringe held up by another person above motor height for good gravity feed. It worked for me to verify consumtion. Just trying to help.
Text Newsprint Paper News Font
 

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I agree, having the oil trickle in the top of the carb is much better than being delivered internal.
Try looking in the manual post above if you don't have a manual. You should find test procedures for your pump in it. Very seldom do pumps fail though. Usually it's an adjustment or hose failure.
Being the ring locating pin is in place. The ring didn't turn and catch.
What was your ring end gap when you rebuilt motor?
Is this a complete new top-end, or just a re-ring of old Pistons?
 

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Discussion Starter · #12 ·
Also to add I didn't have a stx 900 service manual. I referred to the zxi 1100 service manual. I would assume ring gaps would be similar but I could be completely wrong. The major issue I have is that the reason I rebuilt it was because the center piston melted. That same one that had this issue. However I can't think of a reason that it would melt as the carbs were serviced.
 

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.012" is about what I would expect for gap. So that rules ring gap out.
Now the added info on motor having burnt this cyl. Being reason for rebuild. That starts to shed light on the issue. That leads me back towards an air leak on that cyl. Leaning it out. Maybe intake manifold. Reed cage assembly. Case half seal. Cyl base gasket. Only way to know for sure is to pressure test motor.
This also leaves possible issue still with carb.
 

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Discussion Starter · #14 ·
.012" is about what I would expect for gap. So that rules ring gap out.
Now the added info on motor having burnt this cyl. Being reason for rebuild. That starts to shed light on the issue. That leads me back towards an air leak on that cyl. Leaning it out. Maybe intake manifold. Reed cage assembly. Case half seal. Cyl base gasket. Only way to know for sure is to pressure test motor.
This also leaves possible issue still with carb.

So I have some uploaded photos to my drive. I inspected everything yesterday, top to bottom. The only thing that was kinda suspect is the inner gasket to the reeds, was not very... Stuck? All the other gaskets, were very attached to the motor and manifold on both sides. I put a different cylinder ring in the old cylinders, and the center cylinder was like...... .045 thou.... Not sure if I had a mini stroke or if something went really wrong. But that's more then double the maximum gap.

 

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Discussion Starter · #15 ·
So, I ended up putting this back together, and it did it again... so I rebuilt it one more time and started looking at everything. I took the ECU out and found that someone assembled incorrectly and put i the lid and body together pinching the center cylinder signal wire for the coil. so, my leading thought is really high EGT's and it just burned the piston by the exhaust. I also found the fuse was melted and some wires were also melted. So its possible I has some other electrical issues as well. Does anyone have a service manual for the STX 900? I need the electrical schematics for the damaged wires, and some weird looking sensor that just hanging in the bay.

Thanks,
Tim
 

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I don’t have any schematics of yours or a service manual. I do mainly tigersharks but i have to ask? Are any cylinders new? If so, are the ports chambfered to the proper degree? I know that will almost guarantee ring catching mainly on exhaust side plus a out of spec. ring endgap adds to disaster.
 

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Discussion Starter · #17 ·
I don’t have any schematics of yours or a service manual. I do mainly tigersharks but i have to ask? Are any cylinders new? If so, are the ports chambfered to the proper degree? I know that will almost guarantee ring catching mainly on exhaust side plus a out of spec. ring endgap adds to disaster.

Everything is new. the cylinders were sleeved by SBT and I did all the recommendations to assemble it correctly via the documentations supplied with it.
 

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If it were me i would either have a service manual or clymer manual for it. Then tear it down to shortblock, remove from hull. After the crankcase, seals and crankshaft fell within spec.and bearings on crank and needle bearings etc. all looked near perfect i would then go to tophalf. With my micrometer i would prove that all cylinders are std. bore or oversize. Are rings and pistons std. or oversize. If 2 different oversizes are assembled as a whole your doomed. Example Putting 0.010 overbored piston/ring assemblies into a 0.020 oversized cylinder jug. Personally i use a rule of thumb my parents taught me. Replace as a set. Meaning if i have multiples as in cylinders then i replace all to the same. All are std. bore or the same oversize. Can you have some standard and some oversize? Never tried and never will. You can usually see with a magnifier if the cylinder ports are chambfered. In the back of the clymer manuals they have the electrical schematics too. There rebuild info. is more basic compared to service manuals.
 
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