Oh Cr&p!

[soccerdad]

Took the skis out Tuesday. Amazing day. Both GTX's were a dream. Quick starts, and ran great. Took them both out again today. The green, non-Shamu ski was pulled our tube for about 20 minutes or so. My wife was driving it so nothing radical. The girls wanted more fun, so we switched skis. The tow ski would not start. Finally, I got it running, but it had small backfires and sounded really sick. It would not rev over 5500 RPM. I tried the rectifier with no love. Finally we just rode them back to the ramp. I had a heck of a time starting it, but finally it caught and ran fair at 4000 RPM, but not much higher. Did a compression test and found 155 on the Mag and ZERO on the PTO. :facepalm: WTF!!!

The PTO piston is burnt on the exhaust side.



The plugs have me baffled. The dark one is the one that melted and the lighter one is the good Mag plug



The PTO side head doesn't look that bad to me (except for the shrapnel marks) It does look a lot different than the mag side though. The dark side is the Mag good side. It has a bunch of oil on it, but I did ride it back for about 15 minutes at a pretty slow speed. Although that cylinder was the only thing running the ski.



This is the Mag piston



My only thought is something went wrong in the PTO carb and it burned up. Any other ideas? This is my first motor loss and it really bugs me. I should be receiving new needles and seats Monday. I was going to rebuild those carbs again because it was having a warm start issue. GRRRR.

I do have a spare 787 that came out of the part-out ski. I did start it for a few seconds before I pulled it. It has 150 PSI in both jugs. So I guess that goes in. Any of the NC posse have a motor alignment tool that you want to loan to a friend?

 

[soccerdad]

Looking at it closer, is it just me, or does it seem to have a huge amount of piston to wall clearance? I can kind of wiggle the pistons side to side in the bore. Is this normal on these motors? I am used to tighter motors. That said, the good side still had 155 PSI.

 

[falklandsx]

Sorry about your motor loss. Out of curiosity, how does the oil pump look? Not unexpected to see a bad cylinder fouled. I'd speculate that the pump is good (based on the plug.) Guessing your motor just decided to go... pop off pressure on the blown cylinder look good?

 

[falklandsx]

also, that motor is done... dont try a top-end rebuild. At zero compression, guarantee that you have metal fragments in the bottom end. Sorry man!

 

[soccerdad]

I have not checked the oil pump yet. I don't think that was the issue. It didn't seize. But I did pull the RAVE valve. The tip is chewed up some and looking down the rave slot I can clearly see that the ring is broken (missing) in that area and the land area of the piston is actually fractured off and is not melted. So it looks like it snagged a ring. I started unscrewing the rave cap before I just removed the entire assembly. So I am not sure if it was loose or not. BUT if the rave cap that screws onto the valve was loose, would that let the valve hit the piston? Or is the damage on the rave just from broken piston parts flying around? Not sure why it would have snagged a ring at this age???

 

[eddie708]

How many hrs are on the motor? Some times they just decided to pop and there's not a damn thing you can do about it.

 

[Sportster-2001-951C-Stock]

Oh dang, sorry to see. I bet the ring locating pin sheared off and the ring rotated enough to snag ring end in the port?

Aside from that seems maybe you could use a hair more fuel, piston crown fuel wash on the port could show a little more aluminum (less carbon) I think and that plug looks too light to me (where's da brown?). Look @ underside of crown and see if carbon is there. There's no piston wash showing on the MAG so hard to read maybe since it just got dusted?

Q: Do I have enough oil? A: The puddle in the bottom of case should touch crank wheels. While there feel around in the soup you might find gritty kitty litter hanging out down there among the aluminum dust particles?

 

[soccerdad]

Great tips sportster. I was thinking that ring pin as well. Motor will come out this morning and I will look at it some more. Yep, I thought the "good" plug was a little lean too. The plugs were new about 2 hours of run time ago, and it did have to work pretty hard to get me home since it was running only on that cylinder. But I am running all stock stuff with stock carb settings. Not sure why it would be lean. The carbs were rebuilt last spring. I will go through them again.

At this point, I will just pull this motor. Rebuild the carbs. Put in my backup motor and hope for the best. No crying over blown pistons!

 

[Dr Honda]

The wall clearance looking down is deceiving. The Rotax engines use a crown mounted top ring. SO... the ring is exposed to the combustion chamber. (It's smaller than the piston walls)


To me... it simply looks like it was running a tad lean. The piston wash on the good side is minimal.


Sorry about the meltdown.

 

[soccerdad]

The wall clearance looking down is deceiving. The Rotax engines use a crown mounted top ring. SO... the ring is exposed to the combustion chamber. (It's smaller than the piston walls)


To me... it simply looks like it was running a tad lean. The piston wash on the good side is minimal.


Sorry about the meltdown.

Well you are one of the experts so thanks for the diagnosis. Glad to have you back on the forum! You left just as I got into it, but I have read lots of your posts. Motor will be out in about an hour. Then I can get a real look at it. I didn't know that about these pistons. I'll post some more pix after I pull the jug. Not that it really matters at this point, but I want a good feeling of what happened so I can learn, and try to keep it from happening again.

 

[soccerdad]

Here is under the dome



And the exhaust side. I guess it was just a little lean and stuck and then ripped off the ring. The ring pin is still in place for the top ring, but the entire ring, except about two inches worth is gone.

 

[Sportster-2001-951C-Stock]

Sure nuff....... Always interesting to me and I think helpful to others also, are the "lessons learned" post-mortum failure observations.

 

[soccerdad]

A few months ago I noticed that the 97 carb specs for the GTX called for opening the high speed screw 1/4 turn on the PTO side only. I was going to do this as a precaution on my 96 since they are basically identical. I figure they put that change in for a reason. BUT, I forgot and did not do it. Perhaps that would have helped. I think towing tubes puts a fair amount of extra load on the motor so some extra fuel will help. And that is what the pistons are telling me. So I am going to open the HS on the PTO carb about 3/8 of a turn and the mag side 1/8. Looking at the Mikuni manual (great resource) I think this will help and will be insurance. I don't care if it runs a little fat. I carry extra plugs but doubt I will foul them. But I cant carry an extra motor.... My advice to anyone that pulls tubes /wake boards etc, is to do plug checks and at least open the PTO HS screw 1/4 turn. I will post back how it runs once this puppy is in and running.