melted a piston

[93mustang97sea]

ok so i melted a piston today on the ski.. by the way the skis a 97 spx with the 787 in it.. OK so with that said I dont know what I should do should I make it back to stock or go all out with factory pipe and a head and carbs and intake?? I want it to be reliable but fun.. I love speed so I dont know what I should do... What kind of speed am I looking at if I build it up?

 

[LegalizeIt]

No big deal...



What you need to figure out is why the piston melted. Lean mixture? Air leak from a possible crack or loose carb? Was it tuned at a different air temperature (air density)?

 

[93mustang97sea]

I just finished cleaning the carbs by that I mean I replaced the fuel filters and gaskets nothing adjustment wise was touched because it ran perfect until they got clogged so I did that.. I had it back together on Wednesday and was out everyday and then yesterday I was out and I let a friend ride it he filled it up with regular gas at the gas dock... he didnt know i always ran 93 in it since i owned it and then later on he was pushing it back to the beach where i was... so how do i figure out what caused it to happen?

Could it be the change in octane from 93 to 87?

 

[LegalizeIt]

Stock head? It shouldn't matter.



Post up some pictures when you take it apart.

:cheers:

 

[93mustang97sea]

Stock head? It shouldn't matter.



Post up some pictures when you take it apart.

:cheers:

Yes Stock head..

Heres those pictures..

 

[6KJR]

I always run premium 91 in my 787. It has aftermarket FA;s thats it. but 165 compression. A big drop in octane could definitely cause some pre ignition. However, that does NOT mean there were other contributing factors...

 

[93mustang97sea]

So what does anyone think happen to be the cause of my piston and head?

 

[LegalizeIt]

The fact that it's only one cylinder rules out the octane issue.

How's the spark plug tip?

The brass oil injector fittings could possibly have a leak. Or around the bad cylinder. Do a visual inspection.

Check the carbs again. Take them apart and look around in there.

Just a few places to start...

 

[93mustang97sea]

spark plug was crushed had no gap

 

[seadoobutch787]

Hello, Im a newbie to this forum but not a Newbie when it comes to ski's. My experience with Detonation was because i put a load on my 96xp and i wasn't allowing it to rev out all the way. I have a 96xp with a spec 2, novi 42's and a R&D rec b head. I made the mistake of running the wrong pitch on my boat along with a 83mm venturi nozzle. This caused my engine to only rev to about 7000 rpms and that is when the engine just shut off like i pulled the lanyard. The spec 2 like's rpms in the 7400 rpms range anything less than that at top speed your rear piston is toast.

So keep that in mind if you are thinking about putting a factory pipe spec 2 on your 800. That pipe loves eating rear pistons if your carbs are not in order.

So is your piston in question the rear?

with less octane the risk of deto increases IF your engine has other issues like the carbs. When it deto's it melts off the top Crown of the piston and deposits the aluminum on the squish ban which you see in the bottom side of the head. In my case is broke off the tip on the spark plug and took out the porcelin around the electrode.

 

[seadoobutch787]

In the first 2 pics you can see my piston that had detonation, there is melting on the top of the piston crown and down to the top ring, there are pits all over the top also.

In the other 2 pics shows a piston that had a seizure due to a bad base gasket. It caused a lean condition and scored the side of the piston, but if you look at the top of the piston it appears to be in really good condition.


I have learned my lessen with the spec 2 LOL. I run 110 all the time so there is no worry's when im turning 7450rpms..

 

[LegalizeIt]

I think it's a stock ski.

But are you saying the Spec2 is trash?

Someone was ready to buy my Spec1 and Extrude-honed oem manifold, then changed his mind because he thought it was a Spec2...

 

[93mustang97sea]

I think it's a stock ski.

But are you saying the Spec2 is trash?

Someone was ready to buy my Spec1 and Extrude-honed oem manifold, then changed his mind because he thought it was a Spec2...

Yes it is a stock ski..

 

[seadoobutch787]

No not at all! I have a spec 2 and it has kick ass performance. I love it! It just took me some tweeking to get it right. 2 rear pistons to be exact lol.

Im saying the spec 2 will trash rear pistons if you don't get your carbs set right IE..jetting,adjustments.

My first pipe on my 96xp was a Rossier, back then i thought 7050rpms was fast lol... That pipe is way more forgiving and it will lay down before your trash a piston..but the performance isn't there.

The spec 1 uses the same chamber as the spec 2, the only difference is the spec 2 has a bigger headpipe opening and it comes with a fpp manifold.

I have a friend that did the same thing to his spec 1 and he has the same performance as a spec 2.

 

[LegalizeIt]

No not at all! I have a spec 2 and it has kick ass performance. I love it! It just took me some tweeking to get it right. 2 rear pistons to be exact lol.

Im saying the spec 2 will trash rear pistons if you don't get your carbs set right IE..jetting,adjustments.

My first pipe on my 96xp was a Rossier, back then i thought 7050rpms was fast lol... That pipe is way more forgiving and it will lay down before your trash a piston..but the performance isn't there.

The spec 1 uses the same chamber as the spec 2, the only difference is the spec 2 has a bigger headpipe opening and it comes with a fpp manifold.

I have a friend that did the same thing to his spec 1 and he has the same performance as a spec 2.

I think 7400-7500 is what the Spec1 was turning...

Yeah, Extude-honing is a great improvement, but costly.
I think it was 200-250 each for manifold and headpipe, now no one wants the whole system for $250...lol...

 

[LegalizeIt]

Yes it is a stock ski..

Did you do a visual inspection looking for any cracks or signs of leakage?

Is your intake manifold OK?
No cracks on it?

Carb mounting bolts tight?

We're looking for an air leak that effects one cylinder...

 

[rexsteiner]

Most likely not an octane issue. I would lean towards an air leak. Are the cylinders scored? If so where and where are the pistons scored? You can tell a lot from pistons. If the scores go from skirts all the way up then its probably an oil lean. If the scores are mostly on the exhaust with most of the detonation on the exhaust side then its most likely a fuel lean witch is caused by either a improper carb setting a cracked or leaking seal or gasket or a restriction of fuel inside the carb on that cylinder. In your case I would inspect the carb to make sure you didn't get a gasket rolled in or somthing givin the fact that you just had them apart.

 

[seadoosnipe]

No Octane or detonation issue...

Going over the pix, it def looks like something got into your motor or you had a bearing let go.

Your going to have to pull andsplit the casing.Look at all your bearings to see if you had any of the roller balls bop out; should be easy to see. Though breaking off part of the piston skirt would cause more damage, check it too. You may have just chipped a piece off.

But, from what I see, you can rule out detonation and octane.

The octance requirements of our 2 stokes is 87. It does not mean you can not run 89 efficiently, that rating by BRP is only a minimum requirement. You only need to be aware of the heat changes in buring a higher octane fuel and make all changes necessary to burn anything higher than 93.......:cheers:

 

[rexsteiner]

Going over the pix, it def looks like something got into your motor or you had a bearing let go.

Your going to have to pull andsplit the casing.Look at all your bearings to see if you had any of the roller balls bop out; should be easy to see. Though breaking off part of the piston skirt would cause more damage, check it too. You may have just chipped a piece off.

But, from what I see, you can rule out detonation and octane.

The octance requirements of our 2 stokes is 87. It does not mean you can not run 89 efficiently, that rating by BRP is only a minimum requirement. You only need to be aware of the heat changes in buring a higher octane fuel and make all changes necessary to burn anything higher than 93.......:cheers:

I kinda thought the big hunk of piston that's melted away on the exhaust side was a dead give away.