...cast iron cylinders, 1st overbore.

New pistons and rings ordered, but no bulletproofing plan in place.
.

 

I'm just another backyard mechanic, so take this for whatever it is worth. You have two good solid sleds there, and probably a third one if you apply the same principals. I can't speak for the bulletproofing, but learning about these sleds with some basic and affordable tools will hopefully get you close. Find your weakness and build on it. You won't need all the answers, just enough to get you settled in. By the way I have already given you my thoughts in the thread that I linked below.

MELTED

 

For a hint on the oil, you may use the same principal of fuel/oil ratio for a given fuel consumption in mpg. It applies for all three sleds including the Polaris. The easiest way is to build your own chart and graduate the oil tank to then focus on oil versus fuel consumption. This way you can stop at a gas station and have a quick look to know if everything looks right without having to take out the calculator.

 

A compression check came out to 130 PSI in one cylinder and 30 PSI in the other.. He looked in through the exhaust and plug holes. The cylinders looked clean.

We decided to open it up. The low compression piston was concave, bowl shaped.

The good compression side showed signs of something (screw?) being sucked though it.there were marks on top of the piston.

What fuel are you running?

Were the marks on the top of the good compression piston fresh or had they been there for a while.

Was the bowl shaped piston top irregularly shaped....like it had softened or partially melted?

 

91 octane non-ethanol. Highest they had. I did put some Seafoam in with the fresh fill. I added it before I put n the gas, so it mixed. It was most of a whole tank fresh of fuel.

I didn't get a description of if the marks on top of the were fresh. He isn't say anything about the head.

Philosophically, how long would they look fresh after a rebuild and/or 70 imile ride? I was unaware (don't recall if it was mentioned when I bought it) it had ben opened up. I was told the cylinders were "clean" by the mechanic two days ago. ...no scoring.

The bowl shaped piston was described by the word "melted".

I did some reading before I bought both of these. A couple of my 'doo riding friends considered the 500SS a good sled. ....so, we are Skidoo owners.

My mind is wandering.. now.. My wife wants to go for a ride this weekend. The shop might have it back together tomorrow

The white 700 XC has been in the family for over 13 years. It had a bad rectifier once that manifested in some odd behavior. Rode it home with no lights one night. It's FAST. Never had to open the engine. It's been ridden in brutal cold with snow dust. It has electric start, but we never got a battery because after it's going for the season, it starts in a couple pulls. It has a primer which I love. It does not have reverse though. That was our main reason for wanting newer sleds. The XC hand warmers will give you blisters if your gloves are too thin. Last year I replaced the expansion chamber. The old one rusted open after 21 years...

I kind of miss my 2001 XCSP 800 too. It was starting to nickle-dime me and messing up rides due to mechanical/age/high miles problems.

 

If the marks on the top of the good piston were randomly scattered small dots, then that indicates detonation. And the 'melted' sounds right for a dish shape on the bad piston... a lot of heat and probably high pressure, so again, detonation is a likely cause. The 'spirited' riding would be a good way to make that happen, but something had to make the engine prone to detonate.

Some thoughts:

• Possibly some work on the engine previously raised the compression ratio. But the cranking compression on the good cylinder is does not seem out of line. (Unless the rings are damaged and actual compression is a lot higher.)
• Ignition timing set too far advanced... this will certainly cause detonation. But AFAIK, this is all set in the computer.... maybe Daag can comment on that.
• Fueling running lean. Could be all sorts of reason for that: low fuel pressure from bad fuel pump, restrictions or cracks in fuel line, fuel sock in tank partially plugged.

BTW, Seafom does not raise octane; it just absorbs some water in the fule and slows the breakdown of the fuel. Octane is one thing that adds detonation resistance. But if this was indeed detonation, then I think the issue was far beyond just octane.

 

Thank you for the observations.

I've used Seafoam for years, especially seasonal stuff, for cleaning the fuel system.

We did discuss timing during our conversations. Is that ECM driven? Does a program advance it during operation, that is, does it get advanced over a range in RPM?

 

Thank you for the observations.

I've used Seafoam for years, especially seasonal stuff, for cleaning the fuel system.

We did discuss timing during our conversations. Is that ECM driven? Does a program advance it during operation, that is, does it get advanced over a range in RPM?

And I just added the Seafoam comment to be sure no one thinks it is meant add octane rating.

And for my comment on the fuel pressure.... sorry, my brain had Mach Z SDI injection in it, not MXZ. So the comments from letmgrow apply.

From the 2006 manual on this engine, here is it's info for timing. If the engine has been 'into' previously and something timing related got disturbed then not checked, then it could be off. The ECM/MPEM adjusts it vs RPM, but has to use crank sensors (trigger coils) to set the base timing.

Liquid-Cooled Engines
Normally ignition timing adjustment should not be
required. It has been set at factory and it should
remain correctly adjusted since every part is fixed
and not adjustable. The only time the ignition timing might have to be changed would be when removing and reinstalling the magneto housing, replacing the crankshaft, the magneto flywheel, the
trigger coil or the MPEM or ECM. If the ignition
timing is found incorrect, first check for proper
crankshaft alignment. This might be the indication
of a twisted crankshaft. Refer to LEAK TEST AND
ENGINE DIMENSION MEASUREMENT.
The ignition timing can be checked with either the
engine hot or cold. Also, the ignition timing is to
be checked at 3500 RPM with a timing light except on SDI engines. On these engine the ignition timing must be checked at 4000 RPM.
NOTE: Except on the SDI models, between
3000 and 4000 RPM, the spark advance does
not change. So when checking ignition timing at
3500 RPM, a change in engine speed within ±
500 RPM will not affect the timing mark when
checked with the timing light.

I would be more inclined to think your issue is fuel or seal related rather than timing. If it was timing you should see evidence of overheating or melting on the other piston. I'd be checking carb boots and reed gaskets for signs of failure as well as crankcase seals.
Possible restricted main jet or needle out of position?

That is why I asked about the marks on the good piston... if they were scattered tiny marks, like dots. That is a sign of detonation. In my experience, one piston suffers a severe crisis with detonation, while other may or may not show problems, even with detonation from excessively advance timing.

But I'll certainly go along with fueling being the more probable cause.

 

I would be more inclined to think your issue is fuel or seal related rather than timing. If it was timing you should see evidence of overheating or melting on the other piston. I'd be checking carb boots and reed gaskets for signs of failure as well as crankcase seals.
Possible restricted main jet or needle out of position?

 

He is saving the Pistons for me. I will post pictures.

 

 

The bowl shaped one is on the left. iIts the black spot on the side that's towards us as viewing the picture. I was actually picturing something much more severe from the verbal description.

My mechanic said the ring the land had gotten elongated so the ring was slapping back and forth. There's a chunk of carbon crust in the one side also.

 

 

The piston with the dings... Looks like pump cavitation to me from what I've heard described as.

No air filters on these, huh? My wife (500SS) hit this with her ski and dragged it a few hundred yards on the gravel logging road. It threw a big 18" tall rooster tail of snow, brake side, while the ski dragged it. ...trail crud?

Mechanic said tiny gravel would just get crushed and blow through.

He did run a ball-hone to scar up the cylinder to seat the rings. The bolt that holds the clutch on was missing? He replaced that.

...and a new hand warmer.

 

The piston with the dings... Looks like pump cavitation to me from what I've heard described as.

I wonder if this is what it looks like when pockets of glycol auto-ignites.

 

 

Good pix. Here you go..... refer to page 44 for detonation damage. Looks like you won that prize!

https://www.memoparts.com/img/cms/Documents/Piston%20Failue.pdf

Hard to see the 'bowl' but will take your word of it. If the ring land was widening, then the piston top may been getting soft from heat.

 

Anytime I run across a melted piston(s) and I can't find an obvious reason for it I back up and take a close look at the fuel delivery system. I will drain the fuel tank completely and remove the fuel line and fuel filter from the fuel tank. The filter on the end of the pick up line is probably one of the most forgotten items in the whole fuel system! I change my filters periodically just as preventative maintenance. I have never found a satisfactory way to clean these filters. Another thing to check is the fuel line in the tank. These lines can rot away to the point where the filter can become detached from the line. This can present two problems.
# 1, the fuel drawn out of the tank will not be filtered.
# 2 the line may tend to float in the tank and when the fuel level becomes low the line may rise enough where it's drawing in air as well as fuel which will starve the carburetors of fuel. The filter acts as a weight to hold the line low in the tank. These pick up lines can crack over time too allowing air into the system. I will only use BRP Polyurethane lines part # 415005700 in my Mach 1 and Z sleds. Not over the counter gas line. I'm especially particular with our Pro Improved Mach 1 which gets fed VP C 14 fuel. It can be rough on fuel lines.
To be considered: The filters are small in size. Over a period of time these filters can become plugged and limit the amount of fuel which can be drawn through them.
Being hidden inside the tank these items are often forgotten or overlooked. When is the last time you checked yours?
I do my best to try using only filtered fuel. I have a funnel which has a fine mesh screen and a chamber under it which will catch any debris or water which might otherwise end up in my fuel tank. I don't use fuel additives. There is a warning on my funnel with the screen in it which states "additives may allow water to pass through the filter. If you are buying fuel on the trail this isn't practical. You can only hope the tanks owner keeps a vigilant eye on filters etc. to pump only clean fuel. Most of these tanks sit outdoors in all kinds of weather. Hot, cold and everything in between. I'm sure they don't get "Dip" tested like the tanks at a filling station with a long stick with a test compound on the stick.
One other side note about buying fuel "On the trail". You never know what you are getting. A few years back I was riding with my family and we stopped at a popular place for a burger and a drink. This was in the middle of the week and my kids were on winter break from school. I watched as a fuel truck pulled up to the gasoline storage tanks. There were two tanks. One for regular gasoline and the other for premium gasoline. The tanker driver filled the regular gasoline tank then stepped over and began to fill the premium tank. He didn't touch any valves or anything else on the truck. This told me anyone paying for premium gasoline was getting the same gasoline I was when I was filling up the sleds with regular. Anyone buying premium was only paying a premium price for regular grade gasoline.

 

Or people buying regular were getting premium from that load.... LOL

 

Makes me wonder what was in his burger!

 

They used to have horses.

 

Did you know that before you ordered?

 

The Army had worse.

 

There are pictures of both side of the pistons. I had trouble posting. ...posted twice.

 

The skirts look like they get a healthy dose of snow/water. So far I see no clear indication of coolant ingestion. If there was then I would expect the head and piston crown to be noticeably cleaner on the failed side. Does the intake side show the same? It would be nice to inspect the rods and crankcase for what is left of an oil film if any, but I understand you don't have those parts. When I come-up against a type of failure that I am not familiar with, or any failure for that matter, I go through lengthy process of elimination. Even so it may not solve the crater riddle. Were the craters on the mag side?

 

I wouldn't worry too much about owning what happened. The next step is the choice of repair and this is where it gets tough. In season $100 top end ball hone repairs are the toughest needing a degree of luck.

 

I wouldn't worry too much about owning what happened. The next step is the choice of repair and this is where it gets tough. In season $100 top end ball hone repairs are the toughest needing a degree of luck.

And that last phrase is my concern. That is why I asked about everything else.

The new pix of the pistons, picture 2 in post #27, show a lot. The top of the right piston, the one with the dished top, is starting to lift as the area behind it sunk. If you look behind there in the ring groove, you will probably see some distortion in the back of the groove. This piston got hot.... I am aware of one case of the whole piston top pulling off in the case of excessive heat, like this looks like it is starting to do. IDK if the shiney area below the ring is an indication of heat or is just wear from piston rock, but it is pretty odd looking to me.

So detonation still ranks at the top of my list. The reason to ask about all the seals and gaskets is that there could be a crankcase leak that allows the mixture to lean.... causing the detonation. Ditto for any water/coolant in a cylinder. Since it sounds like this engine has been 'into' before, then who know what was done right or wrong in regards to leaks.

Daag, help me here to learn something.... what are the signs of water ingestion that you see?

 

Nm9stheham, the dished top illusion can be seen on the right side piston in the picture below. You might need to zoom into the picture to see the slightest shade.

For the shiny area below the ring, this is indeed wear from excessive piston rock. I find it worsens as the bottom of the skirt deforms. It is an illusion from the shadowns, or does the ring groove appear to have grown?

 

Nm9stheham, the dished top illusion can be seen on the right side piston in the picture below. You might need to zoom into the picture to see the slightest shade.
View attachment 1991947

I am taking the word of the OP and his mechanic that there is a 'bowl' in the top of the one piston. See below....

ANd I don't think there is enough carbon on the pitted piston to reveal a bowl shape once removed.

It is an illusion from the shadowns, or does the ring groove appear to have grown?
View attachment 1991961

The OP's said in pot #13 that his mechanic stated that the groove had a lot of side clearance in it ("gotten elongated"), so I assume this is real and is what he was referring to. That is why I mentioned the top of the piston separating. I know of one case, and have heard indirectly of other cases.

 

Do you have a picture of the cylinder head? Mainly the combustion chambers.

 

It's back together. My mechanic rode it, checked the color of the plugs, light tan. We put about 30 miles on it Saturday afternoon.

So, I had asked him about fattening the jets. His feeling was tha lt the plug color meant it didn't warrant it.

I found a YouTube video the tank filter... Apparently there is also another in-line filter behind the dash...

 

OK, yes that 'stamp' is barely seen in the photo of the carbon covered piston. But I don't think that is what the OP is referring to. I think he means a bowl or concave shaped area on the piston top right behind the area where the ring land is wide. The bowl would be caused by the top edge of the piston bending up as the material behind the ring land was getting soft. At least that is what I think he means!

It's back together. My mechanic rode it, checked the color of the plugs, light tan. We put about 30 miles on it Saturday afternoon.

So, I had asked him about fattening the jets. His feeling was tha lt the plug color meant it didn't warrant it.

I found a YouTube video the tank filter... Apparently there is also another in-line filter behind the dash...

Here we go! Hang on!

Take out the plugs a few times OP and examine the porcelain with a magnifying glass. If you see tiny pepper looking specks, then you are getting into detonation. Example pix:

Spark Plugs

BTW here is a piston pix from detonation that looks like the top of your pitted piston. See the 3rd pix down. There is a lot of stuff in this page that would be good for you to read IMHO:

Two Stroke Detonation

Story from Millenium Technologies - LINK HERE The most common mode of failure for 2-stroke engines is detonation. We asked 3 time AMA pro mechanic of the year, multiple SX/MX championship winning crew chief, and Millennium Technologies Technical Manager, Tom Morgan to put together brief guide...

www.tomdonneymotors.com