[nm9stheham]

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?

 

[nm9stheham]

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.

 

[nm9stheham]

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.

 

[nm9stheham]

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.

 

[nm9stheham]

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.

I hope this are just dumb questions..... Did it all come apart and new gaskets and seals everywhere? And crank bearings inspected? New pistons?

 

[nm9stheham]

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

 

[nm9stheham]

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]

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.

 

[nm9stheham]

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

 

[nm9stheham]

We also ran a couple lakes and roads. It sounds like this happens fast.

500ss Jetting - Lean Midrange

Ok we all know this issue. Lots of info on here about lean mid range on the 600 carb motors. Lots of good info with EGTs and thorough testing even. Now I'm going to try something that I haven't seen anyone try yet. 2006 500ss. I've tried all the variations of needle clip setting, pilot jets...

www.dootalk.com

The final destruction happens quite fast. Usually the stage get sets by conditions, things get good and hot and primed for destruction, and then a sufficiently long burst of WOT running does the deed. I raced a rally car engine for 30 hard miles in the mountains with severely advanced timing without failiing, detonating hard all the while. It finally failed on a 2-3 mile very steep climb up a high mountain, where the engine was at WOT for several minutes. Melto-smelto....! But it was probably all along putting pits in the piston that eventually failed catastrophically. (And no, I did not know it was detonating.... try hearing detonation in a rally car with a helmet on and gravel and rocks constantly banging the bottom of the car! Snowmobile opperation is pretty noisy too.. and you ride with a helmet. Same deal....)

That is why to check the plugs as suggested. This will show you if you are on the edge of detonation and getting enough detonation to put the pepper on the plug porcelain, but before you ran it at WOT long enough to cause the destruction. So please take some short WOT runs and then stop and pull both plugs and look for the pepper signs. Do this a few times under different conditions. And do some searches for other signs of detonation on plugs; the pepper on the porcelain is the one I know of best, but there may be other signs.

BTW, as far as the snow ingestion.... that is a very unlikely cause of detonation. In fact, I would not expect it at all. The snow and ice bits ingested have to get past the crankcase, where they probably will be evaporated. The moisture in the air will moderate the chamber temps and pressures, not raise them. I'll never say never, but unlikely.

BTW #2, the Donnelly motors site is for old Sabb 93 2-stoke engines. But they all work basically the same and you can learn from this info.

It is confirmed that you hit the nail on the head. This is not something that I have ever seen nor would have expected out of an OEM Ski-Doo piston, although their Hypereutectic pistons have been strengthened over the years for the HO High Output 600/800/1000 and once again when it they weren't surviving the 800R E-TEC. So even this being a forged piston, it doesn't speak for its quality. With the crown having curled up from excess heat, I am beginning to see how a cascade of superheated fuel/air pockets could have resulted in a surface moon lol

Well, there is no piston type that is proof against detonation damage and failure. The case I described above was with forged pistons. Being forged probably just extended the time until the final disaster... and that may be the case here. And I would suspect the curling at the top (or the tearing off of the whole top in some cases) probably mostly has to do with the thickness of the casting left behind the groove after the groove is machined. So that is more a casting and groove design matter than the material, IMHO.

Yeah that pitting aropund the edge says the detonation was starting in the squish zone... so it is some sort of very dynamic thing going on in this case. I would guess that the detonation occuring on one side was different than the other side; the failed piston side must have been getting a broad heat zone. No surprise; usually one piston out of many will fail and we immediately shut things down; the other pistons may or may not show signs of distress.

 

[nm9stheham]

Pre-ignition is spontaneous combustion prior to the spark. I suppose that can be considered a subset of detonation, but what most people refer to as detonation is after the spark is initiated. My understanding is that detonation is the more destructive type of spontaneous combustion as it is essentially an explosion (like TNT) rather than the desired steady burn. So while spark plugs are a source, they are not the only source by any means. A hot spot on the head or piston can set it off, or even just too high a localized temperature within the unburned mixture can set it off.

BTW, if your cylinder head had the same pock marks and craters on it as the one piston, then the edges of those will tend to be hot spots. So if the head was pocked, then you need to be vigilant about this.

Detonation is prone to occur only at WOT, when cylinder filling is at is highest and pressures and temperatures.

Too cool of a plug will not burn off the deposits. You just have to try a step cooler at a time and run only moderately to see if they foul up.

The 'reading' of plugs for mixture is not my area of expertise, though I have tuned carbs on cars this way. I look up the picture charts online! What you do is to run hard and then hit the kill switch at the moment of letting off the throttle. (And make sure you don't get thrown off if the sled suddenly decelerates!) Then you pull the plugs and read them for the mixture that they were seeing at WOT. Decelerating to idle and then stopping the engine will change the coloration so that is not the way to do it.

 

[nm9stheham]

Nm9stheham, I see the explanation as a reason that snowmobiles survive despite their apatite for snow ingestion. I wonder about the indigestion when the oil film is steamed <burp> lol

Thanks for the SAAB-O link. There is a quite the 2S following with SAAB.

Well it just makes sense. If an ice particle made its way into the chamber, then it probably would flash into steam once combustion started. The added pressure would increase the likelihood of detonation but the process of melting and then evaporating the ice particle would take a huge amount of heat out of the mixture, at least right near to the particle.

I think the lower heat will win for these engines, but I cannot say 100% for sure. I read as study from the 1980's in car engine research that high humidity in the air acted like as much as 3 to 4 extra points of octane; I assume that is because it moderated the combustion temps. Add enough snow or liquid moisture and it can lower torque output, as the cylinder pressures will be lower. This may be what people experience on snowmobiles.

(By contrast, when diesel engines ingest too much liquid water, then they suffer damage to the engine from excessive pressures. But the burn process is different and the pressure profile versus crankshaft rotation is a lot longer.)

I have not ever thought about effects on the oil film but it seems like it would have to be in a liquid form to effect it. (Like a cylinder head gasket leak.)

Yes those 2 stroke Saab's were pretty quick little cars!

 

[nm9stheham]

Loved your post on the 93's, letmgrow! They were a hot rally car on the 60's.

Never thought of the freewheel for that reason. The early Saab 99 4 strokes had it too.

 

[nm9stheham]

In reply to plugs and A/F, I use BR9 for the heat range and call it day. For the A/F I use stock jetting. For the 800R P-TEK and 600HO I built a Main Jet chart for different air temperature. What is missing is a chart to shim the needle position for the low to upper midrange. I thought of doing this with my Wideband, but it is more difficult than it sounds. The problem with jetting for safe is the noticeably lower power. I would love to jet for economy, but I don't believe this is possible with a carb. Then there is jetting for power which makes the sled responsive, much like it was done for the 600HO linked below. It is worth the read and question what throttle/load range do most problems arise in two stroke snowmobiles. Is it the top end or midrange? For many the midrange would not even be on the list.

Optimizing your 600HO

I would actually put lower and mid RPM ranges well up on the list. The simple matter is that there is more time in the combustion phase for the unburned mixture temps to rise high enough and for things to go wrong. Time is not the only factor by any means but it is A factor.

 

[nm9stheham]

So, I'm asking myself, would more coolant volume capacity in a head make for more heat sink and better cooling? Or, might a larger coolant volume create lower flow and the heat being removed less quicky?

My oldest took Thermodynamics at MTU, but didn't want to hazard a guess.

Interesting thread on all of this. Same things go on over and over in the high performance auto world.

FWIW head cooling is always tricky so I don't blame your son LOL. Interior roughness seems like it would help heat transfer, but at the same time, it makes localized boiling spots worse, so it can hurt overall by making different spots in the head hotter.

As for flow, this is always off the mark. Flow rate by themselves don't really change much; this is becasue heat transfer rates at surfaces don't depend as much on flow rates as they do on surface turbulence as the coolant flows across them This applies in the head, rad and everywhere. Those turbulence issues can be modeled to limited degree (Reynolds numbers is one example), but ultimately, empirical testing is needed.

That was a pretty big change in jet numbers from dyno to outdoors. But, again, nothing new. Changes in temps AND humidity enter into it. The amount of actual water vapor in the air in the dyno area can be hugely greater than outdoors in very cold temps, and water acts like a significant octane boost.

 

[nm9stheham]

I like the camera speed.... 33,000 frames per second!

 

[nm9stheham]

Must.. resist...another... project....

 

[nm9stheham]

Yes. Which product and for what?

 

[nm9stheham]

I have no clue how well it will help something like a switch....