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MS 650 Now Has Zero Compression?

Wood Doctor

Wood Doctor

Edwin
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Post a picture of the spark plug. Was it brown or tanish color? How old was the fuel that you were running in it?
Spark plug was tan to brown in color with no fouling to speak of. The owner and operator (not me) says that he uses the best fuel mix he can buy, either Stihl or NAPA. He operates a logging company and goes through over a gallon of fuel a day with three saws, mixing it in the morning, so the fuel was fresh.

Here are two Pics of the plug, prior to blowing the hole in the piston crown:





FWIW, I notice that the hole in the piston crown is right below the decompression valve opening in the cylinder head.
 
sefh3

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My experience with old two stroke dirt bikes points to preignition burning a hole in the piston. It's usually caused by burning carbon deposits or the hot tip of a spark plug (heat range too hot).
Usually carbon plays a role in detenation.

What does the muffler look like? Is it loaded with carbon?
 
rupedoggy

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Spark plug was tan to brown in color with no fouling to speak of. The owner and operator (not me) says that he uses the best fuel mix he can buy, either Stihl or NAPA. He operates a logging company and goes through over a gallon of fuel a day with three saws, mixing it in the morning, so the fuel was fresh.

Here are two Pics of the plug, prior to blowing the hole in the piston crown:





FWIW, I notice that the hole in the piston crown is right below the decompression valve opening in the cylinder head.
Your FWIW tells me the release was leaking a little bit. It may have sucked in a little air and caused a hot spot/detonation right there. Few 100 thousand up and downs and you got a peek-a-boo piston!
 
Wood Doctor

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I Think You and I Have the Cause

Your FWIW tells me the release was leaking a little bit. It may have sucked in a little air and caused a hot spot/detonation right there. Few 100 thousand up and downs and you got a peek-a-boo piston!
+1. Now we are on track. I believe that is exactly what eventually caused the hole in the piston crown. It could have been a defective design. As much as we like this decomp feature, it promoted a long-term failure. Perhaps Stihl has corrected the situation with the MS 660 and other engines as well, but who knows? Few saws have been used as much as this one has in six years, because this logging company is very active, year round. So, with 4000 starts, this decomp valve could have been pressed and released automatically 10,000 times.

Does the Forum understand what RupeDoggy and I are discovering as a team? Please comment.
 
ZeroJunk

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I don't believe for a minute the decomp had anything to do with it. Everything is headed the other way when it fires. I suppose it could lean it out a tiny bit, but I don't seee it acting unlike any other lean condition.
 
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barneyrb

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I don't believe for a minute the decomp had anything to do with it. Everything is headed the other way when it fires. I suppose it could lean it out a tiny bit, but I don't seee it acting like any other lean condition.
Not true, all timing is BTDC (before top dead center). It fires so many degrees before the piston reaches top and then starts down.
 
CTYank

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Not true, all timing is BTDC (before top dead center). It fires so many degrees before the piston reaches top and then starts down.
The point (likely) is that the decomp valve is an innocent bystander, in that for the situations where air possibly flowing in from outside would NOT have vacuum inside to draw it in to dilute the mix.

Not to mention that closed valve would allow extremely small flow rate at the very best.

Red herring besides.
 
edisto

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Not true, all timing is BTDC (before top dead center). It fires so many degrees before the piston reaches top and then starts down.
I think his point is that it is hard for air to leak in when the charge is being compressed.
 
ZeroJunk

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I think his point is that it is hard for air to leak in when the charge is being compressed.
Yep. I think everybody knows that they usually fire somewhat before they get to the top, but at that point it is even more pronounced that whatever is in there is headed out the decomp, not in. If it can.
 
Wood Doctor

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Historical Failure Data

Yep. I think everybody knows that they usually fire somewhat before they get to the top, but at that point it is even more pronounced that whatever is in there is headed out the decomp, not in. If it can.
Agreed. Also, here's a case where performance history would be helpful--number of piston failures in engines with and without decomp valves. The problem is that in a 10-year span, the number of uses of the valve would vary all over the place from saw to saw and operator to operator. Some burly loggers don't even use the decomp valve when the engine is warm, and some may not even use it when they start cold. Regardless, valve use and engine hours would both have to be considered.

One has to wonder if Stihl or Husky has even collected any data on this since the decomp valve was invented, so I doubt they would even know.
 
fatboymoe

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Just kind of thinking out of the box a little. Maybe the improper use of a metallic piston stop could of damaged the top of the piston that caused this failure later on. Just a WAG on my part.
 
fatboymoe

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Yes, a piston stop. I am siting here looking at Woodland Universal Metal Piston Stop that could damage the top of the piston if it were slammed into from rotating the crankshaft too fast into the stop itself for clutch removal.

It could damage the piston by indenting it and causing a weak spot in the piston crown.
 
Wood Doctor

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Soon as I Get it Removed...

Plus one.
... I'll take a Pic of the removed piston, but it's goiing to take a a little longer than I thought. The special screwdriver I have on board for removing the cylinder base screws just snapped under torsion. Those things are fastened tight as a drum, perhaps even with thread adhesive. One was stronger than my favorite screwdriver. :mad:

When it comes time to work on Stihl engines, you live by the sword and you die by the sword.
 
Wood Doctor

Wood Doctor

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How did they get a pic of the plug before the damage was caused? Someone know the future? Or does he start each day with pics of each saws plug? (doubtful).
Next time I will say "after" rather than "prior". Is that OK with you?

This is the exact spark plug after the fact. Now, what do you have to say to help solve the problem?
 
morgaj1

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Could it be as simple as a casting flaw in the piston, resulting in an area that was slightly thinner than its surroundings? Might not ever show on a normal saw, but showed on this high-hour saw?
 
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